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I a a a a a a a a a a a a a a a a i 

Spaceborne Radar Observations 

A Guide for Magellan Radar-Image Analysis 

J. P. Ford 

H. G. Blom 

J. A. Crisp 

C. Elachi 

T. C. Farr 

R.S. Saunders 

E. E. Thellig 

S.D. Wall 

S. B. Yewell 

December 15, 1989
The research described in this publication was carried out by the Jet Propulsion Laboratory, California 

Institute of Technology, under a contract with the National Aeronautics and Space Administration. 

Reference herein to any specific commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government 

or the Jet Propulsion Laboratory, California Institute of Technology. 

JPL Publication 89-41
Foreword 

N • a a a IT a a I I a a a a a a a a 

The Magellan Mission will provide, for the first time, high-resolution images of almost the total 

surface of Venus. Past observations with Soviet Venera 15 and 16 spaceborne radar and Arecibo, 

Puerto Rico, and Goldstone, California, ground-based radar at medium resolution (a few kilometers) 

over large regions of the planet provided intriguing hints of large-scale morphological features that 

could be associated with impact, volcanic, tectonic, and erosional phenomena. The high resolution (a 

few hundred meters) of the Magellan images should allow clear identification of the processes that 

shaped the surface morphology. 

To facilitate interpretation of the Magellan data, it is important to collect and make widely 

available a comparative data set of images acquired over well-known Earth regions using an imaging 

system similar to that of the Magellan radar. Fortunately, the data available from three Earth-orbiting 

radar missions—Seasat (1978); the Shuttle Imaging Radar A (SIR-A, 1981) and Shuttle Imaging Radar 

B (SIR-B, 1984)—make this possible. These three missions acquired data over a wide variety of 

regions, including some that reflect processes similar to those that might have shaped the Venusian 

surface: impact features, volcanic regions with a wide variety of age and type, regions shaped by wind 

erosion and fluvial erosion, and areas shaped by tectonics. 

This publication provides examples of radar images of well-documented Earth regions to help 

those planetologists who have little familiarity with radar-image interpretation better understand the 

data. We hope this information will increase the number of scientists involved in the analysis of 

Magellan data. 

The examples are grouped under the following general categories: impact structures, volcanic 

landforms, eolian features and subsurface imaging, and tectonic landforms. Each includes a thorough 

analysis of representative radar coverage acquired with Seasat Synthetic-Aperture Radar (SAR), SIRA, or SIR-B, as well as supporting information from field work or geologic maps; a reference list is 

provided for readers interested in further in-depth study. 

In addition to the examples, some fundamentals of imaging radars and signal interaction with a 

planetary surface are provided. Particular emphasis is given to the geologic interpretation of radar 

images, and examples of radar-image anomalies are included to help the interpreter recognize potential 

image artifacts.

li^f_ozz 

111
Acknowledgments 

MORONS OWNERS OWN was 

This publication represents part of the contribution of Charles Elachi and R. Stephen Saunders as 

members of the Magellan Science Team. Their effort was made possible by the Magellan Project's 

Science Office under contract (NAS 7-100) to the National Aeronautics and Space Administration. 

Publication coordination support was provided by the Data Management and Archive Team under L. 

Pieri, Team Chief. We are grateful to A. Richardson for her wizardry in locating and providing necessary negatives for the images. This publication would not be complete without the excellent support of 

M.F. Buehler, R. Chandler, R. Dumas, T. Kiriyama, M. Kirk, A. Riethle, and A. Stein of the JPL 

Documentation Section. Also, we express thanks to the JPL Photo Lab Operations for their valuable 

assistance. Special gratitude is given to D. Fulton who edited this publication. 

1EflIItG PAGE BLANK NOT FILMED
Abstract 

9 9 9 9 9 H 9 H 9 9 H 9 9 9 9 H H H 

Geologic analyses of spaceborne radar images of Earth are reviewed and summarized with respect 

to detecting, mapping, and interpreting impact craters, volcanic landforms, eolian and subsurface 

features, and tectonic landforms. Interpretations are illustrated mostly with Seasat SyntheticAperture-

,Radar and shuttle-imaging-radar images. Analogies are drawn for the potential interpretation of radar 

images of Venus, with emphasis on the effects of variation in Magellan look angle with Venusian 

latitude. In each landform category, differences in feature perception and interpretive capability are 

related to variations in imaging geometry, spatial resolution, and wavelength of the imaging radar 

systems. 

Impact craters and other radially symmetrical features may show apparent bilateral symmetry 

parallel to the illumination vector at low look angles. The styles of eruption and the emplacement of 

major and minor volcanic constructs can be interpreted from morphologic features observed in images. 

Radar responses that are governed by small-scale surface roughness may serve to distinguish flow 

types, but do not provide unambiguous information. Imaging of sand dunes is rigorously constrained 

by specific angular relations between the illumination vector and the orientation and angle of repose of 

the dune faces, but is independent of radar wavelength. With a single look angle, conditions that 

enable shallow subsurface imaging to occur do not provide the information necessary to determine 

whether the radar has recorded surface or subsurface features. The topographic linearity of many 

tectonic landforms is enhanced on images at regional and local scales, but the detection of structural 

detail is a strong function of illumination direction. Nontopographic tectonic lineaments may appear in 

response to contrasts in small-scale surface roughness or dielectric constant. The breakpoint for rough 

surfaces will vary by about 25%,through the range of Magellan viewing geometries from low to high 

Venusian latitudes. Examples of anomalies and system artifacts that can affect image interpretation are 

described.

vii 

PRECEDING PAGE BLANK NOT FILMED

INTENTIy 6Lq4K,
Contents 

X a a a a a a IF a a a a IN IF a a a a 

Synthetic-Aperture Radar Imaging..........................................................................................................I

Introduction......................................................................................................................................i

ImagingGeometry ............................................................................................................................1

Responseto Terrain ..........................................................................................................................2

Slope..........................................................................................................................................2

Roughness.................................................................................................................................2 

MagellanData Characteristics .................................................................................................................5

Introduction..................................................................................................................................... 5

MissionDescription.......................................................................................................................... 5

Data-Acquisition Parameters ............................................................................................................6

Processing.........................................................................................................................................8

DataProducts....................................................................................................................................8

Full-Resolution Basic Image Data Record ................................................................................ 9

Full-Resolution Mosaicked Image Data Record .......................................................................9

Mapprojections .......................................................................................................................10 

ImpactStructures ................................................................................................................................... 13 

Introduction................................................................................................................................... 13 

ImagingCharacteristics .................................................................................................................. 13 

Cratersand Structures ...................................................................................................................... 14 

Simple Terrestrial Structures .......................................................................................................... 15 

Barringer, U.S.A . ..................................................................................................................... 15 

Talemzane, Algeria ................................................................................................................. 16 

VolcanicMaar Craters.................................................................................................................... 17 

Elegante, Mexico..................................................................................................................... 18 

Complex Terrestrial Structures ........................................................................................................ 19 

Elgygytkhyn , U.S.S.R. ............................................................................................................. 19 

Manicouagan, Canada ............................................................................................................. 20 

VenusianImpact Craters ................................................................................................................ 21 

Discussion................................................................................................................................... 22 

Cratermorphology .................................................................................................................. 23 

Radarhalos .............................................................................................................................. 23 

Resemblance to volcanic maars .............................................................................................. 24 

Spatialresolution..................................................................................................................... 24 

VolcanicLandforms .............................................................................................................................. 27 

Introduction.................................................................................................................................... 27 

Morphologic Scales and Radar Observation Parameters ............................................................... 28 

Observations of Terrestrial Volcanic Areas ................................................................................... 30 

Floodbasalts............................................................................................................................ 30 

Iceland.............................................................................................................................. 30 

Columbia Plateau, Northwest U.S.A................................................................................ 33 

Basaltic plains: Craters of the Moon, U.S.A . .......................................................................... 35 

Shieldvolcanoes...................................................................................................................... 39 

KilaueaVolcano, U.S.A . .................................................................................................. 39 

Pinacate Volcanic Field, Mexico ..................................................................................... 41

p90 PA PlANK T FJLME

ix

V i L INTENtwA &Af$ 
Medicine Lake Highland, U.S.A . 42 

FantasticLava Beds, U.S.A . ............................................................................................ 44 

PisgahLava Field, U.S.A . ................................................................................................ 47 

Composite volcano: Mount Shasta, U.S.A . ............................................................................. 48 

Explosivevolcanism ............................................................................................................... 48

VallesCaldera, U.S.A . ..................................................................................................... 49 

Ignimbrite sheets, central Andes, South America ............................................................ 49

Summary ......................................................................................................................................... 51

Sand Dunes and Subsurface Imaging .................................................................................................... 55

Introduction ................................................................................................................................... 55

EolianDeposits ............................................................................................................................... 55

Look-angle effects .................................................................................................................... 55

Illumination-direction effects .................................................................................................. 58

Conclusions ............................................................................................................................. 58

SubsurfaceImaging ........................................................................................................................ 59

Necessaryconditions ............................................................................................................... 59

Variables in interpretation of Magellan images ...................................................................... 59

Effectsof grain size .......................................................................................................... 59

Look-angle considerations ............................................................................................... 60

Application to Magellan images .............................................................................................. 61

TectonicLandforms ............................................................................................................................... 63

Introduction ................................................................................................................................... 63

TectonicStyles ............................................................................................................................... 64

Foldbelts ................................................................................................................................. 64

Southern Appalachian Valley and Ridge Province, U.S.A . ............................................. 64 

Central Appalachian Valley and Ridge Province, U.S.A . ................................................ 72 

SibiTrough, Pakistan ....................................................................................................... 72 

Accretedterranes ...................................................................................................................... 74 

Eastern interior Alaska, U.S.A . ........................................................................................ 74 

Continental boundary transforms ............................................................................................ 75 

SouthernCalifornia, U.S.A . ............................................................................................. 79 

Continentalinteriors ................................................................................................................ 85 

Continentalrifts ................................................................................................................ 85 

Continentalshields ........................................................................................................... 86 

Canyons ............................................................................................................................ 86 

Summaryand Discussion ............................................................................................................... 88

Image Anomalies and System Artifacts ................................................................................................ 91

Introduction .................................................................................................................................... 91

Unique Characteristics of SAR Images .......................................................................................... 91

Radiometriceffects ................................................................................................................... 91

Geometricartifacts .................................................................................................................. 94

SARProcessing Errors ................................................................................................................... 96

Ambiguities ............................................................................................................................. 96

Range-gate misalignment effects ............................................................................................ 99

References ............................................................................................................................................ 101

Glossary ............................................................................................................................................... 115

x 
Appendixes 

A. Orbital and Physical Properties of Venus and Earth ............................................................. 121

B. Magellan Standard Data Products ......................................................................................... 123

C. Magellan Special Data Products ............................................................................................ 125

xl
Synthetic-Aperture Radar Imaging 

I I I I I I I I I I I I I I I I I I I I I 

Introduction 

Synthetic-aperture radar (SAR) transmits 

pulses of electromagnetic energy perpendicular 

to the line of flight of a moving platform 

(spacecraft) and measures and records the 

intensity, time delay, and frequency shift of the 

echoes. Each pulse spreads across the terrain as a 

section of a cone, and echoes are generated as 

the pulse is reflected. Measuring and dividing 

the echo in time gives cross-track (range) 

resolution because the earliest echoes come from 

the closest part of the illuminated terrain, and 

later echoes come from progressively greater 

distances. 

Relative motion between terrain and 

spacecraft in the along-track (azimuth) direction 

induces a position-dependent frequency change, 

or shift, on the echoes. Echoes from terrain 

ahead of the spacecraft have a higher frequency 

and echoes from terrain behind the spacecraft 

have a lower frequency than the transmitted 

signal. Doppler measurement of the reflected 

signal provides terrain location information in 

the flight-track direction as a function of the 

frequency shift. 

Image resolution elements are thus created 

by the combined analysis of time delay and 

frequency shift. Resolution elements are 

illuminated more than once as the SAR flies 

along, and the spatial resolution of a large 

antenna is synthesized by multiple sampling 

from a smaller antenna; this is the syntheticaperture technique. Each resolution element is 

assigned a brightness level corresponding to the 

strength of the echo from that portion of the 

terrain. These concepts are illustrated in 

Figure 1. Further explanation of the technique 

can be found in Elachi (1980 and 1987). 

Imaging Geometry 

It is important to define some of the terms 

that refer to the geometry of radar images, 

because these terms are sometimes used in 

different ways. The terms are illustrated in

Figure 2. Look angle is that angle between the 

sensor nadir and the antenna boresight. It is 

fixed at the sensor and does not vary with terrain 

slope. Look angle is the complement of depression angle, a term often used in describing 

airborne imaging-radar geometry. Incidence 

angle is that angle between the surface normal 

and the radar beam. Consequently, incidence 

angle is a function of both the look angle and the 

slope of the terrain observed. Look angle and 

incidence angle are equal in value only if the 

terrain is horizontal and the radar-platform 

altitude is low enough to make planetary 

curvature negligible. Look angle and incidence 

angle are never the same in spaceborne radar 

images because of planetary surface curvature, 

which increases the incidence angle slightly. 

Figure 3 shows the nominal incidence angle 

of the Magellan imaging radar as a function of 

Venusian latitude. Because many of the image 

examples in this publication are from Seasat or 

Spacecraft 

Flight 

Line -

rT

Lines of 

Equl-Doppler 

Illuminated 

Area

Figure 1. Constant 

time-delay and 

Doppler contour 

lines that form the 

radar-imaging 

coordinate system. 

Image resolution 

elements are defined 

by intersections of 

Doppler resolution 

elements and timedelay resolution 

elements. The 

brightness in each 

image resolution 

element is 

proportional to the 

backscattered 

energy. Figure 

copyrighted 1980 by 

the AAAS (Elach4 

1980).
Figure 2. 

Nomenclature for 

imaging radar 

geometry. For a 

horizontal surface, 

the look angle is 

slightly less than the 

incidence angle 

because of planetary 

curvature when 

viewed from orbit.

11

Depression 

Angle 

Incidence 

Look Angle 

Angle 

Planetary Surface 

SIR-A, their incidence angles are also indicated 

on the figure. The effects of incidence-angle 

variation relative to latitude on feature identification are described and illustrated in several 

examples in the text; these variations must be 

kept in mind when interpreting Magellan radar 

images. 

Response to Terrain 

The factors that control brightness in radar 

images are, in the decreasing order of effect, 

surface slope, surface roughness, and the 

dielectric constant of the material imaged. On 

Earth, dielectric constant is mainly a function of 

moisture content: dry soil has a lower dielectric 

constant than wet soil, and wet soil is a better 

radar reflector than dry soil. The values of the 

dielectric constant of most geologic materials are 

broadly similar (Campbell and Ulrichs, 1969). 

Despite the small differences, variations in 

dielectric constant may have geologic significance (Zisk and Mouginis-Mark, 1980; Jurgens 

et al., 1988), if the subtle effects can be reliably 

detected. The effects of slope and roughness are 

reviewed below. 

Slope 

Very significant in the interpretation of radar 

imagery is the relationship between surface slope

and the incident radar beam. If a surface is tilted 

nearly perpendicular to the radar beam, it will 

reflect the greatest amount of radar energy (i.e., 

echo) and, in turn, produce a very bright area on 

the image. Conversely, a surface that slopes 

away from the radar will produce a weaker echo 

and therefore a darker area on the image. This 

effect emphasizes topography, which can be 

useful for mapping geologic structures. 

If a surface slopes away from the radar at a 

great enough angle, the area will receive no radar 

illumination and a shadow will result. In 

Magellan images, shadows will result where 

surfaces are inclined away from the radar 71 deg 

or more at the minimum look angle of 14 deg 

plus 5 deg for planetary curvature, and 44 deg or 

more at the maximum look angle of 44 deg plus 

about 2 deg for planetary curvature. The 

steepness required to produce a shadow increases 

as the strike of the slope moves farther out of 

alignment with the radar beam. Most terrestrial 

slopes are less than the 44-deg minimum 

indicated above, and so, presuming Venus is 

similar, it is probable that shadows will be uncommon in Magellan images. Because the 

Magellan look angle will vary with latitude, the 

expressions of topography, shadows, and 

variations in backscatter will also vary with 

latitude. This topic is covered in later chapters 

of this publication. 

Roughness 

Brightness variations in SAR images give 

clues about the surface at a scale much smaller 

than the spatial resolution of the radar image. 

When imaged by SAR, a smooth surface such as 

a playa reflects most of the energy away from the 

radar and thus is dark on the image. The highest 

parts of an alluvial fan, on the other hand, have 

rough surfaces composed of cobbles and 

boulders. Thus, although cobbles and boulders 

cannot be resolved on the image, a rough surface 

can be inferred because of the bright area caused 

by strong echoes. 

The surface-roughness response is wavelength dependent. Long wavelengths are not 

backscattered by relatively fine-textured surfaces. The longer the wavelength, the coarser the 

surface must be for backscattering to occur. 

Because imaging radars operate over a broad 

range of wavelengths (from 1 to 75 cm), the 

texture of a given surface can vary from rough
when observed at a short wavelength to smooth 

when observed at a longer wavelength. 

Peake and Oliver (1971) studied the 

relationship between surfaces of known roughness and radar backscatter. Practical geologic 

applications relating surface roughness to radar 

backscatter are described by Schaber et al. 

(1976) in studies of radar response to alluvial 

fans and lava flows. Additional useful discussions can be found in Sabins (1987), MacDonald 

(1980), and MacDonald and Waite (1973). 

Schaber et al. (1976) found good agreement 

between surface roughness measurements made 

in the field and the Rayleigh criterion for the 

transition from slightly rough to smooth (quasispecular reflection) behavior. The Rayleigh 

criterion formula is 

h<?/(8 cos i) (1)

Seasat SAR SIR-A

80 

60 

C) .40 

a) 

•D 

. 20 

'U 

-j 

cn 

cD -20 

-40 

-60 

Figure 3. Variation 

of Magellan nominal 

incidence angle with 

Venusian latitude, 

showing 

corresponding 

nominal values at 

beam centers for 

Seasat SAR, SIR—A, 

and Venera 15 and 

16. Short dashed 

lines indicate the 

latitudinal range of 

Venera coverage. 

Negative latitudes 

denote the southern 

hemisphere. 

where X is the radar wavelength, i is the incidence angle, and h is the mis height. 

Ulaby et al. (1982), however, found better 

agreement using the Fraunhofer criterion in a 

study of bare agricultural fields of known 

roughness. The Fraunhofer criterion formula is 

h < /(32 cos i) (2) 

The use of slightly different formulas 

illustrates the difficulty of modeling a complex 

surface, but the formulas do indicate a range over 

which the transition from slightly rough to 

smooth will occur. 

Peake and Oliver (1971) also defined a 

"rough" criterion, which describes surfaces that 

scatter essentially all of the energy in a diffuse 

manner. The formula is 

h> X /(4.4 cos i) (3) 

These formulas can be used as a guide in the 

interpretation of radar images, provided the 

limitations of modeling natural surfaces are kept 

in mind. 

The Magellan radar has a wavelength of 

12.6 cm. With the Fraunhofer criterion (Formula 

(2)) applied to Magellan images, a smooth 

surface (dark area) will have irregularities less 

than about 0.4 cm when imaged at the minimum 

incidence angle of 19 deg (a 14-deg look angle 

plus the angle added for planetary curvature of

0 10 20 30 40 50 60

Nominal Incidence Angle, deg 

the planet), and irregularities less than about 0.6 

cm when imaged at the maximum incidence 

angle of 45 deg. Use of the Rayleigh criterion 

(Formula (1)) results in values of about 1.7 and 

2.3 cm, respectively. 

Use of Formula (3) for rough surfaces 

indicates that a uniformly bright area will result 

from surface irregularities greater than about 3.0 

cm at a 19-deg incidence angle and greater than 

about 4.1 em at an incidence angle of 45 deg. 

The above discussion demonstrates that 

while radar-image-brightness levels are a reflection of surface properties, the radar response will 

vary with a number of factors that can be poorly 

known or difficult to determine. Also, for a 

given surface, image-brightness levels can vary 

as a function of radar geometry and wavelength. 

For Magellan, the observing wavelength is 

constant, but image geometry will vary with 

Venusian latitude. Accurate interpretation of 

Magellan images may in part hinge on recognition of the factors discussed above and illustrated 

by the examples that follow.

i-i
Apoapsis 

Occultation

80 

60 

tM 40 

0 

0

-20 -j

-40 

-60

- Orbits 

Past 

Venus 

Orbit 

Insertion 

p,hIq[II4AI iiiiitii 1NRI1 ci= 

WI:Mi. W.r, 00 

W;70am-I Wyl51M WW^_ a r141 1 WE 

SOIMIM==Wmc;MGM ' WEIWIM WE 

MW ENSW

ANUM

I 

LWIL 

____

-Days 

Past 

Venus 

Orbit 

Insertion 

-80

240 270 300 330 0 30 60 90 120 150 180 210 240 

Longitude, dog 

Superior

Conjunction

Gap 

28 deg -H HFigure 4. Magellan 

planned coverage 

(unshaded area) for 

Venus: Mercator 

projection, 1.0-km 

contours. 

Magellan Data Characteristics 

Introduction Mission Description 

The Magellan Mission will produce a global 

data set that will be used for geologic analysis 

and comparative planetology for many years to 

come. The acquisition of a global data set for 

Venus is an important part of completing the 

exploration of the inner planets, which will allow 

a detailed comparison of Earth, Mars, and 

Venus. In many ways, the Venusian data will be 

more detailed and more complete than anything 

planetologists currently have for Earth, since the 

largely unmapped ocean floors occupy 70% of 

our planet. The Venusian data will be of great 

interest to all geoscientists, allowing tests of 

many Earth geophysical and tectonic models. 

One of the objectives of the Magellan 

Mission is to produce a data set that will include 

the first systematic radar map of an entire planet. 

The data will be in several forms, but SAR 

images will provide the basis for the mapping.

Magellan will operate from a near-polar 

elliptical orbit with a period of 3.15 h. Periapsis 

will be at lO°N with an altitude of 250 km. The 

look angle at periapsis will be about 50 deg. 

Mapping will proceed from the north pole, with a 

look angle of about 14 deg and an altitude of 

2100 km, down to 70°S. The incidence angle 

will continuously change from about 16 deg at 

the start of a mapping swath to about 45 deg at 

periapsis and decrease again to about 20 deg at 

70°S. The planned coverage of Venus (less the 

polar coverage) is given in Figure 4. Mapping 

characteristics are summarized in Tables I and 2. 

More details of the Magellan mission plan can be 

found in Dallas and Nickle (1987) and the 

Magellan Mission Plan (1988). 

PRECEDING PAGE BLANK NOT FILMED 
Table 1. Magellan Data-Acquisition Parameters 

Parameter Value mapping

characteristics. Mapping frequency, swath/orbit 1

Because radar-image acquisition, processing, and interpretation differ significantly from 

SAR record rate, kbits/s 806.4 the usual optical television-imaging systems used 

•Record duration, mm/orbit 37.2 for planetary mapping, it is necessary to underPlayback rates, kbits/s 268.8 and 115.2 stand something about the design of the MagelPlayback duration, min 90 to 141.5 lan radar system.

Tape recorder capacity, bits 3.6 x 10

The data-acquisition parameters of the 

Magellan synthetic-aperture radar imaging 

True anomaly range during 80 to +80 system are given in Table 3 (also, see Johnson 

mapping, deg and Edgerton, 1985; Johnson, 1989). Several of 

Altitude range during mapping, km 250 to 2100 these parameters interact to control SAR image 

Look-angle range during 14 to 50 quality. The most important are spatial resolumapping, deg tion, the number of looks, amplitude resolution, 

Swath width, km 20 to 25 signal-to-noise ratio, and incidence angle. 

Duration of nominal mapping 243 Spatial resolution is specified in range, or 

mission, days cross track, and in azimuth, or along track. 

Latitude range mapped 90°N to 67.2°S Range resolution is determined by the transmitExpected coverage of Venus, % 71 to 79

ted bandwidth; when projected onto the surface, 

it is called slant-range resolution and thus 

Planet coverage per day, % 0.4 becomes a function of incidence angle. Slantrange resolution decreases as incidence angle 

increases. This effect is seen in the variation of 

Table 2. Mapping

Parameter Value Magellan slant-range resolution with latitude: 

orbital data for the from 120 to 360 m. Azimuth resolution is 

Magellan launch of Period, h 3.15 determined by the length of the synthetic 

May 4, 1989.

Periapsis altitude, km 250

aperture created by the moving spacecraft. In 

principle, azimuth resolution can be as small as 

Apoapsis altitude, km 8029 the antenna. Magellan azimuth resolution will 

Orbital speed at periapsis, km/s 8.4400 be held at 120 in for most of the mapping swath. 

Orbital speed at apoapsis, km/s 3.7771 The number of "looks" is the number of 

Orbital speed at ±80 deg true 6.9062 independent observations made for each resoluanomaly, km/s tion element (see Figure 1 in Synthetic-Aperture 

Venus sidereal period, days 243.01 Radar Imaging). The science requirements for 

Magellan specify a minimum of four looks. This 

Rate of rotation, deg/day -1.4814 minimum may be reached around periapsis, but, 

Rate of rotation, deg/orbit -0.1944 in general, more looks will be achieved. IncreasShift in ground track at equator, 20.53 ing the number of looks reduces coherent "noise" 

km/orbit or speckle that occurs in coherent imaging (see 

Periapsis latitude 1 0.0°N Image Anomalies and System Artifacts). 

Periapsis longitude at arrival 275.450 Amplitude resolution is a measure of how 

well the image data represent the backscatter, or 

reflection characteristics, of the surface over a 

wide range of reflectivity or backscatter 

strengths. Magellan science requirements 

specify that image quality must be maintained 

over a wide range of backscatter strengths. This 

would require a large number of bits per sample, 

more than are available. Therefore, a method of 

data compression called the "block adaptive

6 
quantizer" was developed and built into the flight

Parameter Value

Table 3. Magellan 

hardware. This technique preserves the ampli- SAR imaging system 

tude resolution while allowing the return of a Planned operating altitude, km 250 to 2100 parameters. 

larger quantity of high-resolution data. 

Thermal signal-to-noise ratio (SNR) for Possible operating altitude, km 225 to 3500 

Magellan is designed to be 8 dB because tests Radar frequency/wavelength, 2.385/12.6 

show that this level is approximately at the G Hz/cm

threshold between no noticeable improvement at System bandwidth, MHz 2.26 

higher values and noticeable image degradation Spatial resolution (slant range), m 120 to 360 

at lower levels. The system SNR, about 5 dB in Azimuth resolution, m 120 to 150 

the processed images, is the combination of all 

noise sources including those from radar Number of looks 4 or more 

hardware, processing, and the radio link. Swath width, km 20 to 25 

Incidence angle is the angle between the Antenna diameter, m 3.7 

incoming radar pulse and the normal to the Antenna look angle, deg 13 to 45 

surface. An incidence angle that is constant from nadir 

relative to the mean spherical surface of Venus Incidence angle on surface 16 to 45 

would have been desirable because a given (nominal), deg 

geologic terrain may appear differently at Polarization HH 

different incidence angles. In interpreting Transmitted pulse length, jis 26.5 Magellan images, then, more care and understanding of the effects of the varying incidence Pulse-repetition frequency, Hz 4400 to 5800 

angle, which decreases with increasing latitude, Transmitted peak power, W 350 

are required. Time-bandwidth product 60 

The elliptical orbit of Magellan will place SAR data on-board recorded 750 constraints on the maximum incidence angle that bit rate, kbits/s 

can be used for mapping (approximate values are Quantization, bits 2 given in Table 4). In general, higher incidence

angles result in less distortion in the images, 

especially in rugged mountainous terrain. The Table 4. Radar 

most serious problem arises when the slopes Altitude, Incidence Range Azimuth Looks performance 

exceed the incidence angle and layover occurs. km angle, deg resolution, resolution, 

This was not a severe problem for Venera 15 and ___________________________________________ 

m m parameters.

16 even at a 10-deg incidence angle because of 250 50 115 120 5 

the low resolution (1 to 2 km). That is, few 275 48 120 120 6 slopes of many kilometers extent and steeper 

than 10 deg occur on planetary surfaces. For 500 40 135 120 7 

smaller slope lengths at the resolution of Magel- 1000 31 170 120 8 

Ian images, steeper slopes are resolved, and 1750 20 260 120 12 

hence layover at low incidence angles becomes 2100 17 300 120 16 more of a problem. A reasonable upper limit is

the angle of repose of dry materials; this is about 

30 deg and independent of gravity, so it is the 

same on all planetary surfaces. Magellan science 

requirements specify that the radar must be 

operated at the largest angle of incidence that 

allows all the other image requirements to be 

met. More than 70% of the image coverage will 

be at incidence angles greater than 30 deg.

7 
In general, the SAR may be operated at any 

incidence angle lower than the maximum angle 

required for mapping. Other incidence angles 

will be obtained during the extended mission to 

provide stereoimage coverage. Table 4 shows 

the SAR performance at the planned mapping 

incidence angles. 

The elliptical orbit of the Magellan spacecraft will require frequent adjustment of the radar 

operating parameters because of changing slant 

range (400 to 2200 km) and variations in speed 

(6.9 to 8.3 km/s). The commandable parameters 

(look angle, 14 to 50 deg; pulse-repetition 

frequency, 4400 to 5800 Hz; processed bandwidth, 1300 to 4000 Hz; burst duration, 0.02 to 

0.2 s; and burst period, 0.2 to 1.0 s) must be 

balanced against the many mapping requirements. 

Processing 

The Magellan SAR data will require 

complex digital processing, or correlation, to 

create images. The digital processor used for 

Magellan is called the Advanced Digital SAR 

Processor. This system was developed for use 

by shuttlebome imaging radar systems such as 

SIR-C. 

The SAR Experiment Data Record (SAREDR) is processed by the Magellan SAR Data 

Processing Subsystem (SDPS). The SAR-EDR 

contains SAR echo bursts, orbit ephemeris data, 

antenna pointing information, and engineering 

data. The output of the SDPS is the FullResolution Basic Image Data Record (F-BIDR), 

which is the averaged composite of approximately 6000 single-look framelets, each from 

one SAR burst. 

To process a SAR burst into a framelet, the 

processor performs range compression, azimuth 

compression, radiometric gain compensation, 

geometric rectification, and framelet truncation. 

Range and azimuth compression provide the 

high spatial resolution. Radiometric compensation converts the data number (dn) values into 

the normalized backscatter coefficient. Geometric rectification projects the images into the 

sinusoidal projection and the along-track and 

cross-track (AT/CT) projection. Framelet 

truncation removes pixels that do not have the 

required quality based on processing bandwidth 

and range swath. 

Each burst of data is processed into a onelook image franielet (Figure 5). The framelet 

along-track dimension is determined by the 

processed bandwidth, and the cross-track 

dimension is determined by the collected swath 

width. Processed bandwidth is the range of 

Doppler frequency processed on each burst. 

Data from successive bursts are processed and 

averaged along track to create multilooks. 

Data Products 

Magellan data products will fall into three 

categories: standard products (Appendix B), 

special products (Appendix C), and cartographic 

products. Standard image products will include 

all raw data, correlated image strips, mosaics, 

and ancillary data. Special products will include 

images produced for science analysis and 

reports, and public-information products. 

Figure 5. Burstmode data 

collection.

Planet

Orbit Path 

K+2

Frameiet(K4

1)

D 

The Crosshatched Area Represents 

the One-Look Image Framelet 

Corresponding to Burst K + 1 

Processing 

Bandwidth 

Antenna 

Footprint

EEE K+1 

I / / / /

Frameiet (K .1 

Framelet (K) 

L, W, and D Vary as a Function 

of Orbit True Anomaly 

8
Cartographic products will be intermediateresolution scaled-map quadrangles in standard 

projections. A complete description of the 

Magellan science data products can be found in 

the Magellan Science Requirements Document 

(1987) and Appendixes B and C. 

Full-Resolution Basic Image Data Record 

The F-BIDR will be the fundamental image 

data set and the basis for all the other image 

products. The complete description of this 

product can be found in the Magellan Software 

Interface Specification (1988). The F-BIDR is 

produced by the SDPS. The F-BIDR is available 

in digital form on magnetic tape (2400-ft reels of 

6250-character/in, computer-compatible tape). 

It contains a copy of the ancillary data provided 

in the SAR-EDR, the full-resolution image data, 

processing parameters used to produce the 

image, results of analyses of selected raw data 

and selected single-look images, and the reduced 

radiometer data. One F-BIDR is created for each 

orbit, resulting in nearly 1900 records in the 

nominal mission. One F-BIDR contains approximately 120 Mbytes of image data and approximately 10 Mbytes of ancillary data. The F-BIDR 

label format is F-BIDR.onum;ver; for example, 

F-BIDR.001 ;2 identifies a reprocessed version of 

the data from mapping orbit number 1, version 2. 

The F-BIDR between 70°S and 89°N is in a 

sinusoidal equal-area projection with a localreference meridian that corresponds to the intersection of the SAR antenna boresight intercept 

locus and the equator of Venus. Pixels are 

resampled and placed along latitude lines and 

spaced 75 in apart. An overlapping section of 

each F-BIDR, between 80°N and the north pole, 

is projected in the AT/CT grid system. The AT/ 

CT projection places image lines along great 

circles normal to the spacecraft ground track. 

The orbit plane is determined at 85°N and the 

image line that includes that latitude becomes the 

reference meridian for the AT/CT sinusoidal 

equal-area projection of that orbit. 

Pixel values are in units of normalized radar 

backscatter represented by 8 bits. The pixel 

values are the decibel (dB) representation of the 

ratio of the radar cross-section value obtained 

from the SAR processing and the corresponding 

value in the backscatter-coefficient model:

- 0.Ol88 cos I 

- (sin I + 0.111 cos3 I) 

The steps are 0.2 dB with a range of values 

from -25 dB to +25 dB, using data numbers (dn 

or pixel value) from 1 through 251. A dn of 0 is 

used as a filler value, and numbers 252 through 

255 are data descriptor tags. 

Full-Resolution Mosaicked Image Data Record 

The F-BIDRs will not be in a format useful 

for geologic mapping. Consequently, the FBIDRs will be assembled into Full-Resolution 

Mosaicked Image Data Records (F-MIDRs) 

(Figure 6) by the Magellan Image Data Processing Subsystem (IDPS). In the standard processing for the nominal mission, 15% of the planet 

will be mosaicked into F-MIDRs; this will result 

in approximately 240 full-resolution mosaics. 

To provide coverage for larger areas, the 

data will be averaged in successive operations in 

which a mosaic pixel is replaced by the average 

of nine pixels. The first step of this process is 

the creation of a 3 x 3 compression of the F- BIDR called the compressed BIDR or C-BIDR. 

The C-BIDRs are mosaicked into 8192 x 8192 

arrays called C1-MIDRs. C2-MIDRs and C3-

MIDRs are successive 3 x 3 compressions into 

8192 x 8192 pixel arrays. The top 7168 x 8192 

pixels are image data and the bottom 1024 x 

8192 pixels are annotation. The SAR image 

field contains a scale grid. The image data field 

is partitioned into a 7 x 8 array of 1024 x 1024 

pixel subframes. 

All the data products below 80°N latitude 

will be in sinusoidal equal-area projection. The 

F-BIDRs will be available in this projection up 

to 890N latitude. 

The various MIDR products will be distributed as photographic prints and digital files on 

CD-ROM. The mosaicked data will fill more 

than 100 CDs. In general, the photoproducts are 

spatially filtered to suppress intensity discontinuities at the seams, and the resulting products 

incorporate a contrast enhancement. The 

archived digital data are not altered in this way, 

so any digital standard-product mosaic will have 

the unfiltered, original dn values.

9
[I 

5° 

150 

Figure 6. Magellan 

image data 

products. (1 

C2-MI DF 

(45° x 45 

C1-MIDF 

(15° x 15 

F-M1DR 

(5° x 5°) 

F-E 

(2C 

F-BIDR Full-Resolution Basic Image Data Record 

F-PIDR Full-Resolution Polar Image Data Record 

F-MIDR Full-Resolution Mosaicked Image Data Record 

C1-MIDR Compressed-Once Mosaicked Image Data Record 

C2-MIDR Compressed-Twice Mosaicked Image Data Record 

C3-MIDR Compressed-Thrice Mosaicked Image Data Record 

P-M1DR Polar Mosalcked Image Data Record

The polar AT/CT image data between 80°N 

and 90°N will be incorporated into Full-Resolution Polar Image Data Record (F-PIDR) frames. 

Each strip is cut into 7168 pixel segments and 

placed side by side in frames. 

Map projections 

Magellan is a mapping mission, and 

therefore maps are emphasized in the data 

products. Base maps will be produced at various 

scales using standard projections. These maps 

will be used to present the results of the geologic 

analysis of the mission data (Figure 7).

Because Venus, like Earth, is essentially a 

sphere, map projections developed by cartographers to depict the spherical surface of Earth can 

be used. The problem is one of producing a 

curved surface on a flat sheet while minimizing 

distortion. A globe might appear to be the best 

map to use, but it cannot be made large enough 

for most applications, and it is difficult to make 

measurements on a surface that is not flat. With 

any flat projection, compromises must be made. 

Equal-area maps preserve the area from place to 

place, but distort shapes and scale. Conformal 

maps preserve the shapes of small features, but 

distort large regions to some degree. 

The Magellan Project will use conformal

map projections. The important features of this 

10
True 

1:5M 

Scale True Scale 

 n

75Polar Stereographic 

73.4° At 900 Projection 

27.5° 

75°

T

______ 

/nV15

/_\ 'V2' 

Conformal Conic Projectic 50°

V16 V17 /iT\ //V8 V 

25° I 

180 : 21101, 3001 

25° 

0° 

-25° - 180° 210° 2401 270° 300° 330°

V50 V51 V52 V53 

Lambert 

-2

mal 

-500 V59 V60 V61 

\ -,j1:5M\- /' 

Scale -J 

-27.5° 

-73.41 True Scale I' 

±22.8

Polar RtrnnrnhIr

Figure 7. The 

Magellan Venus 

mapping plan. 

.—' 75° V4 \ 

!650o 

0° 

0° 60° 1200 

V26 V27 .I... V28 V29 V30 I V31 V20 I V21 V22 V23 V24 V25 

True

1:5M

I 

I Mercator Projection 

V38 V39 1V40 V41 V42 V32 V33 V34 V35 V36 V37 

Scale

r - I-25° 

00 30° 600 90° 120° 150° 180° 

¶E5v46v47v8v49 7 .25

62 Sheets

At 90° \..750) Projection 

type of projection are the true angles about a 

given-Point and the constant scale in all directions about a point. This means that lines of 

latitude cross lines of longitude at right angles 

just as they do on the globe. 

Three standard conformal map projections 

will be used for the maps of Venus: Lambert 

Conformal, Mercator, and Polar Stereographic. 

The areas covered by these projections are 

shown in Figure 7. The Mercator projection has 

parallel, equally spaced meridians of longitude 

crossed at right angles by horizontal parallels of 

latitude. The scale is a true 1:5,000,000 at ±22.8 

deg latitude. This projection will cover up to the 

25°N latitude to minimize the kind of distortion 

in some maps of Earth that makes Greenland 

appear so much larger than it is. 

The Lambert Conformal Conic Projection 

will be used between latitudes of 25 and 75 deg, 

both north and south. A true 1:5,000,000 scale is 

placed on parallels of 27.5 deg and 73.4 deg,

north and south. On this projection, parallels of 

latitude are concentric circles while meridians of 

longitude are equally spaced straight lines that 

are radii of the parallels, thus cutting them at 

right angles. 

The Polar Stereographic Projection will be 

used for the polar regions down to 75°N from the 

north pole and up to 75°S from the south pole. A 

true 1:5,000,000 scale is at the pole. On this 

projection, the sphere is projected onto a plane 

tangent to the pole. The point of projection is 

from the opposite pole. 

The cartographic products will comprise a 

series of 62 maps at a 1:5,000,000 scale. They 

are semicontrolled, using the best control grid 

available at the time of production during the 

mission. The final control grid will be available 

for production of additional maps after the 

primary mission. These maps will be produced 

by the United States Geological Survey in 

Flagstaff, Arizona.

11 
Impact Structures 

a a a a a a a a IN a a a a a a a a a a a a 

Introduction 

Investigators have speculated on the 

presence of impact features on Venus (Saunders 

and Malin, 1976). Earth-based radar observations show features on Venus that are morphologically akin to impact craters (Bums and 

Campbell, 1985). Radar images obtained by 

Soviet Venera missions 15 and 16 cover about 

25% of the planetary surface and display a large 

number of craters, many of which resemble 

impact craters on Mercury, Moon, Mars, and 

other planetary bodies. However, the resolution 

of Venera images limits perception to craters 

with a diameter greater than about 8 km. 

The Magellan orbiter includes an imaging 

radar that will cover from 70 to 90% of the 

Venusian surface at a resolution an order of 

magnitude higher than that of the Venera 

missions. Thus, Magellan will provide extensive 

regional coverage and a more detailed view of 

the surface. Closer study of crater origins will be 

possible and smaller craters may be observed. 

However, the interpretation of craters and 

structures of impact origin on synthetic-aperture 

radar (SAR) images depends not only on 

resolution, but also on contrasts in surface 

reflectivity and such imaging characteristics as 

the illumination geometry and wavelength as 

well. To understand the types of information 

about impact craters on Venus that Magellan 

radar is expected to provide, it is instructive to 

review existing spaceborne radar-image observations of impact features on Earth where there is 

good ground-truth data and compare them with 

impact features on the Venera images.

Imaging Characteristics 

The importance of large-scale slope, smallscale surface roughness, and incidence angle in 

interpreting crater morphology and structure 

from radar images is examined below by 

reference to terrestrial examples obtained by 

Seasat SAR and SIR-A. The observations are 

compared with images obtained by Venera 15 

and 16 and discussed in the context of the 

Magellan imaging radar. The essential radar 

parameters of these imaging systems are shown 

in Table 5. Seasat SAR and SIR-A were 

operated at a comparatively long radar wavelength (23.5 cm) and high spatial resolution (25 

to 40 m), with antenna geometries that produced 

different nominal incidence angles. The Venera 

system obtained images at a shorter wavelength 

(8 cm) and lower resolution (1000 to 2000 m), 

with a very low nominal incidence angle. 

Magellan will operate at an intermediate wavelength (12.6 cm) and spatial resolution (120 to 

300 m). However, the nominal incidence angle 

of Magellan will vary significantly with latitude 

on Venus (Johnson, 1989). 

For each system, the specified incidence 

angle (Table 5) is nominal because it is valid 

only for a level horizontal surface. On sloping 

surfaces, the incidence angle decreases at 

foreslopes or increases at backsiopes by the 

amount of the slope in the direction of the 

illumination vector. Thus foreslopes are 

negative and backslopes are positive relative to a 

nominal incidence angle.

Radar system Nominal incidence angle, deg Wavelength, cm Spatial resolution, m 

Seasat SAR 23 ± 3 23.5 -25 

SIR-A 50±3 23.5 -40 

Venera 15, 16 11 to 13±1.5 8.0 1000 to 2000 

Magellan 16 to 45 12.6 120 to 300

Table 5. Spaceborne 

imaging radar 

system parameters. 

13 

PRECEDING PAGE BLANK NOT FILMED

MØ...J.,2JNTENTIONAbLY BLAaI 
Dominant Surface Effect 

Slope Roughness Shadow 

10 I 

\ Smooth 

0;\

Intermediate 

•••.. \ 

%

••.. 

'

\ ••... 

\ \

..Rough 

' •••...

... 

' 

-30

0 20 40 60 80 

Incidence Angle, deg

1976 (geologic surfaces); Ulaby et al., 1982 

(agricultural surfaces)). This illustrates one of 

the difficulties associated with modeling the 

radar behavior of natural surfaces. Regardless of 

the modifying factor that is used, it follows from 

the equations that at any incidence angle a given 

surface is rougher as the wavelength decreases 

and, independent of wavelength, a given surface 

is smoother as the incidence angle increases. 

Using the modified Rayleigh criterion for 

example, Table 6 shows that there is about a 

29% increase in mis surface smoothness and a 

25% decrease in rins surface roughness through 

the range of Magellan nominal incidence angles 

from about 45 to about 16 deg. For the interpretation of Magellan radar images, this has 

important implications that are discussed further 

under Radar halos in this chapter. 

Figure 8. Variation 

of radar backscatter 

with incidence angle 

for surfaces from Im 

rough, through 

intermediate, to 

smooth showing the 

ranges of dominant 

surface effects.

CD -10 

At low incidence angles, up to about 30 deg, 

backscatter is dominated by large-scale surface 

slope, on the order of meters. Small changes of 

slope, and hence incidence angle, produce 

relatively large changes of backscatter. This 

yields strongly contrasting gray levels on images. 

At incidence angles from about 30 to 60 deg, 

backscatter is dominated mostly by the smallscale roughness of a surface, on the order of the 

radar wavelength. At this scale, smooth surfaces 

produce specular reflection and are dark on 

images; rough surfaces produce diffuse scattering and are bright on images. Intermediate 

surfaces produce both types of scattering and are 

gray on images. Backscatter curves for rough, 

intermediate, and smooth surfaces (Figure 8) 

show that in the range of incidence angles from 

30 to 60 deg, changes in surface roughness have 

a greater effect on backscatter intensity than 

changes in incidence angle. At incidence angles 

greater than about 60 deg, backscatter is weak 

and surfaces are dark to very dark on images. 

A breakpoint between smooth and rough 

surfaces (and the corresponding dominant radar 

scattering mechanisms) is loosely implied by the 

Rayleigh criterion. Because the equation for this 

criterion does not consider an intermediate 

category of surfaces between definitely smooth 

and definitely rough, it was modified by Peake 

and Oliver (1971) using factors that define upper 

and lower values for mis surface smoothness or 

roughness (see Synthetic-Aperture Radar 

Imaging). Field measurements of different types 

of surfaces have led researchers to experiment 

with a variety of modifiers (e.g., Schaber et al.,

Craters and Structures 

Characteristically, an impact crater has a 

circular rim and a small depth relative to its 

diameter. Because such craters are radially 

symmetrical, the landforms are ideal for relating 

image responses to radar illumination geometry. 

The geomorphic expression of impact craters is 

most pronounced and widespread on planetary 

bodies that have low rates of resurfacing. For 

example, Mercury and Moon are covered with 

circular craters, which often overlap. This 

indicates successive impact events on surfaces 

that are old and relatively stable. Earth has a 

comparatively rapid rate of resurfacing. Most 

terrestrial impact features have had their original 

morphology modified by erosion; only a few 

recent features retain a distinct craterform. 

Craters range in form from simple, bowlshaped features to complex structures with 

central peaks and/or rings (Grieve, 1987). They 

also exhibit a wide range of degradation states 

from relatively fresh structures, some with 

portions of preserved ejecta, to erosional scars 

known as astroblemes (Dietz, 1961). Simple, 

bowl-shaped craters are confined to diameters up 

to about 2 km in sedimentary rocks and about 4 

km in crystalline rocks. At greater diameters, up 

to about 150 km, terrestrial impact structures 

have a complex form that commonly includes a 

central uplift. Astroblemes expose a root level of 

crater substructure from which the overlying 

craterform has been completely removed by 

erosion. 

14
Wavelength, cm 

Incidence 

angle, Surface 23.5 12.6 8.0 

deg (Seasat, SIR) (Magellan) (Venera) 

smooth <1.0 <0.5 <0.3 

16

rough > 5.6 > 3.0 > 1.9 

smooth <1.4 <0.7 <0.5 

45

rough > 7.6 >4.0 > 2.6

Table 6. RMS 

surface smoothness 

and roughness 

breakpoints for the 

range of Magellan 

nominal incidence. 

angles (from 

modified Rayleigh 

criterion). 

Currently, 116 terrestrial impact structures 

have been verified, and a small number of new 

structures are recognized each year (Grieve, 

1987; Grieve and Robertson, 1987). The 

principal support for an impact origin comes 

from the physical association of meteorite debris 

and/or evidence of shock-metamorphic effects. 

In the absence of such evidence, an impact origin 

may be suspected for a crater on the basis of its 

morphology, provided that other nonimpact 

sources of cratering such as volcanism, tectonism, collapse from underground solution, or 

wind erosion are evidently less plausible in any 

given instance. 

Measurements indicate that the circularity of 

terrestrial impact craters is generally higher than 

that of craters of other origins (Pike, 1974). At 

low incidence angles, pronounced relief displacement creates a major difficulty in determining the precise position of crater rims on images. 

It is convenient here to consider the radar 

responses from crater surfaces in an orthogonal 

frame of reference. Abrupt contrasts in radar 

response to changing slope conditions (hence 

incidence angle) produce sharp tonal boundaries 

normal to the illumination vector; these contrasts 

divide a crater into distal, medial, and proximal 

sectors. This image characteristic provides the 

basis for analysis and interpretation in the 

following examples. 

Simple Terrestrial Structures 

The best currently available radar coverage 

of simple terrestrial structures includes Barringer 

(Meteor) Crater, Arizona, U.S.A., and 

Talemzane (Daiet el Maadna) Crater, Algeria 

(Figure 9). The craters display similar morphology and an approximate radial symmetry. Each 

is located in a comparable geologic setting on a

plateau underlain by flat-lying sedimentary 

rocks. Field descriptions of the craters indicate 

that Barringer is the best preserved and least 

subdued; thus it is thought to be the younger of 

the two. 

Barringer, U.S.A. 

Barringer Crater is located on a plateau 

underlain by sandstone and dolomitic limestone 

formations. The crater is essentially bowlshaped, about 1.2 km in diameter (D) and 180 in 

deep (d). In plan view, it appears to be squaresided with rounded corners (low roundness, but 

high sphericity), which may represent some 

structural control by the regional joint sets. The 

depth-to-diameter ratio (dID) is 0.15. The crater 

has an encompassing rim that rises 30 to 60 m 

above the surrounding plateau. The rim is 

locally overturned and ejecta blocks up to 30 in 

are scattered about the surface close to the rim 

(Shoemaker and Kieffer, 1974). The crater has a 

comparatively level floor and a surrounding 

interior wall composed of talus slopes up to 

about 43 deg. Bedrock outcrops toward the top 

are locally vertical or overturned at the rim. The 

outer surface slopes away from the rim at about 8 

to 10 deg. The rim is outlined by tonal contrasts 

on the Seasat SAR image (Figure 9(a)); the 

contrasts are pronounced in the distal and 

proximal sectors, but subdued in the medial 

sector. The crater floor and the undisturbed 

plateau surface show medium to dark image 

tones because they are mostly level and comparatively smooth to the 23.5-cm wavelength (Lband) radar. There is no evidence of a radar halo 

on the image. 

The illumination geometry shown in Figure 

9(a) falls mostly in the range where backscatter 

is dominated by large-scale slope effects, which 

result in notable relief displacement. For 

instance, the negative incidence angle on the

15 
e, deg 58 20 50 80 42 

a, deg =8 -30 0 30 -8 

(b) 

0, deg = 31 -20 23 66 15 

a, deg =8 43 0 43 -8 

(a) 

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH 

Figure 9. Simple, 

terrestrial impact 

structures: (a) 

Seasat SAR image of 

Barringer Crater, 

Arizona, U.S.A.; (b) 

SIR-A image of 

Talemzane Crater, 

Algeria. The craters 

are divided by strips 

into sectors that are 

distal (0), medial 

(M), or proximal (P), 

relative to the 

illumination vector.

N / --- Illumination 

0 1

'km

N"-. - Illumination 

01 L_._J km 

Note: The proximal sectors of the crater rims are indicated by dashed lines. The schematics show the average 

surface slope angles (x) and corresponding radar incidence angles (0) at five locations across the diameter of 

each crater in the direction of radar illumination. Negative slope angles represent foreslopes. The negative 

incidence angle in (a) indicates radar foldover of the crater wall and rim in the distal sector. Note that the 

contrasting intensities of responses around the halo in (b) are bilaterally symmetrical in the direction of 

illumination. There is no vertical exaggeration. 

crater wall in the distal sector indicates that the 

upper part of the slope was imaged by the radar 

before the base. This compresses the slope to the 

point of foldover, causing saturation on the 

image and obscuring part of the crater floor. 

Further contributions to saturation are provided 

by corner reflections from within the crater. 

Forward scatter from relatively smooth surfaces 

on the crater floor and from the proximal sector 

of the wall are doubly reflected as backscatter 

from appropriately oriented facets on the wall 

and rim in the distal sector. Consequently in this 

sector, the wall and the interior facing portion of 

the rim appear very bright, rim structure is 

imperceptible, and the position of the rim on the 

image is indeterminate. 

In the proximal sector, the outline of the 

crater rim is more clearly marked. The rim 

structure on the outer slope is bright from 

inclined surfaces of overturned strata that locally 

produce specular returns, and from randomly 

distributed meter- and decameter-size ejecta

blocks on the rim. The crater wall slopes away 

from the radar illumination in this sector and is 

dark because of low backscatter at the large 

incidence angle (- 66 deg). The rim shows a 

significantly greater radius of curvature than that 

in the opposite distal sector. 

In the medial sector, the surfaces of the 

crater wall and the outer slope on each side are 

aligned essentially parallel to the illumination 

vector. In this orientation, slope changes 

produce lower backscatter contrasts. Tonal 

contrasts are subdued and the edge of the rim is 

not sharply defined on the image. The flanks of 

the distal sector appear compressed relative to 

the flanks of the proximal sector. This lowers 

the apparent circularity of the crater outline in 

plan view and imparts a bilateral symmetry in 

the direction of the radar illumination. 

Talemzane, Algeria 

Talemzane Crater is located on a plateau 

underlain by limestone strata. It forms an ap16
proximately circular depression about 1.75 km in 

diameter and 67 in in depth (dID ratio = 0.04). It 

is encompassed by a rim uplifted some 27 in 

above the surrounding plateau. The crater floor 

contains an asymmetrical area of fine-grained 

alluvial sediments to the south. The crater wall 

slopes all around at angles between 22- and 35 

deg. Limestone beds are more steeply inclined 

or vertical in the upper part of the wall, and in 

places they are overturned (Lambert et al., 1980). 

However, the wall is not as steep and The edge of 

the rim is not as sharply defined as the wall and 

rim of Barringer Crater. The outer surface slopes 

away from the rim at 8 to 10 deg. It shows a 

radar halo on the SIR-A image (Figure 9(b)) that 

varies systematically in brightness around the 

rim (McHone and Greeley, 1987). Rocks 

exposed on this surface are mainly mixed 

breccias that closely resemble those in the crater. 

They form a nearly continuous annular blanket 

that is concentric with the crater and that extends 

about 400 in from the rim. The size of the 

fragments ranges up to 10 in and decreases with 

distance from the rim. The outline of the rim 

(Figure 9(b)) is marked by less pronounced tonal 

contrasts than that of the Barringer Crater 

observed on the Seasat SAR image (Figure 9(a)). 

Dark image tones on the crater floor and most of 

the plateau surface north and east of thë crater 

represent low backscatter from level and 

relatively smooth sandy surfaces. - The illumination geometry shown in Figure 

9(b) lies mostly in the range where backscatter is 

dominated by small-scale surface roughness. 

Moderate to relatively high incidence angles on 

the outer slope in the medial and proximal 

sectors result in bright image tones dominated by 

the small-scale roughness of the coarse-grained 

ejecta fragments. This produces the radar-bright 

halo seen on the image out to about one third of 

the crater diameter (Figure 9(b)). The halo is 

comparatively bright in the proximal sector, but 

appears darker in the distal sector where there is 

less radar signal return as the incidence angle 

approaches 60 deg. Note that equivalently 

rough, blocky, weathered surfaces of undisturbed 

limestone bedrock on the level surface of the 

plateau south and west of the crater produce 

image tones comparable to those in the radarbright halo. However, the low incidence angle 

on the distal sector of the wall and on adjacent 

segments of the uplifted rim results in bright 

image tones dominated here by large-scale slope.

Additional radar brightness may be provided 

from energy scattered forward off the crater floor 

and then backscattered from appropriately 

oriented facets on the rim. 

In the medial sector where the walls are 

essentially in line with the illumination vector 

and slope changes provide little contribution to 

backscatter, the rim and the outer slope appear 

equally bright on the image because of diffuse 

backscatter from the rough surface. This 

roughness-related brightness merges imperceptibly with slope-related brightness on the image 

toward the distal sector of the crater wall. The 

transition between dominance of backscatter by 

surface roughness and dominance by slope is not 

readily apparent from the image. However, the 

curvature of the rim appears to be comparable in 

both proximal and distal sectors and the outline 

of the crater retains a radially symmetrical form 

on the image. 

Volcanic Maar Craters 

Though the morphology of the small craters 

described above is typical of fresh, simple 

impact structures on Earth, it is not unique to an 

origin by impact. Among the various crater 

types that do not originate by impact, volcanic 

maar craters have morphologic characteristics 

that most closely resemble those of small impact 

craters. Maar craters originate from shallow 

volcanic eruption and collapse. They have an 

uplifted rim structure and- a circular form in plan 

view that approaches the circularity of impact 

craters. In the absence of regional geologic 

information, the two types of craters can be 

confused on radar images (Greeley et al., 1987). 

Maar craters were imaged by both Seasat 

SAR and SIR-A in the Pinacate Volcanic Field, 

Sonora, Mexico. This is a Quaternary basaltic 

shield of numerous lava flows and cinder cones, 

as well as eight maar craters and a partially 

collapsed tuff cone. The craters, which range up 

to 1.6 km in diameter, expose steeply sloping to 

vertical interior walls with stratified volcanic 

rocks (Jahns, 1959; Gutmann and Sheridan, 

1978). Though the maars in the Pinacate field 

display equal or greater depth relative to diameter (dID ratio) and steeper interior slopes, they 

show radar responses similar to those of the 

impact craters discussed above.

17
Elegante, Mexico 

Elegante is the largest maar crater in the 

Pinacate field and the most nearly circular in 

outline; also, it has the simplest rim, with a 

symmetrical outer slope. It is a flat-floored, 

essentially cylindrical depression approximately 

1.6 km in diameter and 244 m deep (dID ratio = 

0.15). The crater walls form a continuous ring of 

cliffs up to 130 m high with an average slope of 

about 80 deg. At the base of the cliffs, an apron 

of talus with a more gentle slope (about 24 deg) 

extends to the base of the crater. The rim rises

approximately 50 m above the surrounding plain 

and slopes outward from its crest at 9 to 15 deg. 

It exposes a steeper inner slope, with tuff breccia 

up to 70 m thick. The tuff breccia thins as it 

encompasses the crater out to about one-half the 

diameter. The circular pattern of the rim crest is 

interrupted to the southeast by a depression 

known as the Scallop (Gutmann, 1976). 

The Seasat SAR and corresponding SIR-A 

images of Elegante Crater (Figure 10) provide 

good examples for contrasting the dual-incidence-angle viewing effects and for comparing 

Figure 10. Elegante 

Crater, Pinacate 

Volcanic Field, 

Sonora, Mexico: (a) 

Seasat SAR image; 

(b) SIR-A image. 

The crater is divided 

by strips into 

sectors that are 

distal (D), medial 

(M), or proximal (P), 

relative to the 

illumination vector.

W 0, deg 32 -58 23 47 8 

. a,deg= 9 -81 0 24 -15 

ORIGINAL PAGE

Illumination BLACK AND WHITE PHOTOGRAPH 

• i' 

k•..•4

0, deg 59 -31 50 74 35 

a, deg = 9 -81 0 24 -15 

.4 

N / "Illumination 

0

1 km 

Note: The proximal sector of the crater rim is indicated by a dashed line on each image. The schematic sections 

are in the direction of the Seasat SAR illumination; the average slope angles (a) and incidence angles (8) 

correspond to five locations across the crater diameter. Negative slope angles represent foreslopes. On both 

images, the radar brightness that extends to the south from the gentle outer slope denotes the steeper erosional 

slopes of a small breached cinder cone. There is no vertical exaggeration. 

18
these effects to the observations by Seasat SAR 

at Barringer Crater and SIR-A at Talemzane 

Crater. Though the Seasat SAR and the SIR-A 

images of Elegante Crater were obtained from 

azimuthal directions 40 deg apart, the essentially 

radial symmetry of the crater renders it virtually 

isotropic with respect to the illumination 

azimuth. Thus the contrasts in radar signal 

intensity around the crater relate mostly to 

different Seasat SAR and SIR-A incidence-angle 

effects. 

The slope of the crater wall produces 

negative incidence in both Seasat SAR and SIRA images; the negative incidence results in 

foldover and saturation around the distal sector 

of the wall in both images. The saturation 

obliterates signal from the inner slope of the 

upturned rim, and the position of the rim crest in 

this sector is indeterminate on both images. In 

the proximal sector, the steep crater wall is in 

shadow on both images. Here the rim crest is 

outlined by the subdued tonal contrast between 

its comparatively steep inner slope and its more 

gentle outer slope. Foldover in the distal sector 

and shadowing in the proximal sector combine to 

distort the radial symmetry of the rim crest on 

both Seasat SAR and SIR-A images and to 

produce an apparent bilateral symmetry parallel 

to the illumination vector. 

In the medial sector, the Seasat SAR image 

(Figure 10(a)) shows the outline of the rim crest, 

which is normal to the craterward limit of 

numerous small radial gullies that extend across 

the outer slope. The gullies are highlighted in 

this sector, where they are normal or highly 

oblique to the illumination vector. However, 

they are obscure in the proximal and distal 

sectors, where they are parallel or gently inclined 

to the illumination vector. At the lower resolution of SIR-A (Table 5), the rim crest is unclear 

and individual gullies are imperceptible on the 

SIR--\ iiiage Figure I 0(b). 

Complex Terrestrial Structures 

A small but varied assemblage of complex 

terrestrial structures including astroblemes was 

imaged by Seasat SAR and SIR-A. The structures range widely in size and level of preservation, but mostly the craterforms have been 

removed by postimpact erosion. The best 

spaceborne radar image coverage of known

structures includes Elgygytkhyn, Chukotka, 

U.S.S.R, and about 60% of Manicouagan, 

Quebec, Canada. Elgygytkhyn is the largest 

terrestrial impact structure that has the erosional 

remains of the original crater rim. Manicouagan 

presents topography that has been profoundly 

modified by glacial erosion. 

Elgygytkhyn, U.S.S.R 

Elgygytkhyn Crater (also referred to as 

Elgygytgyn) is located on the upland of the 

Anadyr' Plateau in eastern Siberia. A possible 

impact origin was suggested by Zotkin and 

Tsvetkov (1970). An origin by impact is 

supported on morphological grounds by the 

remarkable circularity of the ring of eroded 

mountain peaks that surround the crater, and the 

lack of collapse scalloping around the crater 

margin. These crater features have been shown 

to be in sharp contrast with caldera morphology 

as observed on Landsat images. Furthermore, 

Elgygytkhyn does not display the geomorphic 

aspect of a caldera (Dietz and McHone, 1976). 

The Seasat SAR image of Elgygytkhyn 

clearly shows the ring of mountains that surrounds the crater; the ring has a diameter of 

about 18 km (Figure 11). Nevertheless, the rim 

crest is so deeply eroded that significant contrasts of radar response in different sectors of the 

crater are not apparent. If a central uplift is 

associated with the structure, it is obscured 

entirely by the lake.

- 

I

Pot?., 

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,.

' 

LI'P f

Jr \A1*2Ik/,'1 

N ' --- Illumination 

0 10 

I I i km 

ORIGINAL PAGE

aLAckc AND WHITE PHOTOGRAPH

Figure 11. Seasat 

SAR image of 

Elgygytkhyn Crater, 

Chukotka, U.S.S.R. 

The dashed line 

indicates the crater 

rim. The form of the 

mountains and 

valleys on the image 

gives no evidence of 

significant glacial 

erosion in the area 

outside the crater.

19
Manicnuagan, Canada 

Manicouagan is one of the most intensively 

studied complex impact structures on Earth. It 

has concentrically arranged morphological 

elements that were defined by Floran and Dence 

(1976) and have more recently been elaborated 

by other authors. The morphology consists of an 

inner central region, an inner plateau, and an 

annular trough, and a group of outer elements 

that include an inner fractured zone, outer 

disturbed zone, and outer circumferential 

depression. These morphological elements 

represent structural levels much below the 

original form of the crater (Grieve and Head, 

1983). The Seasat SAR image (Figure 12(a)) 

covers all but the western sector of the Manicouagan structure. However, it displays varying 

proportions of each of the morphological 

elements shown on the sketch map 

(Figure 12(b)). 

The three outer morphological elements of 

the structure are best defined to the west, in the 

portion not covered by the Seasat SAR image. 

The distal boundary of the inner fracture zone is

perceptible on the image (Figure 12(a)) to the 

north where it coincides with sharp bends in 

most streams that flow inward to the annular 

trough. East of the annular trough, the outer 

elements have not been defined. An area of 

distinctive coarse-grained texture on the image 

east of the trough corresponds to massive 

crystalline rocks (gabbro). Conjugate sets of 

linear topographic features denote fracturing in 

this area. The higher density and closer spacing 

of these features for about 20 km outward from 

the trough probably indicate the inner fractured 

zone.

Concentrically aligned valley segments are 

prominent from 10 to 50 km east and southeast 

of the annular trough. The orientation suggests 

that they are structurally related to the Manicouagan feature. Locally, the concentric valleys 

are truncated against prominent topographic 

alignments that radiate outward from the annular 

trough for tens of kilometers. The radially 

aligned features denote the extensive fracture 

halo that is reported to have a diameter of about 

150 km (Grieve et al., 1988). 

Figure 12. 

Manicouagan 

structure, Quebec, 

Canada: (a) Seasat 

SAR image; (b) 

sketch map. Note 

the annular trough, 

about 15 km wide 

and 65 km in 

diameter, that 

contains the 

Manicouagan 

Reservoir.

(•. '• :;;.-

-

, .. . ,J E.• 

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5 

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Note: The sketch map shows (1) Mont de Babel (MB) in the central peaks, (2) the inner plateau, (3) the annular 

trough, (4) the inner fractured zone, (5) the outer disturbed zone, and (6) the outer circumferential depression. 

Note the north-south orientation of glacial topography at the center, the high density of conjugate linear 

features east of the annular trough, and the extensive radial alignments of valley segments outward from the 

trough.

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH
Venusian Impact Craters 

Venera radar images reveal about 150 

impact craters with diameters from 8 to 144 km 

that are distributed over less than 20% of the 

Venusian surface (Ivanov et al., 1986; 

Basilevsky et al., 1987). Smaller craters have 

been reported, but diagnostic features are not 

resolved. The size—frequency distribution of the 

craters shows a maximum in the 16- to 32-kmdiameter interval. The craters have been 

classified into morphological types that change 

progressively with increasing diameter. Most 

morphological types show contrasts in levels of 

preservation from fresh through mature to 

degraded. Apparently this represents a degradational sequence that is independent of crater size 

(Basilevsky et al., 1987). 

Venusian impact craters range from bowlshaped types, which are the smallest observed on 

Venera images, through knobby-bottom to 

central-peak and multiple-ring types, which are 

the largest. Each of the first three types is 

represented in the maximum size—frequency 

interval, though the central-peak type predominates (Basilevsky et al., 1987). Fresh craters 

have sharp rims and outlines that suggest many 

of them are radially symmetrical. The diameters 

of craters in the maximum size—frequency 

interval range from 10 to 20 times larger than 

those of Barringer, Talemzane, and Elegante 

observed by Seasat SAR and/or SIR-A, but 

because the spatial resolution of Venera images 

ranges from 70 to 20 times lower than that of 

Seasat SAR or SIR-A, the scales of perception of 

the small terrestrial craters and the larger 

Venusian craters are comparable. 

Examples of prominent central-peak-type 

craters were selected from the Venera 15 and 16 

coverage for this analysis. The craters range in 

diameter from 24 to 34 km (Figure 13) and 

exhibit fresh to mature morphology with sharp, 

presumably circular, rims (Basilevsky et al., 

1987). Though it is not possible to determine 

surface slope values from the images, the slopedominated outlines of the crater walls are 

analogous in each case. In the distal sectors, the 

walls appear saturated and compressed due to 

foldover. Evidently the slope of the walls equals 

or exceeds the nominal incidence angle here. In 

the proximal sectors, the crater walls are dark, 

and the sharp rim crests are drawn toward the 

illumination vector. In the medial sectors, the

distortion of the crater rims is minimal, but the 

outlines are weak. These responses impart an 

appearance of bilateral symmetry to the craters 

that is further enhanced by the outlines of the 

central peaks, notably on Rudneva and Zvereva 

(Figure 13(b) and (c)). Comparable image 

characteristics were observed on the Seasat SAR 

images of Barringer and Elegante Craters 

(Figures 9(a) and 10(a)). 

Fresh craters are reported to have radarbright halos that commonly show bilateral 

symmetry on the images. The halos are thought 

to denote surface roughness that is related to 

ejecta blankets. Mature craters show no apparent 

halo; this condition is thought to represent terrain 

modification, perhaps by eolian erosion and 

deposition (Ivanov et al., 1986). On images of 

Kemble, Rudneva, and Golubkina (Figure 13(a), 

(b), and (d)), the radar halos are brightest in the 

proximal sectors, but comparatively subdued in 

the distal sectors. A comparable effect was 

noted on the SIR-A image of Talemzane Crater 

(Figure 9(b)), where angular fragments of mixed 

breccia are reported to form a nearly continuous 

annular blanket that extends up to 400 in from 

the crater rim (Lambert et al., 1980). 

In the case of Zvereva (Figure 13(c)), the 

proximal sector appears bright, but there is no 

evidence of a halo in the medial and distal 

sectors. As this brightness contrast occurs along 

a mosaic boundary on the image, it is probably a 

processing artifact. 

In addition to obvious impact craters, many 

circular features have been noted on the Venera 

15 and 16 images (Nikolaeva et al., 1986; 

Basilevsky et al., 1987). While some of these 

features have morphology suggestive of a 

volcanic or tectonic origin, a number of circular 

features are believed to be highly degraded 

impact craters. Nevertheless, a clear Venusian 

analogue to the degraded terrestrial Elgygytkhyn 

Crater has not been reported. 

The largest and morphologically most 

prominent impact crater reported on Venus is a 

double-ring type named Kienova. It has outer 

and inner rings with diameters of 144 and 

105 km; these rings consist of discontinuous 

ridges 10 to 20 km wide (Figure 14). The 

surface inside the inner ring is knobby at the 

perimeter and smooth toward the 15-kmdiameter central hill. The crater is surrounded by 

a ringlike zone of smooth terrain 40 to 60 km 

wide. This is probably the ejecta blanket, which

21
Figure 13. Soviet 

Venera 15 and 16 

radar images of 

central-peak-type 

impact craters with 

sharp rims, radarbright halos (a), (b), 

and (d), and a 

missing halo (c). 

The craters are 

divided by strips 

into sectors that are 

distal (D), medial 

(M), or proximal (P) 

relative to the 

illumination vector. 

Identities and 

diameters (km) are: 

(a) Kemble, 24; (b) 

Rudneva, 34; (c) 

Zvereva, 24; and (d) 

Golubkina, 28.

I 

It 4*WV 

pro-

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dl 

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Illumination 

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4

Illumination 

Discussion 

At low incidence angles, Seasat SAR images 

of terrestrial impact craters and Venera 15 and 

16 images of Venusian craters (obtained at a 

shorter wavelength) show similarities in the 

radar responses to crater morphology. SIR-A 

images show notable differences in the responses 

at high incidence angles. These observations 

have important implications for the interpretation 

of Magellan images, which will be obtained 

through a wide range of incidence angles at a 

wavelength that is intermediate between that of 

Seasat SAR and SIR-A at one extreme and 

Venera 15 and 16 at the other. 

*. ..^ " j 

•X4

Illumination 

overlies the ridge-and-band pattern of the target 

terrain (Ivanov et al., 1986; Basilevsky et al., 

1987). The overall dimensions of Klenova are 

comparable to the terrestrial Manicouagan 

structure (Figure 12). However, the two features 

expose different structural levels. At the Venera 

resolution, there is insufficient morphological 

detail in the image (Figure 14) to make further 

comparisons. 

ORGNAL PAGE

SLACK A1 WHITE PHOTOGRAPH 

22
Because of orbital considerations, the 

nominal incidence angle of Magellan radar at the 

center of the swath will vary from about 45 deg 

at l0°N latitude to about 16 deg at the north pole 

and at 70°S latitude (Johnson, 1989). Figure 3 of 

Synthetic-Aperture Radar Imaging shows that at 

latitudes from 20°N to 10°S, the viewing 

geometry will approach the SIR-A configuration. 

North of 60°N and south of 40°S, the viewing 

geometry will be analogous to that of Seasat 

SAR. At the latitudinal extremities of imaging in 

both hemispheres, the viewing geometry will 

approach the Venera 15 and 16 configuration. 

Thus, radially symmetrical landforms such as 

impact craters, central peaks, cones, and other 

circular features can be expected to show varied 

outlines on Magellan images at different latitudes. 

Impact craters commonly occur as circular 

features. Degradation or multiple impact may 

impose a secondary asymmetry on the landforms. It is important on radar images to 

distinguish between departures from crater 

circularity that are real and image distortion that 

relates to the scene illumination.

. •'1i .• . - -r • .• - 1• 

-.

T: 

4 jy pr'. I •,1f' • .'' , .& I •1 

9*

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)J,f o .;Il 

Illumination 

impact crater distortion will be greater on images 

obtained at higher latitudes. Essentially similar 

morphologic features that appear circular at low 

latitudes can be expected to show a distinctly 

different outline at high latitudes. If the outline 

of a crater at high latitudes is bilaterally symmetrical parallel to the illumination vector, it is 

likely to be an artifact of the viewing geometry.

Figure 14. Klenova, 

shown in this Soviet 

Veneta 16 radar 

image, is the largest 

and morphologically 

most prominent 

multiple-ring type of 

impact crater 

observed on Venus. 

Crater morphology 

At any radar incidence angle, the distortion 

of a crater outline is minimal across the medial 

sector, in a direction normal to the illumination. 

Crater diameters can be measured accurately 

only in this direction, provided the outline of the 

rim is distinguishable. This yields the radius of 

the rim and the center of a circle for comparison 

to the crater outline on each image. In cases 

where the slope of the crater wall equals or 

exceeds the nominal incidence angle, the 

imaging beam produces foldover, as at Barringer 

and Elegante on the terrestrial images (Figures 

9(a) and 10) and Kemble, Rudneva, Zvereva, and 

Golubkina on the Venusian images (Figure 13(a) 

through (d)). In such cases, the outline of the 

crater corresponds to the circle in the median 

sector, but is comparatively prolate in the 

proximal and oblate in the distal sectors. The 

former is drawn out and the latter is compressed, 

so the craters appear to show bilateral symmetry 

parallel to the illumination vector. Where the 

incidence of the beam does not produce foldover, 

as at Talemzane (Figure 9(b)), the outline of the 

crater rim corresponds to the inscribed circle 

with comparatively little distortion. The variable 

viewing geometry of Magellan indicates that

Radar halos 

The radar brightness of crater halos varies 

with the mis surface roughness, which is 

dependent on both the wavelength and the 

incidence angle (e.g., Table 6). The radar-bright 

halo on the SIR-A image (23.5-cm wavelength) 

of Talemzane (Figure 9(b)) represents a surface 

with mis roughness about three times greater 

than that of the radar-bright halos on the Veneta 

images (8-cm wavelength) of Kemble, Rudneva, 

and Golubkina (Figure 13(a), (b), and (d)). 

However, the halo brightness varies systematically around the craters and shows bilateral 

symmetry relative to the illumination vector in 

each case. If the radar-bright halos denote the 

small-scale roughness of the ejecta blankets, it is 

likely that either the ejecta are not uniformly 

rough around the craters, or the slopes of the 

ejecta blankets provide a sufficient range of 

incidence angles to produce a notable difference 

in backscatter intensity from the foreslopes to the 

backslopes.

ORrL PAGE 

BLAcFç AND WHFrE PHCTOGRAJ

23
At Talemzane Crater, the ejecta are reported 

to form a nearly continuous annular blanket of 

mixed breccia that extends out to 400 in from the 

crater rim (Lambert et al., 1980). In this instance, the symmetrical variation of the halo 

brightness around the crater probably represents 

contrasted incidence-angle effects on the image 

(Figure 9(b)). 

Assuming the slopes around Kemble, 

Rudneva, and Golubkina craters are radially 

symmetrical, the relations between symmetry, 

brightness, and the illumination vector suggest in 

each case that there may be no significant 

difference in roughness around the ejecta 

blanket. The brightest responses from the 

proximal sector of the halos suggest foreslopes 

where the incidence angle approaches zero and 

the backscatter is dominantly specular; consequently, the radar return has the greatest intensity 

here. Less intense responses from the backslopes probably indicate a contrasting incidenceangle effect. It has been proposed that the bright 

halos may result from fresh ejecta material with 

a comparatively high Fresnel reflectivity at the - 

low Venera incidence angles (Jurgens et al., 

1988). However, this does not account for the 

symmetrical reduction in brightness observed 

around the halos from the foreslopes to the 

backsiopes. 

Given the variable viewing geometry of the 

Magellan radar system, it follows that systematic 

changes must occur with Venusian latitude in the 

rms values that define the breakpoints between 

radar-smooth and radar-rough surfaces. From 

Table 6 and Figure 3 of Synthetic Aperture 

Radar Imaging, it is evident that the breakpoint 

of rms smoothness at high incidence angles in 

the low to middle latitudes will be rougher by 

about 28% at low incidence angles in the polar 

latitudes. Similarly, the breakpoint of rms 

roughness at polar latitudes will be smoother by 

about 25% at the low to middle latitudes. This 

limits the ability to compare directly the rms 

surface-roughness characteristics of ejecta 

blankets or other types of surfaces to narrow 

latitudinal ranges on the planet. At widely 

separated latitudes, rms surface roughness 

comparisons must allow for the contrasting 

effects of different incidence angles. Furthermore, the criteria used to evaluate mis surface 

roughness breakpoints assume that reflection 

occurs at a free-space boundary without modification by fine-grained, unconsolidated deposits 

at the surface. The conditions that permit radar 

penetration of such fine-grained deposits and the 

perception of subsurface roughness cannot be 

discriminated in images obtained at a single 

radar wavelength. Corresponding pairs of 

images obtained at widely different wavelengths 

are needed to verify radar penetration (Blom, 

1988). Thus, in addition, the distinction between 

surface reflections and possible subsurface 

returns will not be feasible in the Magellan 

images. 

Resemblance to volcanic maars 

The similarities in the radar responses of 

small terrestrial impact craters and volcanic maar 

craters of comparable dimensions indicate 

difficulty in distinguishing the different origins 

of these craters from the images. On Earth, the 

regional setting of the craters in volcanic or 

sedimentary terrains can provide helpful discriminatory information. (However, this 

distinction cannot apply on Venus, where there is 

no comparable sedimentary cover.) 

In all the terrestrial craters mentioned above, 

the radar responses have been related to field 

observations of slope, surface roughness, and 

structure. In Venusian examples, where direct 

field observations cannot be made, slope 

characteristics can be approximated from the 

image responses by analogy with terrestrial 

examples, and roughness characteristics can be 

related to the observing wavelength and the 

incidence angle (Table 6). 

Spatial resolution 

Large terrestrial impact structures with 

diameters up to 150 km retain little or none of 

their original crater form. At the high resolution 

of Seasat SAR, they show details of the structurally related morphologic elements. Correspondingly large Venusian impact features at the 

Venera resolution show only the broadest 

morphologic outlines. The spatial resolution of 

the Magellan radar system is coarser than Seasat 

SAR or SIR-A, but finer than Venera by about 

an order of magnitude (Table 5). Magellan 

images will provide the morphologic detail 

necessary to verify an impact origin or to 

identify a plausible alternative for large and 

24
degraded Venusian craters comparable to those 

observed on Venera images. 

Magellan images should enable the discrimination of craters with diameters below the limits 

of reliable Venera resolution (about 8 km) and 

possibly as small as 2 km, if they exist. This in 

turn would provide a more definitive indication 

of the ability of comparatively small bolides to 

penetrate the Venusian atmosphere. However, 

the difficulties in discriminating an impact origin

from a volcanic origin for small terrestrial craters 

on higher resolution Seasat SAR and SIR-A 

images have been described above. Similar 

difficulties will probably apply in discriminating 

the origin of small Venusian craters. In any 

event, the extended coverage of Magellan radar, 

up to 90% of the Venusian surface, will yield a 

more comprehensive understanding of the 

size—frequency distribution of impact craters on 

Venus.

25
Volcanic Landforms 

N N N N N N N N N N N N N N N N N N N N N 

Introduction 

The present knowledge of Venusian 

volcanism is restricted by existing coverage to 

the distribution and large-scale morphology of 

features greater than several kilometers. Nearglobal coverage of Venus by Magellan will allow 

the planet-wide distribution of volcanic landforms to be mapped. Improved discrimination 

will allow a better assessment of most landforms, 

including enigmatic features such as ovoids 

(coronae), arachnoids, and small domes. The 

global distribution of volcanic centers, the style 

of volcanism, and the relations of volcanism to 

tectonic features should help define the mechanisms of heat loss from the interior of Venus, 

including the potential for hot-spot volcanism, 

lithospheric conduction, and plate recycling 

(Solomon and Head, 1982). Following a brief 

review of recent findings relevant to volcanism 

on Venus, this chapter discusses the different 

scales of volcanic morphology that may be 

interpreted from Magellan observations and 

presents examples of radar data on terrestrial 

volcanic features.

On Earth, the evolution of individual 

volcanic centers is studied to gain insight into the 

compositional and thermal history of a magma 

source. Inferences about the size and depth of 

magma chambers are drawn from caldera 

geometry. In a regional context, the orientation 

of dikes, volcanic vents, and volcanic centers in 

rift zones provides information about tectonic 

stresses and their changes over time. Magma 

composition and the distribution and types of 

volcanic landforms are associated with varied 

plate tectonic settings such as convergent 

(subduction zones/volcanic arcs), divergent 

(spreading centers) or transform plate boundaries, hot spots, or intracontinental rifts. 

On a local scale, lava-flow morphology may 

provide information on the relative ages, 

composition, rheology, and effusion rates of lava 

flows (e.g., Hulme and Fielder, 1977; Malin, 

1980; Wilson and Head, 1983; Fink and Zimbleman, 1986; Baloga and Pieri, 1986). The size 

and distribution of pyroclasts from explosive 

eruptions can be used to infer the eruption 

parameters. Figure 15 shows the main types of 

QUALITY _________ __________ SMALL - QUANTITY OF MAGMA - LARGE TYPE OF 

OF MAGMA ACTIVITY 

I LOW SHIELD BASALTIC i

FLOOD FLUID, VERY EFFUSIVE 

HOT, MAFIC LAVA FLOWS I SHIELDS i VOLCANOES 

I

PLAINS BASALTS 

*

Pisgah Kilauea, Pinacate, Craters of I I

Iceland 

Iceland J

the Moon CRB 

I

TEPHRA AND I COMPOSITE VOLCANOES I STRATOSPATTER CONES I I VOLCANIC 

* Mount Shasta CHAIN 

INCREASING EXOGENOUS DOMES 

GAS AND THICK FLOWS 

CONTENT MIXED 

Mount Shasta, AND SILICA Valles Caldera, Medicine Lake NONE KNOWN 

MAARS AND TUFF CONES 

I

Pinacate, Kilauea, Iceland 

I CALDERAS AND I 

I IGNIMBRITE SHEETS I VOLCANOVISCOUS DIATREMES I TECTONIC 

Valles Caldera, DEPRESSIONS COOL, SIUCIC Central Andes I

EXPLOSIVE

Examples In most of the areas discussed

Figure 15. The types 

of volcanic 

landforms as a 

function of the style 

of eruption and the 

physical properties 

and quantity of 

magma erupted 

(modified from 

Rittmann, 1962, and 

Greeley, 1977). 

Each area discussed 

in the text contains 

several different 

types of volcanic 

features and is listed 

under the 

appropriate 

categories in the 

diagram.

27 

PRECEDING PAGE BLANK NOT FILMED FAG' ' - INJENTIOMAb j 
volcanic landforms as a function of basic 

controlling parameters and notes the terrestrial 

examples discussed in this chapter. 

The interpretation of volcanic features 

depends partly on understanding the planetary 

environmental effects on the processes involved. 

Although Venus is similar to Earth in mass, 

density, and diameter (Appendix A), the dense 

Venusian atmosphere results in high surface 

temperatures (650 to 750 K) and pressures (4to 

10 MPa) that affect eruption temperatures, 

cooling rates, and exsolution of volatiles. 

Although a complete understanding has not been 

reached, theoretical studies by Wilson and Head 

(1983) and Head and Wilson (1986) predict no 

significant systematic differences in eruption 

rates or durations, lava-flow geometry, or 

differences in the relations between eruption 

temperatures and liquidus temperatures from 

those in terrestrial flows of similar composition. 

Because of the high atmospheric pressure, 

explosive eruptions would be rare on Venus 

unless magmatic volatile contents are greater 

than about 2.5 weight %, and the size and 

distribution of pyroclastic material would differ 

from similar deposits on Earth (Garvin et al., 

1982; Head and Wilson, 1986). 

The presence of volcanic materials on Venus 

is implied both by radar and by surface geochemical data (Barsukov and Basilevsky, 1986; 

Basilevsky and Head, 1988). A variety of 

volcanic landforms was revealed on highresolution radar images of parts of the planet; the 

images were obtained by Earth-based observations (I- to 3-km resolution) and by Venera 15 

and 16 SAR (1- to 2-km resolution). Radarbright and radar-dark lobate patterns on these 

images have been interpreted as lava flows and 

occur within plains units and around large, 

gently sloping domical landforms (Saunders and 

Malin, 1977; Campbell et al., 1984; Barsukov et 

al., 1984a, 1984b, and 1986). Radial lobate 

features, the presence of summit craters, and 

altimeter measurements of low slopes suggest 

that the domical features are shield volcanoes, 

some of which are associated with highland areas 

interpreted as possible rift systems (Saunders and 

Malin, 1977; McGill et al., 1981; Schaber, 1982;

Campbell et al., 1984; Barsukov and Basilevsky, 

1986). Long, lobate radar-bright features within 

plains provinces could be high-effusion-rate, 

low-viscosity flood basalts (Masursky et al., 

1980; Barsukov and Basilevsky, 1986; Stofan et 

al., 1987). Results from gamma-ray spectrometer and X-ray fluorescence experiments on 

Soviet landers support these conclusions and 

suggest that the upland rolling plains are 

chemically consistent with alkaline basalt and 

the flat lowlands are comparable to tholeiitic 

basalt (Surkov et al., 1983 and 1984). The 

slopes of Aphrodite Terra also have a composition similar to tholeiitic basalt (Surkov et al., 

1986 and 1987). ' Small domes (<20 km 

diameter) occurring in plains units may be 

volcanic in origin. Within Ishtar Terra, two 

circular depressions surrounded by radial lobate 

patterns resemble calderas (Masursky et al., 

1980; McGill et al., 1983; Barsukov et al., 

1984b). 

Morphologic Scales and Radar 

Observation Parameters 

The nature of landforms produced by 

volcanism is largely governed by the volatile 

content and viscosity of the magma and the 

quantity of material erupted. The style of 

volcanism can be determined from the type of 

vent, the nature and lateral extent of the deposits, 

and the resultant landforms. The composition 

and particularly the volatile content of magma 

are crucial in determining the style and size of 

explosive eruptions and distribution of the 

resultant products. Volcanic landforms and 

surfaces in radar images can be classified 

according to a scale that includes megamorphic, 

mesomorphic, and micromorphic features. 

Megamorphic features are at scales of 

kilometers or hundreds of kilometers. Examples 

include individual shield volcanoes and the 

products of high-volume eruptions. Observations by the Pioneeer Venus Orbiter and the 

Venera 15 and 16 missions have yielded information at this scale. The relation of such 

features to faults, rift valleys, and possible plate 

boundaries or hot spots should be apparent at the 

kilometer scale. Deposits from large-scale 

eruptions should also be distinguishable at this 

scale; however, flood basalts and pyroclastic 

flows produce similar landforms, making it 

28
necessary to use surface micromorphology to 

distinguish the two. 

The overall distribution of image tones and 

patterns provides interpretive data for megamorphic features, which should be readily detectable 

on Magellan images because at this scale the 

features are at least an order of magnitude larger 

than the Magellan spatial resolution (120 to 300 

m). The geometric distortion inherent in radar 

images will vary with the latitude of the Magellan observations and must be considered when 

making measurements of volcano width, height, 

volume, and slope—especially with regard to 

symmetry. Geometric distortion will be most 

pronounced at the higher latitudes. Altimetry 

measurements can be used to constrain general 

topographic distortions and steepness of slopes. 

Mesomorphic features are at scales from 

meters to hundreds of meters. In this range, 

volcanic features include lava flows, pyroclastic 

deposits, and constructs such as cinder cones, 

maar craters, collapse craters, lava ridges, 

channels, tubes, fissures, flow margins, and 

certain types of vents. Such landforms can be 

used to interpret the style of eruption, the flow 

emplacement mechanism, and the lava rheology 

(e.g., Hulme, 1974; Moore and Schaber, 1975; 

Moore et al., 1978; Fink, 1980; Basaltic Volcanism Study Project, 1981; Theilig and Greeley, 

1986). Some mesoscale volcanic features may 

be near the limits of resolution of Magellan's 

SAR data. 

Detection of mesoscale volcanic features on 

radar images depends on resolution, look angle, 

and azimuth angle, the first two of which will 

vary with latitude for Magellan SAR images (see 

Figure 3 of Synthetic-Aperture Radar Imaging). 

The look angle for Magellan will vary from 

about 50 deg in the equatorial regions to about 

14 deg near the poles. At small look angles, 

gentle slopes (< 30 deg) on cinder cones, flow 

margins, and ridges result in incidence angles 

approaching 0 deg, causing specular reflections 

and bright returns (Elachi et al., 1980; Fan et al., 

1981). This also causes the entire echo from a 

slope to return at the same time so that the slope 

is collapsed to a thin line in the azimuth direction. Foreshortening at small look angles may 

make geometric relations difficult to establish, 

and, in some cases, it may not be possible to 

determine relative relief. At large look angles, 

steeper slopes, such as those in crater interiors or 

in collapse depressions, cause strong radar

returns if oriented toward the radar (Fan et al., 

1981; Greeley et al., 1987). 

The azimuth angle is important in the 

detection of mesoscale topographic features on 

SAR images. Linear features such as ridges, 

leveed channels, lava tubes, fissures, and flow 

margins are more easily detected if oriented 

nearly perpendicular to the radar illumination. 

Detection of subresolution-size elements may 

occur if they are strong scatterers because of 

extreme roughness, because they act as corner 

reflectors, or because they have slopes normal to 

the incident radiation (Schaber et al., 1980; 

Greeley et al., 1985; Blom, 1988). Alternatively, 

combined reflection from numerous slopes, such 

as parallel ridges, may cause a higher return for 

an extended area even if individual components 

cannot be discerned (Malin et al., 1978). 

Micromorphic features are represented by 

surface roughness at the centimeter to meter 

scale, which is below the spatial resolution of 

most imaging radars. However, for surfaces with 

low slopes and similar electromagnetic properties, radar backscatter and image brightness 

levels can be related to the average small-scale 

surface roughness in each image resolution cell. 

For extended volcanic surfaces, this allows 

estimation of the texture in lava flows, the grain 

size in pyroclastic deposits, and the extent of 

modification by nonvolcanic processes on weathered surfaces. Average small-scale roughness is 

used to classify volcanic surfaces and deduce 

their origins (Macdonald, 1953; Swanson, 1973; 

Peterson and Tilling, 1980). The simplest 

classification of lava-flow types—into pahoehoe, 

aa, and block (Macdonald, 1953)—is based on 

the surface expression of flow rheology and 

mechanical history (Walker, 1970; Peterson and 

Tilling, 1980; Kilbum, 1981). Factors such as 

effusion rate and topography combine with the 

lava rheology to determine whether a given flow 

will be pahoehoe or aa, or whether a transition 

will occur in the middle of a flow. 

In Magellan images, brightness should be 

proportional to centimeter- to meter-scale surface 

roughness. The 12.6-cm wavelength of the 

Magellan SAR is shorter than that used by Seasat 

and the SIR-A and -B missions, and consequently it will be sensitive to smaller-scale

29
0 10 20 30 40 50 60 

Incidence Angle, deg 

Figure 16. 

Backscatter 

coefficient 

measurments made 

by Johnson Space 

Center airborne 

scatterometer over 

Pisgah Lava Field 

using C-band (6.3-

cm wavelength) and 

HH polarization.

average surface roughness. The reverse will be 

true in comparing Magellan data with the 8-cm 

wavelength Veneta 15 and 16 data. The effect of 

wavelength on backscatter has been discussed by 

Dellwig (1969), Schaber et al. (1980), Farr and 

Engheta (1983), Blom et al. (1986 and 1987), 

and Greeley and Martel (1988). As backscatter 

varies with incidence angle (Figure 16), the 

changes in Magellan incidence angle with 

latitude (see Figure 3 of Synthetic-Aperture 

Radar Imaging) and with topographic slope 

variations will cause variations in image brightness regardless of surface type. Hence, incidence angles must be taken into account when 

mapping surfaces that have similar small-scale 

surface roughness. 

The average small-scale surface roughness 

may decrease with time through weathering and 

depositional processes (Wells et al., 1985; Fan 

and Anderson, 1987). Possible modification of a 

flow surface must be taken into account when 

interpreting the original eruption conditions on 

Venus. However, weathering conditions and 

depositional processes on Venus are not well 

known. Chemical weathering (Nozette and 

Lewis, 1982) caused by the hot atmosphere 

0 

0 

b 

w 

U 

0 0-10 

I0 

U) 

I-'a 

'a 

cc

-20 

Note: The aa lava has a flatter curve than the pahoehoe because the rough surface scatters more diffusely. Because of this effect, the aa will appear 

darker than pahoehoe at incidence angles less than 

about 15 deg.

probably occurs, but the absence of water may 

lower the rate of such reactions. Low-speed 

winds that could form eolian deposits (Greeley et 

al., 1982; Williams and Greeley, 1982) have 

been observed at the surface of Venus (Keldysh, 

1977). Similarly, explosive volcanic activity 

would deposit pyroclastic material on near-vent 

surfaces (Head and Wilson, 1986); however, it is 

not known if such activity occurs on Venus. 

Although a correlation may exist between 

decreased radar backscatter and increased 

surface smoothing, a direct correlation with age 

cannot be made in the absence of knowledge 

about the original surface texture and the 

weathering process. 

Observations of Terrestrial Volcanic 

Areas 

Here we present volcanologic and radar 

remote sensing data for several well-known 

areas. Specific examples are given and literature 

is cited on the applications of the principles 

discussed above to the interpretation of radar 

images of basaltic flood deposits, complex 

basaltic plains, shield volcanoes, composite 

volcanoes, and products of large-scale explosive 

eruptions. Spatter/tephra cones, lava flows, and 

ash deposits are discussed in several of the 

examples. 

Flood basalts 

High-volume, low-viscosity flood basalts 

are common on the terrestrial planets and may 

form most of the Venusian plains. Two examples of this style of volcanism are provided: 

young quasi-flood basalt in Iceland and Miocene 

flows of the Columbia Plateau. 

Iceland. Iceland is a complex volcanic 

plateau with examples of flood basalts, shield 

volcanoes, calderas, tephra cones, pyroclastic 

deposits, and lava flows of different compositions and surface textures. Analogous plateaus 

have been postulated for the western Aphrodite 

Terra on Venus (Head and Crumpler, 1987; 

Crumpler and Head, 1988). The tectonic setting 

of Iceland, a hot spot on the axis of a midocean 

ridge, is favorable for high rates of magma 

supply and eruption. This area is included here 

because it provides one of the few Holocene 

examples of flood basalts; however, other 

volcanic landforms are also discussed. 

30
Many volcanic units, faults, linear features 

and drainage networks can be distinguished on 

Seasat images of Iceland (McDonough and 

Martin-Kaye, 1984). Figure 17 displays the 

central portion of the Icelandic east rift zone 

where eruptive products are primarily tholeiitic 

basalt. The oldest rocks in the area are Tertiary 

Flood Basalts that underlie the dissected basalt 

plateau and Tertiary—Pleistocene Old Gray 

Basalts (Figure 18). A widespread occurrence of 

the Pleistocene Palagonite Formation results 

from subglacial basaltic eruptions of voluminous 

proportions (number (1) in Figure 18). Young 

Gray Basalts are interglacial eruptive products 

that were not palagonitized. A swarm of fissures 

and normal faults (Sigurdsson and Sparks, 1978) 

at Sveinagja is oriented 6 to 13 deg from the 

illumination direction of Figure 17. Lobate flow 

margins west of Kollottadyngja are prominent on 

the image probably because they are nearly 

perpendicular to the illumination. Three areas of 

Holocene volcanism described below include the 

Askja Caldera, the Trolladyngja Volcano, and 

the Myvatn area. 

The Askja area has undergone basaltic 

fissure eruptions,.the formation of a central 

volcano, late-stage shallow silicic volcanism, 

caldera collapse, and late-stage basaltic flows 

(e.g., Sparks et al., 1981). An historic, 11-km2 

caldera contains a lake (Oskjuvatn) and forms an 

oblong radar-dark area because of forward 

specular reflection off the smooth lake surface. 

The small caldera is nested within an older 

45-km2 caldera that is visible as a broad, radarbright crescent northwest of Oskjuvatn. High 

reflectivity from the floor of the older caldera 

comes from an aa flow that is partly mantled by 

pumice and ash (Malin et al., 1978). 

The most prominent lava flows are radarbright, late-stage basalts on the north, west, and 

south sides of the younger caldera. Despite 

differences of small-scale surface roughness, 

Mackenzie and Ringrose (1986) found that 

pahoehoe flows (typically 15 in wide and 50 in 

long) within the main aa flow of 1961 cannot be 

discriminated in the Seasat image. Strong 

returns from the aa were thought to be due to 

rough surfaces combined with a highly irregular 

local relief. Strong returns from the pahoehoe 

were related to the undulating nature of the 

smoother surfaces, which produced specular 

returns from appropriately oriented facets. It

was noted that both effects occur at a scale much 

below the image resolution. 

Both aa and pahoehoe were found to be 

rougher and brighter in radar than pumice 

deposits, which, in turn, are rougher than ash 

deposits. An inverse relation was found between 

the radar return and the amount of pumice and 

ash covering a lava flow. Malin et al. (1978) and 

Evans (1978) discriminated the surface roughnesses of nine radar backscatter units using 

parallel- (HH) and cross- (HV) polarized L-band 

airborne SAR with look angles from 0 to 45 deg. 

A different style of volcanism occurred at 

Trolladyngja (Figure 19). Magma was fed from 

a deep source, beginning with initial fissure 

basalt eruptions and followed by the buildup of a 

shield volcano. The fissures on the southwest 

side of Trolladyngja are oriented within 10 deg 

of the illumination direction. With one exception denoted by (2) in Figure 18, the fissures are 

not visible on the image. The 500-rn-diameter 

crater at the summit is visible. Trolladyngja and 

the adjacent shield volcanoes (e.g., Ketildyngja, 

Kollottadyngja, Urderhals) show an asymmetric 

appearance on the radar image with bright 

compressed foreslopes and dark stretched 

backslopes. This results from the approximate 

equivalence of both the Seasat SAR look angle 

and the slope angles of the shields. Thus, the 

incidence angle at the foreslopes is about 0 deg; 

at the backslopes, the incidence angle is equivalent to the look angle plus the ground slope. 

The Barthardalshraun Lava Flow originated 

both from Trolladyngja and from a set of fissures 

to the south. It is over 100 km long and 10 in 

thick and is considered a quasi-flood basalt. 

Because it was topographically confined by river 

channels (note the narrowness of the flow to the 

north in Figures 17 and 18), it is not as thick as 

typical flood lava deposits (Greeley and Sigurdsson, 1980). The flow is composed of an 

older bright unit on and to the north of Trolladyngja and a younger unit with lower reflectivity 

south of the shield. The age relations of the two 

flows are not obvious from their radar reflectivities. However, cross-cutting relations relative to 

the two flows indicate a younger age for the 

Urderhals Volcano.

31
ORIGINAL PAGE

SLACK AND ffiHLIE pHo1QGRApd 

Figure 17. Seasat 

SAR image of eastcentral Iceland 

covers prominent 

shield volcanoes 

and shows lava 

flows and flow 

edges, dissected 

flood basalts, and 

the Askja Caldera. 

The major features 

of interest are 

outlined in Figure 

18.

•

.t- ;. ,

. 

Aa ... I 

£ _ a 

W. 

AL 

.

'I •', k'

. - 

IV 

•

..

; - 

N I I i km Illumination 

Radar-dark, roughly parallel streaks that cut 

across the Barthardalshraun Lava Flow ((3) in 

Figure 18) probably do not represent primary 

flow morphology. They appear as low-albedo 

features in aerial photographs and could represent tephra or deposits of eolian origin that 

subdue the surface roughness of the underlying 

flows. 

Some of the individual 100- to 200-rn-wide 

lava flows on the slopes of Trolladyngja are 

darker than the surrounding terrain (Figure 

19(a)), perhaps because they are pahoehoe flows 

that are smoother than the surrounding aa field. 

Festoonlike transverse ridges observed in the 

field by Greeley and Sigurdsson (1980) were

used by Theilig and Greeley (1986) to estimate 

the interior viscosity of the flow when the ridges 

formed. Although the transverse ridges are 100 

m apart and up to 5 m high, they are not resolved 

in the Seasat image. Either the ridges are below 

the resolution of the image or the radar backscatter may not vary sufficiently from the surrounding terrain to provide a detectable contrast. 

The Myvatn area appears in the upper left 

corner of Figure 17. Two basaltic tephra rings 

with central craters and a small rhyolite flow are 

distinguishable near Myvatn (Saemundsson, 

1977) and shown in Figure 20. The tephra rings 

are about 1 km in diameter with pyroclastic 

deposits dipping 7 to 42 deg outward (Thorar32
" Sveinagja fissure swarm 

Flow

unger) 

MLa)xa

Ketlldyngja 

(volcano) 

Laxa Flow 

Myvatn (older) 

(lake) (:1119- Koltottadyngja 

(volcano) 

Lava flow edges

Askja 

(caldera)

Oskjuvatn 

Old (lake) 

Gray 

Basalts

Barthardalshraun 

Lava Flow (older) 

Basalt 

Plateau

1 

Trolladyngja Urderhals (volcano) (volcano) 

1 Young Gray 

Basalts 

Barthardalshraun 

Lava Flow (younger)

Figure 18. Sketch 

map of major 

volcanoes, lava 

flows, flow edges, 

and notable features 

shown in Figure 17. 

Small-scale features 

are (1) Palagonite 

Formation (Moberg), 

(2) fissure source 

of younger 

Barthardalshraun 

Flow, and (3) dark 

streaks on older 

Barthardalshraun 

Flow. Details of the 

Myvatn area (upper 

left) are given in 

Figure 20. Names 

of features and 

geologic units are 

from Kjartansson 

(1965). 

N\ L 

insson, 1960). Numerous 2-rn- to 300-rndiameter craters occur in the Myvatn area, but 

are not visible on the Seasat image. 

The 3500-year-old Laxa pahoehoe flow that 

surrounds Myvatn is brighter in the radar image 

than the approximately 2000-year-old Laxa aa 

unit (Figures 17 and 20). A row of craters in the 

center of the older flow is visible in Figure 17 as 

a line of low reflectivity. The source of the older 

flow is the Ketildyngja volcano (Thorarinsson, 

1953; Preusser, 1976). The youngest lava flow 

in the vicinity is the brightest in the image. Age 

relations and sources for these flows cannot be 

ascertained from the radar image.

20 

-J km 

Columbia Plateau, Northwest U.S.A. The 

Columbia Plateau in the northwestern United 

States is one of the larger flood-basalt provinces 

on Earth, with a total areal extent on the order of 

2 x 101 km2 . The Miocene Columbia River 

Basalts (CRB) erupted from north-northwest 

trending linear vents and attained a maximum 

thickness of about 5 km. The Grande Ronde 

Basalt, which constitutes approximately 85% of 

the total volume of the CRB, was erupted over a 

2-My period (Reidel et al., 1982). On the eastern 

plateau, both the basalts and the thick Quaternary 

loess deposits that overlie them have been eroded 

by cataclysmic floods.

33 
N 

V

• y% ? 

V

V 

. V_s:V •V'

•V• 

ç 

0 2 

I 1 km 

G'kAL PACE 

LAGK AND WHITL HO TOGRAPH.

Figure 19. 

Trolladyngja 

Volcano: (a) 

enlargement of part 

of the Seasat image 

shown in Figure 17; 

the box indicates the 

location of (b), an 

aeriEl photograph 

(courtesy of Ronald 

Greeley, Arizona 

State University). 

Because of 

geometric distortion, 

Figures (a) and (b) 

cannot be compared 

directly. Darker 

areas in both images 

probably represent 

pahoehoe. In (b), 

the summit is snow 

covered.

N

0 10 

I I I km

/1

Illumination 

34
#11 

Lake 

Myvatn .. • r 

:1 ...

1 ( 

1. L 

Lakes 

F]Lava - 1725 to 1729 A.D.

LII Laxa Flow, 2,500 to 1,000 yr old 

F—ILaxa Flow, -3,500 yr old 

Ludent Flow, >6,000 yr old 

Rhyolite Flow 

Undifferentiated 

Fissures and Crater Chains 

Tephra Rings and Cones 

• Pseudocraters

Figure 20. Geologic 

sketch map of 

Myvatn area 

(modified from 

Thorarinsson, 1967, 

0

as presented by 

Preusser, 1976). 

N

0 4 I i km 

The basalts of the western plateau have been 

deformed into the Yakima Fold Belt, a series of 

asymmetric anticlines separated by broad, 

relatively undeformed synclines (Reidel et al., 

1984). A Seasat SAR image covers a portion of 

the Yakima Fold Belt, bounded to the west by 

the Cascade Range (Figure 21). Two anticlinal 

structures are visible in the image but, because 

the illumination direction is roughly coincident 

with the trend of the anticlinal ridges, incidenceangle effects are minor. The style of deformation of CRB is not uncommon on the terrestrial 

planets. The anticlinal ridges of the Yakima 

Fold Belt are similar in morphology and dimension to first-order ridges in wrinkle-ridge 

assemblages that occur in mare basalts on the 

Moon and smooth plains material interpreted to 

be flood basalts on Mars and Mercury (Watters, 

1988). Analogous features may also exist on 

Venus and are suggested on Venera 15 and 16 

radar images by long, narrow, high-relief 

features near Metis Regio (Watters, 1988).

Basaltic plains: Craters of the Moon, U.S.A. 

Basaltic-plains volcanism is characterized 

by low shield constructs, tube-fed flows, and 

multiple flow units erupted from point sources 

aligned along fissures (Greeley, 1982). The 

Craters of the Moon (COM) Lava Field is one of 

the most recent fields composing the Snake 

River Plain, Idaho, which is the type locality for 

basaltic plains volcanism. 

The Snake River Plain is an arcuate topographic depression 50 to 100 km wide extending 

approximately 650 km across the southern part 

of Idaho. The central and eastern parts of the 

Snake River Plain consist of a broad, flat plain 

composed of Quaternary lava flows. Fissures are 

associated with rift zones trending perpendicular 

to the axis of the eastern part of the Snake River 

Plain. The Great Rift of the Idaho Rift System is 

a 3-km-wide zone of en echelon fissures trending 

N35°W and is the primary source for the COM 

Lava Field (Prinz, 1970). COM provides an 

opportunity to study a variety of basaltic lava

35
ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

Figure 21. Seasat 

SAR image of part of 

the Yakima Fold 

Belt. Two anticlinal 

structures, the 

Ahtanum ridge (1) 

and the Toppanish 

ridge (2), appear as 

narrow, relatively 

high-relief 

Iandforms. Mount 

Adams (3), 

composed of 

overlapping cones 

erupted from several 

vents along a fissure 

trending northnorthwest, is part of 

the Cascade Range 

(Harris, 1980). 

(Image courtesy of 

Tom Watters.)

r' 

4ik S 

-s' . . •- ••.S•.._ 

N 6f 

(1 

NT 0 20 km - Illumination 

flows and vents (Figure 22(a)). Over 60 flows 

and flow units cover 1,600 km2 with more than 

30 km3 of lava to form the largest predominantly 

Holocene lava field in the conterminous United 

States. This field consists of pahoehoe, aa, and 

block lava flows erupted during eight periods 

from 15,000 to 2,000 years ago (Kuntz et al., 

1983, 1986a, 1986b, and 1988) (Figure 22(b)). 

Vents within the field include 55 cinder cones 

and 14 fissures, many with spatter cones (Papson, 1977). Discontinuous deposits of loess, 

eolian sand, and alluvial fan material are 

interbedded with the lava flows. 

Jet Propulsion Laboratory (JPL) aircraft and 

Seasat SAR images (Figures 23(a) and (b), 

respectively) provide comparative backscatter 

information on the COM Lava Field at look 

angles ranging from 10 to 50 deg. The variation 

in gray levels in these images is related to the 

type and age of the lava flows and to the look 

angle of the radar (Elachi et al., 1980). Young 

flows are typically free of loess, soil, and 

vegetation and have higher backscatter. The 

extremely rough surfaces of aa flows consisting 

of blocky surface texture and irregular topography with several meters of relief result in these 

flows being the strongest radar scatterers in 

Figure 23(a). The effect of look angle on lavatype identification can be demonstrated by 

comparing a pahoehoe flow with an aa flow in 

the two images (Figures 23(a) and (b)). The 

pahoehoe appears in the aircraft image (45-deg 

look angle) as a mottled unit darker than the 

nearby aa flow, but in the Seasat image (20-deg 

look angle), the pahoehoe flow appears uniform 

and brighter than the aa. This behavior is related 

to the dominant scatter mechanism (see Figure 8 

in Impact Structures) at different look angles as 

demonstrated by backscatter curves for pahoehoe 

and aa flows (Figure 16). Within or below the 

range of 15 to 25 deg, specular reflection from 

the smoother pahoehoe surface produces a 

stronger radar return, whereas the rough aa 

surface scatters energy and less is returned to the 

receiver. At greater look angles, the opposite is 

true; the smooth surface reflects energy away 

and the rough surface scatters relatively more 

energy back to the receiver. 

Mantling of older flows by loess and 

volcanic ash attenuates the initial surface 

roughness and produces lower backscatter 

(Rothery and Lefebvre, 1985). Because smallscale surface roughness has been preferentially 

smoothed by mantling and weathering processes, 

Blom et al. (1986) found that flows of different 

ages but similar initial surface texture could be 

36
LJ -15,000 yr B.P.

le 

Figure 22. Craters of 

the Moon Lava Field: 

(a) Landsat image; 

(b)generalized map 

of the lava flows 

within the Craters of 

the Moon field 

(modified from 

Kuntz et al., 1986b). 

Undifferentiated 0 10 Flow Contact 1-1 km

37
ORGNAL PACE 

BLACK A1'W WHITE PHOTOGRAPH

0 20 

1 k 

• i... 

.. 

- *4 •

I 

I

J . 

gal

:. 

Figure 23. SAR images of the 

Craters of the Moon Lava Field: 

(a) JPL aircraft LHH mosaic 

acquired in 1977, with some 

swaths illuminated from the 

upper left and some from the 

lower right; note that the 

pahoehoe flow is mottled and 

slightly darker than a nearby aa 

flow; the area shown in (b) lies to 

the left of the jagged line. (b) 

Digitally correlated Seasat image 

of the northwest part of the field; 

note that the pahoehoe flow is 

brighter than the rough aa flow.

0 10 

N^ I I I km Illumination 

38
discriminated based on backscatter coefficients 

derived from scatterometer data. Caution must 

be exercised, however, when attempting to 

estimate lava flow age from radar backscatter, 

because the original surface textures may not be 

the same and flows may weather differently. 

Within the Craters of the Moon, aa flows 

weather more slowly than pahoehoe because the 

surface is rubbly and very permeable, allowing 

water and sediment to be transported into the 

interior of the flow rather than collect on the 

surface (Lefebvre, 1975). 

Two major vent types, cinder cones and 

fissure vents, are present in the COM Lava Field. 

Fissures are almost parallel to the radar illumination direction and are not visible in the SAR 

images. Spatter ramparts built along some of the 

fissure vents are several meters high and consist 

of agglutinate. These features cannot be seen in 

L-band data but are distinguishable in X-band, 

probably as a result of scattering of the smaller 

radar wavelength by the centimeter-scale 

agglutinate spatter or alternatively because of 

increased spatial resolution (Greeley and Martel, 

1988). Other geomorphic features that can be 

distinguished include buttes, lava lakes, and lava 

tubes that have coarse rubble at their margins 

(Elachi et al., 1980). 

L- and C-band (19- and 6.3-cm wavelength, 

respectively) HH scatterometer data can be used 

as a surrogate for Magellan S-band (12.6-cm 

wavelength) SAR data. A scatterometer data set 

from COM has been used to investigate the identification and interpretation of lava flow types 

and ages based on backscatter coefficients 

obtained at different incidence angles (Theilig et 

al., 1988). Results of this study suggest that 

young pahoehoe flows are more distinct at 

smaller look angles, whereas older, modified 

pahoehoe flows can be discriminated only at 

larger angles. Intermediate age surfaces are 

difficult to separate at most look angles. 

Shield volcanoes 

Several examples of shield volcanoes are 

included to illustrate the associated variety-of 

morphologic features and eruptive products. 

These examples include Kilauea Volcano, 

Pinacate Volcanic Field, Medicine Lake Highland, Fantastic Lava Beds, and Pisgah Lava 

Field. The latter example can be classified as a 

low shield volcano and is similar in morphology

and origin to small volcanoes in the Snake River 

Plain, Idaho. 

Kilauea Volcano, U.S.A. The Shuttle 

Imaging Radar-B (SW-B) imaged the southeastern part of the island of Hawaii at look angles of 

28 and 48 deg in October 1984. Kilauea Crater 

and the rain-shadowed Kau Desert to the 

southwest are shown in Figure 24. The most 

prominent features in these images are the 

caldera of Kilauea Crater, several normal faults 

in the Kau Desert, aa lava flows, and pyroclastic 

deposits. A good summary of the geology of the 

area is given in Holcomb (1987). The SW-B 

images allow a direct comparison of the effects 

of look angle on the ability to identify and map 

lava flows and volcanic features where other 

system characteristics (e.g., signal-to-noise ratio 

(SNR), digitization, and altitude) and geometric 

characteristics (e.g., azimuth) are held relatively 

constant. 

Identification of vents within the SIR-B 

coverage is dependent on the size and morphology of the feature. Kilauea Crater, about 3 km in 

diameter, is easily visible in both images because 

the southeast rim is highlighted and the northwest rim is partly shadowed. Pit craters, such as 

Halemaurnau, at the southwestern edge of 

Kilauea Crater, and those along the Chain of 

Craters extending to the southeast, are also 

visible. Some of these steep-sided depressions 

are as small as 200 to 300 m. Cinder cones, 

spatter ramparts, and fissures are smaller and 

could not be readily identified. In general, 

topographic features with less than about 10 in of 

relief cannot be resolved in the SW-B coverage 

(Gaddis et al., 1989). 

Fault scarps are detected in the images as 

either dark or bright curvilinear features, 

depending on their orientation. The Koae Fault 

System is a series of scarps up to 10 in high, 

trending east-west, and facing north; they appear 

as thin bright linear features south of the Kilauea 

Caldera. In contrast, south of the Koae faults 

and parallel to the coast, the 350-rn-high fault 

scarp of Hilina Pali faces southeast so that the 

angle of incidence is large and it appears dark 

(Gaddis et al., 1985). Both radar images are 

equally good for discriminating these features.

39
ORGTNAL PACE

SLACK AU WH[E PHOTOGRAPH

Figure 24. SIR-B 

images of the central

.) .. ' parts of the swaths 4 .,

r 

over Hawaii, . •. . - .. - -. 

including the Kau 

Desert and Kilauea ^z F 4^ 

Crater: (a) image i4, : 44: 

obtained at 28-deg (iI I 

at 48 deg look angle.-ç?7

Note differences in ' 

look angle;

Q W 

the response of

vegetated areas to

the right of Kilauea

Crater. These : , . 1 •':'•- 

r 

images have not " • .

., -'% .

- -! : 

been corrected for * 

scale-factor ... . - •

. - A 

differences.

(b) J ri 1 rG -. 

£ - + g 

-

'4 _'f• -' d'

1 

.

:• 

' 

..•,. ... . • ' . ., -1 -. 

4 4 I 

•

.!" •. Jer + •.. .'% s.. 

) *

.

p 

N —p' km Illumination 

Both effusive and explosive eruption (1985), Denyberry et al. (1985), and Gaddis et 

products can be mapped on the images, with al. (1989) have evaluated the potential for using 

different degrees of success. The larger look the SIR-B images for mapping flows of different 

angle (48 deg) appears best for discriminating aa types and ages and have found that young aa 

flows of differing roughness. This may be flows are brightest, followed by older aa flows, 

because backscatter from rough surfaces tends to which are similar to some young pahoehoes, 

be similar at small angles. Farr and Massonnet with older pahoehoes being the darkest of the 

lava flows. Low backscatter from smooth 

pahoehoe surfaces is comparable to that of 

adjacent pyroclastic deposits and could be 

misinterpreted. Slightly greater discrimination 

40
among the pahoehoe flows is evident in the 

image acquired at the 28-deg look angle than in 

that at the 48-deg look angle (Gaddis et al., 

1989). Derryberry et al. (1986) used various 

empirical and theoretical scattering models to 

estimate radar scattering coefficients, ;, for aa 

and pahoehoe surfaces in the SIR-B images. 

Explosive volcanic deposits are also 

observable on the images. The Keanakakoi ash 

of 1790 covers a large area to the south of 

Kilauea Crater (Wentworth, 1938; Powers, 1948; 

Malin et al., 1983; Holcomb, 1987). This ash 

obscures underlying lava flows and, because it 

consists of fine-grained material, it is smooth and 

therefore dark in the radar images (Gaddis et al., 

1985 and 1989). In contrast, the explosive 

eruption of Halemaumau in 1924 produced 

coarser material near the vent (Wentworth, 1938; 

Powers, 1948) and is visible as a bright halo 

around the crater in the radar images (Gaddis et 

al., 1985). 

Based on SIR-B image data alone, the 

general volcanic history and style of Kilauea 

could be ascertained; however, similar low radar 

returns from both ash flows and pahoehoe lava 

flows could lead to misinterpretation of the 

dominant eruptive style and flow emplacement 

conditions (Gaddis et al., 1989). Erroneous 

identification of smooth pahoehoe flows as finegrained ash flows would suggest a greater 

occurrence of pyroclastic activity than actually 

occurred. Determination of lava flow dimensions also is affected by the inability to delineate 

pahoehoe flows, particularly lava flows that 

change from pahoehoe to aa. The inability to 

resolve mesoscale vent structures obscures part 

of the history and evolution of the Kilauea 

summit area and may contribute to underestimating the importance of effusive activity. 

Pinacate Volcanic Field, Mexico. The 

Pinacate Volcanic Field in northwestern Sonora, 

Mexico, is a broad, complex basaltic shield 

composed of three separate shield structures. 

From the Pleistocene through late Holocene 

approximately 240 to 290 km 3 of hawaiite 

magma and 13 km of pyroclastic deposits were 

erupted to form more than 650 lava flows, 500 

cinder and agglutinate cones, and 9 maar or 

maarlike craters (Donnelly, 1974). The flows are 

5 to 15 km long, 1 to 10 in thick, and predominantly aa. Encroachment by eolian and alluvial 

deposits, particularly to the south and west, has 

formed a mantle. that thins toward the interior of

the field. Figure 25 shows Seasat and SIR-A 

images of the Pinacate Volcanic Field. The relationship of radar units within the SIR-A image to 

geologic and geomorphic units has been studied 

by Greeley et al. (1985). 

A map of the distribution of lava flows can 

be made from backscatter contrasts between 

adjacent flows in either image because the 

response is not look-angle sensitive. Backscatter 

curves for these flows remain relatively parallel 

with increasing look angle, unlike the Fantastic 

Lava Beds, where the curves converge at high 

angles (Blom, 1988). Hence, the relative 

brightness differences between the flows is about 

the same in the two images. Although backscatter variations between lava flows can be related 

to the degree of modification by weathering and 

mantle deposits, a direct correlation to flow age 

cannot be made (Greeley et al., 1985). Older 

flows not mantled by eolian sediments can have 

higher backscatter than younger, mantled lavas. 

Identification of at least one heavily modified, 

radar-dark lava flow is enhanced by strong radar 

returns from blocky flow margins. Multiple 

scattering from the blocky edges results in the 

high signal return from the flow margins over a 

range of orientations relative to viewing direction, not just from radar-facing slopes. Since the 

surrounding desert is relatively smooth and 

therefore dark on the image, there is no confusion between lava flows and the surrounding 

area.

Numerous cinder and agglutinate cones 

occurring throughout the Pinacate Volcanic Field 

can be seen on both the Seasat and SIR-A 

images. They are slightly easier to detect in the 

Seasat image; however, the small look angle 

results in geometric distortion, and cone shapes 

are more reliably determined in the larger-angle 

SIR-A image. Hence, in studies where shapes of 

volcanic features are important (such as assessing wind direction during eruption, or directed 

blasts), large look angles are superior. 

The Pinacate Volcanic Field provides an 

excellent example of maar craters. Formed by 

phreatic or phreatomagmatic explosions and 

enlarged by postexplosion collapse (Jahns and 

Fielder, 1952; Gutmann, 1972), these features

41
ORGNAL PAGE

BLACK AND WHITE PHOTOGRAPH 

Figure 25. Pinacate 

Volcanic Field in 

Sonora, Mexico: (a) 

SIR-A image; 

(b) Seasat image. In 

this case, detection 

of lava flows is not a 

function of look 

angle. Cinder cones 

are more easily 

detected in the 

Seasat image, but 

are distorted. Note 

slight differences in 

perceived shapes of 

craters in the two 

images. Bright rings 

outline maar craters.

q

,

[

v4.. 

N\

0

 

10 

km t Illumination 

are raised-rim craters 0.6 to 1.5 km in diameter 

and 30 to 250 m deep. Their distinct radar-bright 

ring pattern is related to rim morphology 

(Greeley et al., 1985 and 1987). Because they 

are geomorphically similar to impact craters, it is 

important to be able to distinguish the two types 

of Iandforms. Radar response from maar craters 

is discussed in more detail and compared with 

that from impact craters in the chapter Impact 

Structures. 

Medicine Lake Highland, U.S.A. Medicine Lake Highland is a forested shield volcano 

located in northern California, about 55 km east 

of Mount Shasta. The central part of the volcano

collapsed to form a caldera, and ring fractures 

around its periphery became the sites of eruptions of viscous andesitic lavas and cones. Later, 

more silicic dacites, rhyodacites, and rhyolites 

erupted on the edges and floor of the caldera. 

The composite, scoriaceous dacitic and rhyolitic 

flows of Glass Mountain on the east rim of the 

caldera and the blocky dacite of Medicine Lake 

Flow on its floor are examples of these late-stage 

silicic flows. Concurrent with the summit 

eruptions, parasitic basaltic aa flows erupted on 

the flanks of the volcano to the north in the Lava 

Beds National Monument area (e.g., Callahan 

Flow) and to the south (e.g., Burnt Lava Flow). 

42
ORGNAL PAGE

BLACK AND WHITE PHOTOGRAPH 

. I 

t 

...: . 

; 

c)

.k;.. 

-. ..(...

I I

- -, 

_3 .- p, 

I, 

A ?

- I

-. 

1

T' S

" •

*7f, .' . ? . 

All 

^rf^ 

............... 

....,

V4ft4-V

1' 

....

: 

N\ 0 10 I 'km Illumination 

Numerous cinder cones and lava tubes also 

formed at this time (Anderson, 1941). The 

variety of features in this area allows study of 

radar response to volcanic landforms as a 

function of magma composition. 

The following is summarized from Farr et 

al. (1981). Cinder cones are detectable on the 

radar image (Figure 26) because of their topographic expression and the presence of a summit 

crater (e.g., Caldwell Butte, to the east of the 

Callahan Flow). Since their slopes are near the 

Seasat look angle of about 20 deg, the illuminated slope is severely foreshortened and the geometry of the cone is distorted. Lava tubes are

visible in the Seasat image as bright curvilinear 

features (e.g., Modoc Crater Lava Tube in 

Hatheway, 1971). The strong return from the 

lava tubes is caused by collapsed sections of roof 

that leave rough rubble enclosed by steep walls. 

Lava flows can be detected on the basis of 

topographic expression and brightness differences. The more silicic flows are thicker and 

have a greater topographic expression than the 

basaltic flows. The Medicine Lake, Glass 

Mountain, and Little Glass Mountain Flows 

show highlighted flow fronts up to 30 m high, 

and both Glass Mountain and Little Glass 

Mountain exhibit multiple overlapping flow

43
ORIG I NAL PAGE 

BLACK AfkD WHITE PHOTOGRAPH 

Figure 26. Seasat 

SAR image of 

Medicine Lake 

Highland.

¶ •

AIr 

k All . 

4 4

46- 

k IL 

iv

ik

0 10 

• :

% 

I

4 

*

t 

:

. 

- , t•l p ;;

1 4

-

I 

4 

! 1

...

.; 

A1

I i km Illumination 

fronts. The Medicine Lake Flow with block 

surface texture is of intermediate brightness 

because the block faces are oriented nearly 

vertically, providing few faces perpendicular to 

the look angle of the Seasat system. The basaltic 

Callahan and Burnt Lava Flows lower on the 

volcano tend to be brighter because of high 

backscatter from their craggy aa surfaces. These 

flows also form distinct flow lobes. Flows in 

Lava Beds National Monument exhibit a variety 

of brightnesses that may be due to differing

amounts of weathering or tephra cover, which 

tend to smooth the original surface texture 

(Donnelly-Nolan and Champion, 1987). 

Fantastic Lava Beds, U.S.A. The three 

spaceborne radar images in Figure 27 illustrate 

the effect of a variable look angle on the ability 

to discriminate volcanic features. These images 

are of the Fantastic Lava Beds in Lassen National Park, northern California, a forested area 

with a young lava flow (most recent eruption, ca. 

200 years ago), ash dunes, a flat ash field, and a 

44
N,;

-

Illumination 0 4 

I I km 

S •S

BLACK AUD WTHUE EHUTOGW-6 

ly

s'

•(c 4 

y 

: 

p 

5,' 1 cr .,. 

• ...... •4•' 

A. 

T :à -'. • 

-,;. •'••i'Ati•.. r •I... 5; FI 

-: .-.;•' 

•. •;iu. .

.• -._. 5,• 

(d) 

[III Lakes 

LIII Ash 

LIII Lava 

LII Ash Dunes 

Cinder Cone 

Faults 

Forests

Figure 27. 

Spaceborne radar 

images of Fantastic 

Lava Beds in Mt. 

Lassen National 

Park, California. 

Most features of 

interest are 

distinguishable in 

the 20-deg-lookangle Seasat image 

(a). Feature 

discrimination 

systematically 

decreases as look 

angle increases to 

about 30 deg in the 

SIR-B image (b) and 

to about 47 deg in 

the SIR-A image (c). 

Major features are 

indicated in the 

sketch map (d). 

-'-S

cinder cone. No significant changes to the area 

occurred over the years intervening between the 

Seasat (1978), SIR-A (1981), and SIR-B (1984) 

images. The principal variable is the look angle 

at which the area was observed. Illumination 

direction has generally not been found to be 

important in the observation of distributed 

targets such as lava flows and natural forests, 

because these features usually lack strong 

directional elements. 

Each image has been digitally contrastenhanced (the SIR-A original negative was digitized) to best display available information. In 

the small-look-angle Seasat image, it is possible 

to identify nearly all geologically important

features. Confusion in this image might arise 

only in discriminating lakes and barren ash, both 

of which have smooth (dark) surfaces. As look 

angle increases, the ability to separate these 

surfaces decreases. At the SIR-A look angle, 

there are only two significant image brightness 

levels representing all materials: the forests and 

lava are bright while the ash dunes, lakes, and 

ash beds are dark and cannot be discriminated. 

Look-angle-induced geometric distortion of 

features with significant topography can also be 

evaluated with these images. In the SIR-A 

image, the cinder cone is difficult to detect 

because it is dark and the rim of the crater at the 

top of the cone is black, contrasting little with

45
ORIGI NAL PAGE 

BLACK ANJ WJHJI HOIi 

(a)

4 LIII Cinders 

Third Eruptive Phase Lavas (Pahoehoe) 

Second Eruptive Phase Lavas (As) 

El First Eruptive Phase Lavas (Pahoehoe)

Fault, Dashed Where Inferred 

Mantled _.._......E7c 

- Surface

Li km 

\N 0 4 

I i km Illumination 

Figure 28. Pisgah 

Lava Field: (a) 

geologic map 

(modified from Wise, 

1966, and Deliwig 

and Moore, 1966); 

(b) mosiac of two 

SAR L-band, HHpolarized images 

obtained by a NASA! 

JPL airborne radar 

system in 1985; (c) 

Seasat image. On 

the Seasat image, 

the bright linear 

feature northeast of 

the Pisgah Lava 

Field is a railroad 

track and the linear 

arrangements of 

bright point returns 

represent power 

lines that cross the 

area.

- '••• •• ,t.__•_ 

4 

p - I, 

4 N I I km Illumination 

46
the cone itself. Prior to digital enhancement of 

this image, the cinder cone could not be detected. 

Despite the difficulty of detection, distortion of 

the cone shape is minimal. The cinder cone is 

distorted in the Seasat image due to the small 

look angle combined with the steepness of the 

cone slope. The cinder cone is easily detected, 

however, because of bright specular reflections 

from the outer edge and inner rim of the crater 

atop the cone where the incidence angle is near 

zero. In the Seasat image, more surface materials are discriminable and the cinder cone is more 

obvious. Distortion induced by imaging geometry must always be kept in mind when precise 

knowledge of the shapes of features is critical. 

In view of the above observations, if one 

were assigned the task of assessing the level and 

recency of volcanic activity in this area, completely different conclusions based on separate 

analyses of the Seasat, SIR-A, and SIR-B images 

could be supported. A single radar image from 

one look angle in the absence of other supporting 

data may therefore be inadequate for geologic 

assessment of a region such as this. In the 

analysis of Magellan images then, the possibility 

that different types of surfaces may have the 

same backscatter at a given look angle should be 

kept in mind. 

Pisgah Lava Field, U.S.A. Pisgah Lava 

Field, a low shield volcano, is located in the 

Mojave Desert about 61 km east-southeast of 

Barstow in San Bernardino County, California. 

Lava erupted onto alluvial-fan and playa-lake 

deposits to form a compound flow 22.5 km long 

and 6.4 km across at its widest point. The 

geology of the Pisgah Lava Field and the 

surrounding area (Figure 28(a)) was mapped by 

Wise (1966) and described by Deliwig (1969). 

Three eruption episodes produced lava flows 

from vents in the vicinity of Pisgah Crater (Wise, 

1966). These units were mapped on the basis of 

phenocryst variations and, as such, do not 

necessarily correspond to different surface 

textures; however, some correlation exists. 

Phase 1 and 3 lavas are predominantly pahoehoe, 

whereas Phase 2 lavas, particularly east of the 

vent, are partly aa. The dominant mesoscale 

morphology within all three units is a ridge-andvalley topography composed of tumuli and 

pressure ridges up to 10 in high.

Alluvial and eolian processes have modified 

the margins of the lava field. Eolian deposits, 

derived from alluvium to the northwest and 

transported- by the dominant winds in this area, 

are concentrated on the western side of the flow. 

Windblown material is deposited in the low areas 

between tumuli and ridges and, locally, is almost 

2 in deep (Dellwig, 1969). Subsequent drainage 

has ponded against the lava and deposited fluvial 

sediments that bury the edge of the lava field and 

fill depressions within the flow adjacent to Lavic 

Dry Lake. 

Sunshine Cone Lava Field is associated with 

a north-northwest trending fault zone south of 

the Pisgah flows (Gawarecki, 1964). This field 

consists of two eruptive units and is slightly 

older than the Pisgah flows (Wise, 1966). 

The Pisgah area has been a test site for 

remote sensing studies since the early 1960s, and 

radar investigations have been conducted by 

Deliwig and Moore (1966), Moore et al. (1968), 

Dellwig (1969), Farr and Engheta (1983), and 

Theilig et al. (1988). Experiments to determine 

an empirical relationship between radar 

backscatter, surface roughness, and eolian 

roughness also have been conducted at Pisgah 

(Greeley et al., 1988). 

Aircraft and Seasat L-band HH radar images 

of the Pisgah area are shown in Figures 28(b) 

and (c), respectively. In the images, the Pisgah 

and Sunshine Cone Lava Fields are brighter than 

the surrounding alluvial material. Variations in 

backscatter exist across the lava surfaces and, for 

the Pisgah flows, correspond predominantly to 

mantled versus unmantled areas. Lower 

backscatter from mantled surfaces is particularly 

noticeable on the long extension of the lava field 

to the northwest and at the southeast end of the 

flow bordering Lavic Dry Lake. The fact that 

the mantled areas are still brighter than the 

surrounding alluvial terrain suggests that only 

the small-scale roughness (< 1 cm) has been 

masked or that the radar penetrates the sediments 

(Elachi et al., 1980). Alternatively, backscatter 

and specular reflection from the unburied ridges

47
and mounds may produce a large radar return 

from the mantled areas. 

Other variations in radar brightness on the 

flow can be related to geologic boundaries 

between the products of the three eruptive 

episodes, but the correlation is inconsistent. In 

both images, the Phase 2 aa unit is slightly 

brighter than the surrounding areas but cannot be 

differentiated from the pahoehoe surface 

southwest of Pisgah Crater. Elachi et al. (1980) 

noted that specular reflection from the low slopes 

of ridges and mounds in the hummocky pahoehoe could be responsible for the high returns 

from that area. 

Pisgah Crater, a cinder cone standing 98 in 

high and having a diameter of 488 in at the base, 

is the most prominent feature on the lava field. It 

is composed of cinders and small bombs 

interbedded with a few layers of agglutinate. 

The cone is visible in both radar images (Figure 

28(b) and (c)) because specular reflection from 

slopes normal to the incident wave creates large 

tonal variations. In addition, the cone appears 

elongated in the Seasat image because of 

geometric distortion caused by foreshortening. 

Airborne L- and C-band (19- and 6.3-cm 

wavelength, respectively) HH radar scatterometer data of the Pisgah Lava Field have been 

analyzed to determine the ability to discriminate 

lava flows, based on backscatter coefficients 

obtained at different incidence angles (Theilig et 

al., 1988). Mantled surfaces have the lowest 

backscatter at all angles for both wavelengths 

and can be distinguished easily from unmantled 

surfaces. The difference between the energy 

returned from bare versus mantled lava tends to 

increase with incidence angle. Phase 3 pahoehoe 

is not easily separated from Phase 2 aa at any 

angle. The difference in backscatter values 

between the two units is greater in C-band than 

in L-band; this suggests that surface roughness is 

similar for both surfaces at the 20-cm scale but 

varies at the 6-cm scale. 

Composite volcano: Mount Shasta, U.S.A. 

Mount Shasta, which rises about 3,000 m 

above the surrounding California terrain, is the

largest composite volcano of the High Cascade 

Range (Figure 29). Volcanic activity began 

about 500,000 years ago, but most of the volcano 

was emplaced within the last 250,000 years. At 

least four overlapping cones composed of 

pyroxene andesite flows, block and ash flows, 

lahars, and dacite domes constitute most of 

Mount Shasta (Christiansen and Miller, 1976). 

The most recent cones, Shastina and Hotlum, 

were built within the last 10,000 to 12,000 years, 

after the last major glacial episode. Only the 

southern slopes and parts of the eastern and 

western slopes of the volcano are heavily 

modified by erosion. Today, five small glaciers 

are present on the mountain. 

Mount Shasta, a nearly circular topographic 

feature, was imaged at three different look angles 

by SIR-B, and thus provides an excellent 

example of how look angle and illumination 

direction affect the appearance of a particular 

feature. SIR-B images at look angles of 28, 51, 

and 57 deg (Leberl et al., 1986) illustrate how a 

similar volcano on Venus would appear in the 

Magellan data at mid- to high latitudes and at 

equatorial latitudes (Figure 29). The most 

striking effect is that of greater geometric 

distortion with decreasing look angle. Many 

volcanic landforms can be identified at all three 

look angles. Moderate to high reflectivity from 

flow surfaces and the highlighting of mesoscale 

lava levees and steep flow margins contribute to 

the detection of thick andesitic lava flows on the 

northwest and southeast flanks of the volcano. A 

north-south line of cones and domes is seen as 

indistinct mounds on the images. Again, 

geometric distortion at smaller look angles is 

evident, particularly on the side of the volcano 

facing away from the radar. The effect of 

illumination direction is exhibited by the variable 

appearance of canyons, which are more distinct 

when oriented normal to the illumination. The 

dark halo around the summit of Mount Shasta 

marks the area above the treeline. 

Explosive volcanism 

Theoretical studies suggest that large-scale 

explosive volcanism is unlikely on Venus 

because of inhibited vesiculation and absence of 

water. However, the possibility cannot be 

discounted in view of the limited data available. 

Two examples of landforms produced by this 

style of volcanism are discussed below. 

48
Valles Caldera, U.S.A. The Valles Caldera 

within the Jemez Mountains volcanic complex is 

a classic example of a resurgent cauldron as 

defined by Smith and Bailey (1968). Situated at 

the western margin of the Rio Grande Rift, the 

volcanic complex overlies Paleozoic and 

Mesozoic sedimentary rocks to the west and is 

interbedded with Cenozoic continental sediments 

to the east. Volcanic activity began with the 

building of a broad highland of basalts, andesites, dacites, and rhyolites erupted between 

about 13 and 3 Ma (Doell et al., 1968; Gardner 

and Goff, 1984). Large-scale explosive eruptions between 1.4 and 1.1 Ma produced the 

numerous rhyolitic ash flows comprising the 

Bandelier Tuff (Smith and Bailey, 1966). As 

large volumes of material were evacuated, the 

roof of the magma chamber collapsed along ring 

faults and formed calderas. The most recent of 

these is the 20- by 25-km-wide Valles Caldera. 

Following collapse, increased pressure in the 

magma chamber uplifted the caldera floor about 

1 km to form Cerro Redondo, a resurgent dome. 

Later eruptions were localized along faults with 

the final activity between 0.9 and 0.15 Ma, 

producing a discontinuous circle of tephra cones, 

domes, and lava flows in the moat between Cerro 

Redondo and the caldera walls (Smith and 

Bailey, 1966; Self et al., 1986 and 1988). 

Many of the volcanic elements of this area 

are depicted in the Seasat SAR image and the 

accompanying sketch map (Figure 30). Structural features and their tectonic implications are 

discussed in Tectonic Landforms. The megamorphology of the broad shield extending away from 

the central caldera is evident in the radar image. 

Valles Caldera, the resurgent dome Cerro 

Redondo, and the postcaldera ring of cones, 

domes, and lava flows can all be distinguished in 

the radar image. The western fault of a graben 

that formed across the resurgent dome during 

uplift is visible because of specular reflection 

from the radar-facing slope. Alluvial deposits on 

the caldera floor are smooth to L-band radar and 

form distinct dark patches in the northern and 

western parts of the depression. The Toledo Embayment, a poorly defined semicircular scarp 

adjacent to the northwest rim of Valles Caldera, 

marks a possible remnant caldera or tectonic 

collapse, modified by subsequent erosion and 

eruptions (Self et al., 1986). On the Banco 

Bonito Flow—located on the southwest side of

the Valles Caldera—mesoscale, parallel, 

festoonlike ridges are highlighted along radarfacing slopes. 

The gently sloping surface of the Bandelier 

Tuff has low relief and is exhibited best to the 

south and west. Deep canyons formed by subsequent erosion cut these surfaces and are highlighted by reflection from radar-facing canyon 

walls. Because of the small look angle, canyons 

appear asymmetric, particularly to the north and 

south where they are approximately perpendicular to the radar look direction. To the east and 

west, the canyons parallel the illumination and 

appear shallower and less well defined. 

Ignimbrite sheets, central Andes, South 

America. Ignimbrite sheets are indicative of 

large explosive volcanic eruptions. Examples of 

unmodified ignimbrite plains are not common on 

Earth and the initial morphology of these 

deposits is not completely known. Older 

Cenozoic pyroclastic flows in the central Andes, 

South America, have been imaged by SIR-B and 

provide examples of modified ignimbrites with a 

distinctive erosion pattern. The following 

discussion is based on a study by Fielding et al. 

(1986). 

Cenozoic volcanism has covered much of 

the Puna/Altiplano, a high plateau in the central 

Andes, with dacitic to rhyolitic ignimbrites and 

andesitic composite volcanoes. Large pyroclastic flows were emplaced predominantly during 

two major periods between 10 and 7 Ma and 

between 5 and 3 Ma (Kussmaul et al., 1975; 

Coira et al., 1982). Minor eruptions producing 

small ignimbrite sheets have occurred within the 

last 3 Ma (Schwab and Lippolt, 1976). 

Ignimbrite sheets are strong scatterers and 

appear bright on the SAR images (Figure 31). 

Weathering of these pyroclastic deposits results 

in variable scales and shapes of roughness that 

enhance radar backscatter. Ignimbrite deposits 

in Figure 31(a) and (b) are roughly similar in age 

to ignimbrites in Chile that have a reticulate 

network of low ridges about 10 to 30 cm in 

height and up to a meter in width (Guest, 1969). 

These ridges are close to the scale of the radar

49
4.

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH 

•

--

Figure 29. SIR-B 

images of Mount 

Shasta at look 

angles of (a) 28 deg, 

(b) 51 deg, and (c) 57 

deg, corresponding 

to expected 

Magellan data from 

mid- to high and 

equatorial latitudes 

of Venus. Geometric 

distortion increases 

with decreasing look 

angle. Most lava 

flows and domes are 

detectable on all 

three images. (d) 

Generalized 

geologic map of 

Mount Shasta 

(modified from 

Wagner and 

Saucedo, 1987).

- Equivalent 

Latitude 

-' on Venus 

-

Mid to High 

--r- :

-A }+)r

-- , --'

"i 1 

,,.

`tz I.- - 

----- •• 

-: 

- 

AL

 Equatorial 

- I - - I - 

-

, I

2 

7 ..t 

- 

N\ Illumination 

0 20 I I 1 k 

50
Lava FIoi 

LII Pyroclasi 

Quaterna 

and GIac 

Cones an 

,-- - Canyons 

(d)

N\ 0 20  1km 

wavelength and could contribute to the high 

backscatter. Older ignimbrites (Figure 31(c)) are 

characterized by meter-sized blocks and resistant 

ledges a few centimeters to a few meters in size. 

These features form facets that, if oriented perpendicular to the radar beam, reflect energy back 

to the receiver. 

Parallel bright and dark lines characterize 

these deposits on the SAR images and represent 

a distinct erosional pattern composed of quebradas—linear steep-sided gullies and canyons. The 

friable nature and thermal disaggregation of 

ignimbrites enhance the ability of the wind to 

abrade these deposits. Flowing water contributes 

to the erosion process but probably follows 

preexisting channels incised by the wind. Steep 

gully walls are responsible for the alternating 

dark and bright bands within the SAR images, 

with the bright lines representing specular 

reflection from radar-facing slopes. In the

examples used here, the SAR look direction is 

almost perpendicular to the quebrada trends. If 

oriented parallel to the radar illumination, they 

may not be detectable. 

Summary 

Magellan radar observations will increase 

our understanding of volcanism on Venus and 

provide new insight into the evolution of 

geologic processes on Earth's sister planet. 

Diverse volcanic processes on Earth produce 

distinct types of volcanoes and associated 

features that can be detected on SAR images and 

interpreted in terms of styles of eruption and

51
ORIGINAL PAGE 

SLACK AND WHITE PHOTOGRAPH 

Figure 30. Jemez - Mountains volcanic '.; 

complex in the Rio

- Grande Rift, New

Mexico: (a) Seasat 

mosaic of the - - - - -3

region. The , NN 

megamorphologyof

:. 

surrounded by a 

shield-like apron of

- pyroclastic deposits

is visible in the 

Generalized map of 

volcanic units

(modified from Ross - •1 , .())(ç(. 

etal., 1961). The .- . . 

extent of this map is 

indicated at the t 

margins of the - q 

image. -J - 

A

: *

\

- : 

— \ 

N\

0 20 I i km -- Illumination 

52
0 

0 

Cl)

o

in 

qoT

U, 

C.) 

0 

C.) 

U) 

Cl) 

106045' 106030' 106015' 

N

0 15 'km 

emplacement, rheology, and tectonic setting. For 

example, it is possible to discriminate plains 

units emplaced as flood basalts (e.g., Columbia 

River Basalts) from units emplaced as a series of 

overlapping tube-fed flows (e.g., Craters of the 

Moon) or low shields (e.g., Pisgah Lava Field). 

The presence of flank lava flows serves to 

discriminate Askja-like calderas (Iceland) from 

large-scale explosive silicic ones such as the 

Valles Caldera. 

Geometric distortion that is high at low look 

angles decreases noticeably as the look angle 

increases above about 35 deg (e.g., Mount 

Shasta, Fantastic Lava Beds, Pinacate Lava 

Field). However, there are inconsistensies in the 

relations between feature discrimination and look

angle that remain unexplained (e.g., systematic 

decrease in feature discrimination with increasing look angle at the Fantastic Lava Beds as 

compared to no noticeable contrast in feature discrimination with increasing look angle at the 

Pinacate Lava Field). 

The global coverage of Magellan will 

provide a basis for comparative planetology at 

multiple spatial scales and further our understanding of relations between volcanism, 

tectonism, and thermal evolution at the planetary 

scale. In addition, altimetry data will help define

53
I 

I

Illumination 

0

I 5km I

ORIGINAL PAGE

SLACK AND WHITE PHOTOGRAPH 

Figure 31. SIR-B 

images of ignimbrite 

sheets: (a) and (b) 

in Bolivia; (c) in 

Argentina. High 

backscatter from the 

rough erosional 

surfaces 

characterizes the 

ignimbrite deposits. 

Linear light and dark 

bands represent a 

pattern that maybe 

characteristic of the 

eolian erosion of 

ignimbrites along 

cooling or tectonic 

joints. A dark sawtooth band in (b) 

marks the contact 

between two 

ignimbrite sheets. the morphology of megascale features such as 

shield and composite volcanoes. 

The increased resolution afforded by 

Magellan SAR will allow the identification of 

mesoscale features and reveal more information 

about the morphology of individual flows, vents, 

and related constructs. It will also allow analysis 

and interpretation of small-scale surface roughness characteristics. This information will help 

elucidate the evolution and relative ages of 

individual volcanic centers, characterize the 

styles of volcanism, constrain the composition of 

the deposits, and evaluate evidence for postemplacement weathering. However, experience 

with terrestrial examples in high-resolution 

spaceborne SAR images has shown limitations in 

the ability to discriminate certain eruptive 

products (e.g., ash vs. pahoehoe at Kilauea; aa 

vs. pahoehoe at Askja Volcano) and the age 

relations and source of multiple lava flows (e.g., 

Myvatn, Iceland). These examples should stand 

as a reminder of limits that may exist in the interpretability of Magellan images. 

54
Sand Dunes and Subsurface Imaging 

N N N N S N S S 1 3 3 if S S S 

Introduction 

This chapter reviews the radar imaging 

characteristics of eolian deposits and the phenomenon of subsurface radar imaging. The 

combination of these topics in a single chapter 

arises from the fact that wind-blown material is 

the most common radar-transparent material at 

Earth's surface. One of the criteria for radar 

penetration is fine-grained homogeneous cover 

material, and eolian sorting readily achieves this. 

Some eolian deposits have scattering properties 

that are highly directional and thus impose 

geometric constraints on the imaging system if 

they are to be detected. 

Eolian Deposits 

Eolian deposits (such as sand dunes and 

sand sheets) cover a large percentage of the 

surface of both Earth and Mars (McKee, 1979; 

Cutts and Smith, 1973; Breed et al., 1979), and 

may occur on Venus (Greeley and Iversen, 

1985). Should eolian deposits occur, detection 

and study of them would provide insight into the 

geologic processes on Venus. Since it is 

necessary to image the surface with radar, and 

since terrestrial eolian materials have peculiar 

radar backscatter characteristics, it is important 

to understand these characteristics to enable 

interpretation of possible eolian features on 

Venus. 

The following review summarizes studies of 

radar images and scatterometer data of terrestrial 

sand dunes reported in the literature (Blom and 

Elachi, 1981 and 1987). The essential conclusions of these studies are quite simple, but they 

have significant implications for the detection 

and study of possible eolian terrains in radar 

images of Venus. 

While the geometry and other characteristics 

of eolian landforms on Venus are likely to be 

different from terrestrial examples, the radar 

response is likely to be similar. This is due to 

the fact that wind-transported material is fine 

grained by nature, which means essentially that

all eolian surfaces are smooth at radar wavelengths. The possible eolian landforms to be 

encountered on Venus are not discussed here as 

this is a different problem than that addressed. 

Experimental and theoretical results about 

potential eolian material and landforms on Venus 

can be found in Greeley et al., 1980 and 1984; 

Greeley and Marshall, 1985; and White, 1981. 

Terrestrial sand dunes are quasi-specular 

reflectors at all wavelengths studied (75 cm 

through 2.25 cm, Blom and Elachi, 1987). These 

wavelengths bracket the Magellan wavelength of 

12.6 cm. Because of this quasi-specular behavior, a radar echo from a dune is possible only 

when a dune face several wavelengths on a side 

is nearly perpendicular to the imaging radar. If 

such is not the case, dunes are imaged as a dark 

area and the true nature of the landscape is not 

disclosed in the radar image. Radar echoes from 

sand dunes are highly directional and sensitive to 

the imaging geometry of the radar system. To 

illustrate the effects that can be encountered, the 

following examples show extreme changes in the 

appearance of dunes as a function of the look 

angle and illumination direction of the radar. 

Look-angle effects 

Airborne 3.0-cm wavelength (X-band) and 

Seasat and SIR-A 23.5-cm wavelength (L-band) 

images of the southern portion of the Algodones 

Dunes (southeastern California) are shown in 

Figure 32. Both of the spaceborne images have 

been digitally contrast stretched (the SIR-A 

original negative was digitized). This portion of 

the dune field consists of large barchanoid dunes 

enclosing areas of desert floor (interdune flats). 

In the Seasat image and the low-look-angle 

airborne image, the sand dunes are quite bright, 

texturally distinctive, and easily distinguished 

from the darker image tone of the desert floor. 

In the larger-look-angle airborne and SIR-A 

images, however, the dunes are black and the 

desert floor remains dark, though brighter than 

the dunes. Thus the dunes themselves are not 

detectable at the larger look angle, and there is a

55
(a)

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

(Cl 

.

4 

N .-

Canals 

Dune

Interdune 

Canal 

lnterdune

Dune 

Figure 32. 

Algodones Dunes, 

California: (a) SIR-A 

and (b) airborne 

images were 

acquired at look 

angles greater than 

the angle of repose 

for dunes; (c) Seasat 

and (d) airborne 

images were 

acquired at look 

angles less than the 

angle of repose. In 

the Seasat and lowlook-angle airborne 

images, sand dunes 

are visible because 

of quasi-specular 

reflection from 

favorably oriented 

dune slopes, where 

the incidence angle 

is zero. In the SIR-A 

and large-look-angle 

airborne images, the 

dunes are dark 

because the look 

angle is greater than 

the angle of repose.

9 0km 

56
Dune 

Interdune 

75 cm HH 

Dune 

Interdune 

19 cm HH 

-

Dune 

Interdune 

6.3 cm HH 

Dune 

Interdune 

I I I 

•1

- -

0 

-5 

-10 

-15 

-20 

6-25 

Z -30 

w 

C,

-35 

a) 

0 

C) 

0)0 

C 

I-w -5 

cn -10 C, 

I..

-15 

cc

-20 

-25 

-30 

-35

contrast reversal between the two surfaces when 

viewed at these different look angles; this 

reversal is independent of wavelength. The large 

dunes would probably be undetected if an 

analysis were based on only the larger-lookangle images. 

The seemingly unusual backscattering 

behavior of sand dunes is due to the fact that 

dunes consist of a set of smooth surfaces with a 

nonzero slope. The dunes are bright in the lowlook-angle images because of quasi-specular 

reflection from smooth dune faces that are nearnormal to the radar beam, where the incidence 

angle at the dune face is zero. Because terrestrial 

eolian sand has an angle of repose of about 33 

degrees, no dune slopes exceed this value. 

Therefore, radar backscatter from a dune is 

possible only at look angles less than 33 degrees 

where a dune slope may be normal to the 

incident beam. If this is not the case, the echo is 

specularly reflected away from the radar antenna, 

and the black image tone seen in the high-lookangle images results. Thus, to image terrestrial 

dunes with radar, a necessary (but not sufficient) 

condition is a look angle less than 33 deg. (Note

that, in the case of spaceborne SARs, the 

effective look angle is slightly increased by 

planetary curvature.) Venusian dunes may have a 

different slope geometry, but they will almost 

certainly have radar-smooth surfaces. Hence, the 

look-angle constraints may be different, but the 

mechanism by which dunes are detected will be 

the same. 

To reinforce this important point, consider 

the scatterometer data shown in Figure 33. The 

graphs show backscatter plotted as a function of 

look angle, and show the surface response as the 

look angle changes. Scatterometer data can be 

used to predict the appearance of various 

surfaces in radar images as a function of frequency and geometry. Here, horizontally 

polarized transmit and receive data at wavelengths of 75.0, 19.0, and 6.3 cm are shown. 

Also, vertically polarized transmit and receive 

2.25-cm wavelength scatterometer data are 

plotted (horizontally polarized data at this 

wavelength are not available). These wavelengths bracket the Magellan and Veneta radar 

wavelengths, and also the Seasat and SIR 

systems used to image the Earth. When examinFigure 33. 

Scatterometer data 

for a large dune and 

interdune flat (desert 

floor), Algodones 

Dunes, California, at 

four wavelengths 

from 75 cm to 2.25 

cm. Dunes are 

darker than interdune 

fiats at larger angles 

and brighter at small 

angles, as in Figure 

32. Backscattering 

characteristics of the 

dunes are not 

functions of the 

observing 

wavelength. Minor 

differences in curve 

shapes for different 

wavelengths are due 

to varying fields of 

view for different 

5 10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50 scatterometers. 

Look Angle, deg

57
Sun Azimuth Illumination Illumination 

'4"T -

I 

ing the plots, keep in mind that the Magellan 

radar will have a look-angle variation with 

latitude that nearly equals the plotted range. 

Figure 33 shows that dunes are brighter than 

desert floor at small look angles and darker than 

desert floor at higher look angles, duplicating the 

response seen in the radar images of Figure 32. 

This behavior is independent of wavelength. 

Hence, the detection of sand dunes requires a 

radar with a look angle less than the angle of 

repose. Note that dune response is not sensitive 

to wavelength over the region considered here 

since the backscatter curves have similar characteristics. Should Venusian dunes have geometric 

characteristics similar to those of terrestrial 

dunes, images of them at high latitudes would be 

comparable to the low-look-angle images in 

Figure 32, while low-latitude dunes would look 

like those in the high-look-angle images in 

Figure 32 (see Figure 3 of Synthetic-Aperture 

Radar Imaging). It would therefore be very 

difficult to image such dunes using the Magellan 

radar at low latitudes, and a false impression of a 

latitudinally varying distribution of dunes might 

be given. 

Illumination-direction effects 

Dunes are also directional features, and so

must be oriented approximately perpendicular to

the radar beam, or no backscatter will occur. 

Thus, the morphology of the dunes as revealed 

by the radar could be misleading in regard to 

dune form and wind regime. These effects are 

illustrated in Figure 34, which shows a Skylab 

visible-wavelength image and two Seasat radar 

images—from different illumination directions—

of the Mohawk Dunes in Arizona. The dunes 

have a network pattern in the Skylab photograph. 

Each Seasat image reveals only half of this 

network pattern. Each radar image, by itself, 

could easily lead to a misunderstanding of the 

true dune forms and the wind regime responsible. 

Conclusions 

Terrestrial eolian features are quasi-specular 

radar reflectors and hence impose stringent 

geometric constraints on any radar system, if 

they are to be detected. Radar look angles must 

be less than the angle of repose for dunes, and 

the radar illumination direction must be approximately perpendicular to the dune trends. Should 

eolian features exist on Venus, their detection 

will likely impose similar constraints on the 

radar system. 

The radar system configuration must include 

the ability to view the area from more than one 

illumination direction. Dune response to radar is 

Figure 34. Mohawk 

Dunes, Arizona: (a) 

Skylab photograph; 

(b) Seasat ascending 

image; (c) Seasat 

descending image. 

In the Seasat 

images, only one set 

of the intersecting 

dunes shows in 

each image because 

of the orientation of 

the radar 

illumination. The 

spacing of the linear 

dune crests is about 

130 m. 

58

N/ 2P km 

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPh
not sensitive to wavelengths over the region 

studied (75 to 2.25 cm). 

In the interpretation of Magellan images, 

then, inferences about the presence or absence of 

eolian landscapes and their forms must be made 

very cautiously, with both the characteristics of 

the response of eolian material to radar and the 

viewing geometry constraints imposed by the 

Magellan orbit kept in mind. 

Subsurface Imaging 

Although the imaging of shallow subsurface 

features by radar has occurred on Earth, it is a 

rare phenomenon. Subsurface imaging may also 

occur in Magellan images of Venus. It is 

therefore useful to review the circumstances 

under which subsurface imaging can occur and 

the resulting response on the radar image. Since 

it is difficult or impossible to determine, based 

on a radar image alone, that imaged features are 

surface or subsurface, surficial processes that 

created now-buried landforms might be misinterpreted as current geomorphic processes. For 

example, if only SIR-A radar images of the 

Egyptian Sahara were available, the obviously 

fluvial landforms seen in the radar images might 

be mistakenly identified as the result of current 

geologic processes. In this environment, the 

overlying cover of eolian deposits is largely 

transparent to the SIR-A radar. 

The possibility of subsurface radar imaging 

through dry surficial materials has been long 

known, but only recently demonstrated. Shuttle 

Imaging Radar-A images of Egypt and Sudan, 

acquired in November 1981, revealed a largely 

unknown fluvial landscape beneath an eolian 

cover (McCauley et al., 1982). In this case, subsurface radar imaging occurred on a regional 

scale. Figure 35 from Elachi et al. (1984) shows 

the dramatic contrast in landscapes perceived by 

radar and optical sensors in the Sahara. Imaging 

of subsurface features through dry fine-grained 

material has now been documented for Seasat, 

SIR-A, and SIR-B radar images of very and 

areas (McCauley et al., 1982; Blom et al., 1984; 

Elachi et al., 1984; Berlin et al., 1985 and 1986; 

McCauley et al., 1986).

Necessary conditions 

The conditions under which subsurface 

radar imaging can occur must be met simultaneously. (These conditions are reviewed in a 

geologic context by Blom et al., 1984.) The 

conditions are: 

(I) The cover material must be radar 

smooth, fine grained, homogeneous, 

and not too thick. (At a 23.5-cm 

wavelength, 2 m of penetration have 

been documented; 4 to 6 in may be 

possible under conceivable conditions.) 

(2) The cover material must be extremely 

dry. (Moisture content must be less 

than about 1%; this criterion is most 

difficult to satisfy on Earth.) 

(3) The subsurface must be rough enough 

to generate backscatter. 

Simultaneous satisfaction of these conditions on Earth is unusual; however, the sand 

sheets in hyperarid regions such as the Sahara 

are ideal. On Earth, the most limiting factor is 

the moisture constraint—few locations are as dry 

as the Sahara. (See "Response to Terrain" in 

Synthetic-Aperture Radar Imaging). On Venus, 

the surface temperature eliminates the moisture 

constraint, and fine-grained areas, if they exist, 

present the possibility of regional subsurface 

imaging as in the Sahara. 

Variables in interpretation of Magellan images 

Effects of grain size. For a radar signal to 

penetrate a medium without substantial attenuation, the particle size of the medium must be 

small relative to the wavelength, and the cover 

material must be homogeneous. Roth and Elachi 

(1975) found that losses from the scattering of 

radar signals were considerable for grain sizes 

larger than about one-fifth of the wavelength, 

whereas these losses were small for grain sizes 

less than about one-tenth of the wavelength. 

Because wavelength shortens upon penetration 

of a denser medium, the size of the particles in 

the cover material must be less than 0.8 cm to 

prevent scattering losses for an S-band system 

such as Magellan. Roth and Elachi (1975) also 

indicated that the presence of only a few particles exceeding the one-fifth-wavelength-size 

criterion for each radar resolution cell results in 

scattering sufficient to largely obscure subsurface features. Therefore, only those environ59
IGNAL PAGE 

WHITE PHOTOGRAPH 

I 

•

* I "r - - .•4_• .,.. 'I 

15. W^P^ ^ 

OR 

BLACK AND

Figure 35. A 50 x 

100 km area at the 

Egyptian/Sudanese 

border: (a) Landsat 

MSS (multispectral 

scanner) band 6 

image; (b) SIR-A 

image. The Landsat 

image shows a landscape dominated by 

eolian processes. 

The Selima Sand 

Sheet blankets the 

underlying material 

to a few meters in 

thickness. Presently 

active dunes are 

visible as diagonal 

streaks near the 

image center. In 

contrast, the SIR-A 

image reveals a 

subsurface landscape carved by 

fluvial processes. 

The confluence of 

two large river 

channels is in the 

center of the SIR-A 

image. The dunes in 

the Landsat image 

have no expression 

on the SIR-A image.

--

- 

N/ 0

i km Illumination 

ments where geologic processes produce 

homogeneous fine-grained cover materials are 

favorable for the detection of subsurface features 

by radar. Landscapes dominated by eolian 

processes are most suitable for subsurface 

imaging. Grus from equigranular granitic rocks, 

volcanic ash, and fine-grained impact ejecta are 

other likely possibilities. 

Look-angle considerations. Because the 

Magellan look angle will vary with latitude (see 

Figure 3 of Synthetic-Aperture Radar Imaging), 

a discussion of the effects of this variation on 

subsurface imaging is warranted.

The first consideration is summarized from 

Elachi et al. (1984). With respect to Magellan 

images, the important conclusion reached is that 

the rate of decrease in backscattered signal 

strength with increasing incidence angle is less 

for subsurface features than for the same features 

exposed at the surface. The cause is refraction at 

the surface interface, which reduces both the 

incidence angle and wavelength. Because radar 

backscatter curves have, typically, a steeper 

slope at smaller incidence angles, the consequence of a reduced incidence angle is an 

appreciable increase in echo strength. The 

wavelength reduction also increases potential 

echo strength because it allows satisfaction of the 

Rayleigh roughness criterion with a smoother 

subsurface reflector. Elachi et al. (1984) showed 

60
High Look Angle Low Look Angle 

4 

iiougn cuosiraze

that under extreme conditions, a subsurface 

feature can yield an actually stronger echo than 

that from the same feature exposed at the 

surface. 

A second consideration is the possible 

interaction of the radar signal with a fine-grained 

cover over a rough substrate where cover 

thickness and slope vary. The following 

example refers to an eolian terrain, but it is 

applicable to other fine-grained material as well. 

A terrain of low sand dunes on a flat sand 

sheet allows imaging of the surface dunes at 

small look angles and the subsurface features at 

larger look angles. The reasoning follows. 

Because sand dunes can be imaged only at look 

angles less than the angle of repose, dry dunes 

are seen only on images taken at small look 

angles, although a specular echo always occurs 

from their surfaces. A rough subsurface, 

however, will backscatter at all look angles. 

Hence, if the dunes and sand sheet are not too 

thick and otherwise favorable for subsurface 

imaging, the dunes would be imaged at small 

look angles and partially mask the subsurface 

image; at larger look angles, the rough subsurface would be seen but not the dune cover. 

In Figure 36, the incoming high-look-angle 

radar beam (1) strikes the smooth sand sheet and 

is specularly reflected forward (thin arrowed 

line); no energy returns to the radar, and a dark 

image area results. In this case, the sand is too 

thick for the energy that penetrates the surface to 

be scattered from the subsurface. Energy that 

strikes the sand dune (2) is partially reflected 

forward off the smooth dune face and partially 

refracted into the sand. As this energy continues 

down, it strikes the rough substrate and is 

diffusely scattered. If the overburden is not too

thick, some of this energy could return to the 

surface and be received at the radar, resulting in 

intermediate brightness. At (3), rough material is 

exposed at the surface and a strong diffuse echo 

occurs, resulting in a bright image. In the next 

four cases ((4), (5), (6), and (7)), the incoming 

signal is at a lower look angle. In (4), the signal 

is specularly reflected off the face of the sand 

dune, and, because of the geometry, a strong 

signal is returned. Some of the signal penetrates 

the sand and is diffusely scattered off the rough 

substrate; this much weaker subsurface signal 

may also be recorded. Case (5) shows that even 

with a low look angle, geometry unfavorable to 

image formation can occur in sand dunes. Radar 

energy is here specularly reflected away from the 

sensor, and the backsides of the dunes are not 

imaged. Also, the combination of the dune and 

sand sheet is too thick here, and the signal that 

penetrates is not strong enough to reach the 

substrate. In case (6), some energy is partially 

reflected away at the sand sheet, and the rest is 

refracted and then diffusely scattered off the 

rough substrate. Here, the subsurface will be 

imaged, if the sand sheet is not too thick. Case 

(7) shows that the exposed rough surface is also 

a diffuse scatterer at low look angles, and a 

bright image results. In these different situations, the surface, subsurface, both, or neither 

may be detected in the resulting images. 

Application to Magellan images 

While geological processes on the surface of 

Venus are not well known at this time, recent 

reports by Pettengill et al. (1988) and Jurgens et 

al. (1988) indicate two factors of interest here. 

First, there are areas where the dielectric 

constant is very high compared to that of 

Figure 36. Possible 

circumstances 

under which the 

surface and 

subsurface could be 

variously detected in 

the same image (S = 

specular scattering, 

D = diffuse 

scattering).

61
ordinary rocks sampled in the laboratory by 

Campbell and Ulrichs (1969). The Venusian 

values are also much higher than those for Moon 

and Mars, for which much better knowledge of 

the surface is available. Second, the analysis of 

Pettengill et al. (1988) indicates that Venus is 

probably not substantially covered with soil or 

dust relative to the coverings on Mars and Moon. 

They state, "...most of the Venus surface is 

overlain, at most, by only a few centimeters of 

soil or dust..." If generally true, this would 

indicate a cover medium that would permit 

subsurface imaging because the "dust" may be 

radar transparent. Jurgens et al. (1988) suggest 

that some dark areas in their radar images of 

Venus may be mantled by fine-grained ejecta 

from impacts. Such fine-grained material, if it 

exists, might also be radar transparent, but 

subsurface features beneath it would not be 

detected because the other criteria are not 

satisfied. 

Because of the high surface temperature on 

Venus, there will be no moisture to raise the 

dielectric constant, although both Jurgens et al. 

(1988) and Pettengill et al. (1988) demonstrate 

that there are areas of very high dielectric

constant. They suggest the phenomenon may be 

the result of a concentration of electrically conducting materials such as iron sulfides. Such 

areas have anomalously high radar reflectivity. 

The point of emphasis is that a single radar 

image does not contain information sufficient to 

determine whether the radar is detecting surface 

or subsurface features. For example, if only SIR 

images of the Sahara were available, the fluvial 

features observed by McCauley et al. (1982) 

could have been misinterpreted as features of the 

current landscape, rather than those of a relict 

subsurface landscape. For an analysis of such 

terrain, multi-look-angle and multiwavelength 

radar images would be most desirable (the latter 

because penetration is a function of wavelength, 

and a short wavelength radar would penetrate 

less), but complementary visible and nearinfrared images (such as those of the Landsat 

Thematic Mapper) would suffice because they 

view only the surface. Such images are not 

possible from an orbit around Venus because of 

the cloud cover. In interpreting the Magellan 

images, the possibility that subsurface rather than 

the surface features are revealed must always be 

kept in mind. 

62
Tectonic Landforms 

Introduction Terra) (Schaber, 1982). Examination of the 

Venus and Earth are regarded as sister 

planets because they are comparable in diameter, 

density, and proximity to the Sun (Appendix A). 

However, they have evolved along different 

paths and are known to have widely contrasting 

surfaces and atmospheres. Accordingly, the 

tectonic character of Venus is vitally important 

to comparative studies of planetary evolution 

among the terrestrial planets. On Earth, the 

concept of plate tectonics, which embodies 

segmentation, spreading, and recycling of the 

lithosphere through time, has provided the 

necessary intellectual framework for studying 

global tectonic history (Cox and Hart, 1986). 

Plate boundaries are highly mobile and much of 

Earth's surface is comparatively young. Observations over the past 25 years have indicated that 

planetary bodies with a diameter less than one 

half that of Earth (e.g., Mars and Mercury) are 

characterized by unsegmented, globally continuous lithospheres that expose an extensive record 

of early bombardment and early volcanism. The 

surfaces of these bodies are relatively old and 

stable. 

Venus is thought to be a dynamic planet 

with a surface that is comparatively young 

(Basilevsky and Head, 1988). The density of 

impact craters observed by Soviet Venera 15 and 

16 imaging radar in the northern hemisphere of 

Venus suggests that the age of the surface could 

range from about 4.5 Ma (Schaber et al., 1987) 

to about 1 Ga (Basilevsky et al., 1987; Ivanov et 

al., 1986), depending on the modeling assumptions used. However, the Venera images cover 

only 25% of the planet, and the cratering history 

of Venus is not known with confidence. 

The global tectonic environment of Venus is 

uncertain. Low-resolution radar reflectivity and 

altimetry measurements in the Pioneer Venus 

experiment enabled the recognition of lowlands, 

rolling uplands, and highlands on a near-global 

scale (Masursky et al., 1980; Pettengill et al., 

1980). Long, global-scale zones of disturbance 

have provided evidence of limited lithospheric 

extension in the Equatorial Highlands (Aphrodite

topography in western Aphrodite Terra led Head 

and Crumpler (1987) to suggest that the area 

displays the characteristics of oceanic divergent 

plate boundaries and is the site of crustal 

spreading on Venus. 

Earth-based radar observations have 

provided comparatively high-resolution images 

of geologic structures in local areas. Banded 

terrain (ridge and groove) reported by Campbell 

et al. (1983) to represent tectonic folds and faults 

was interpreted by Kotelnikov et al. (1984) to 

indicate horizontal deformation. 

Tectonic terrains mapped in northern and 

midlatitudes from the Venera 15 and 16 imaging 

missions (Basilevsky et al., 1986) reveal (1) 

large areas of plains that show flowlike features, 

streaks, grooves, and low ridges; (2) intersecting 

sets of ridges and grooves (parquet) known as 

tesserae; (3) subparallel ridges and grooves that 

extend over long, relatively narrow belts, but 

may occupy broad zones; and (4) concentric 

patterns of ridges and grooves (coronae) called 

ovoids. Basilevsky et al. (1986) concluded that 

terrestrial-style plate tectonics does not exist and 

also that ancient terranes are not observed. 

Using Venera image data, Crumpler et al. (1986) 

showed the presence of fold and fault structures 

in patterns suggestive of orogenic belts. More 

recently, an interconnected transpolar disruption 

zone about 60,000 km in length has been 

reported by Kozak and Schaber (1989). Though 

this feature is similar in scale to Earth's interconnected oceanic ridge system, it is reported to be 

markedly different in other respects. 

Radar reflectivity, altimetry, and image data 

obtained to the present have suggested both 

horizontal and vertical movements on the surface 

of Venus. The general tectonic style in the area 

imaged by the Venera missions is quite different 

from that on present-day Earth and the smaller 

terrestrial planets. Few features on Venus

63
compare directly to the essential plate tectonic 

elements on Earth. High-resolution images (100 

to 200 m) and global coverage are needed to 

determine the overall character, distribution, and 

tectonic origin of landforms on Venus, and to 

distinguish volcano—tectonic features from 

impact craters. The advent of Magellan radar 

coverage will provide continuing impetus to the 

search for a paradigm that explains the evolution 

of the Venusian surface. 

Tectonic Styles 

Tectonic landforms are underlain by structural features produced by crustal deformation. 

The landforms result from differential erosion 

acting on structures produced by endogenic 

processes of regional extent. Different styles of 

regional deformation tend to reflect pronounced 

contrasts in tectonic settings. However, similarities among certain types of deformation packages, such as compressional folds and thrusts or 

large-scale strike-slip faults, enable us to extrapolate from well-studied areas to lesser-known 

ones. Spaceborne images offer a synoptic basis 

for observing and interpreting regional structural 

relations from tectonic landforms. 

Radar imaging of Earth's tectonic landforms 

is restricted to surface features of the continental 

crust. Plate tectonic features of the oceanic 

crust, such as ridges, oceanic transforms, and 

trenches, are underwater and not perceptible to 

radar. Radar images reviewed here are arranged 

according to tectonic style, with illustrations 

taken from prominent fold belts, accreted 

terranes, a boundary transform zone, and 

comparatively stable continental plate interiors. 

These images were obtained by Seasat SAR and 

Shuttle Imaging Radar (SIR). 

Fold belts 

Numerous fold belts are distributed in linear 

zones that coincide with mountain chains in the 

continental crust. In the nineteenth century, the 

deposition and deformation observed in the 

central Appalachian fold belt of eastern North 

America provided the basis for the concept of the

geosyncline (Hall, 1859). Like many orogenic 

belts, the Appalachians have a long and complex 

history that includes several widely separated 

episodes of deformation. In the framework of 

plate tectonics, the Appalachians provide the 

type example for the closing and reopening of an 

ocean (Wilson, 1966). The thrust sheets that 

characterize Appalachian tectonics are mainly 

décollements in which older sequences have 

overridden younger rocks. Imbricate thrust 

slices are bounded at the surface by high-angle 

reverse faults that change to low-angle thrusts at 

depth. Some effects of the continental collision 

between the African and North American plates 

that accompanied the closing of the Iapetus (preAtlantic) ocean in the Alleghenian (Permian) 

orogeny are illustrated in the deformation of the 

valley-and-ridge portion of the fold belt. Thrust 

slices are exposed at the surface in the south. In 

the central portion, the thrusts are buried by 

kilometers of moderately to strongly folded 

layered rocks (Gwinn, 1964). The contrasted 

topographic expression of thrust faults and 

folded terrane is clearly demonstrated in the 

different landscapes of the southern (Figures 37 

through 40) and central (Figures 41 and 42) 

portions of the Appalachian Valley and Ridge 

Province. 

Southern Appalachian Valley and Ridge 

Province, U.S.A. In the southern Appalachians, 

the valley-and-ridge geomorphic province is 

dominated by reverse faults that flatten at depth 

to form low-angle bedding plane thrusts and by 

folds not connected to the basement. To the 

north, the Cumberland Plateau is underlain by 

relatively undeformed, gently dipping strata. 

Seasat SAR coverage of a portion of the province is shown in Figures 37 and 38. The Pine 

Mountain Thrust carried a thin sheet of rocks 

northwestward over the relatively undeformed 

Cumberland Plateau for a distance estimated at 

6.4 km (Harris and Milici, 1977). The terrane is 

divided by major thrust faults into northeasttrending slices, with strata folded and faulted in a 

regular fashion. The topographic trend parallels 

the structural strike. The unconnected structure 

of the basement has been inferred from magnetic 

and gravitational data (Watkins, 1964). Subsequent work has demonstrated the existence of 

large crystalline detachments (Cook et al., 1979; 

Harris et al., 1981). 

Structural mapping is feasible from the 

Seasat SAR images where the structures 

64
JA 

•i •;'. 

1 :... 4..'' 

2 

N/

ORIG1 

BLACK AND WHITE PtluIOGRApH

40 ..j km - Illumination 

Figure 37. Seasat 

SAR image of thrustfaulted, valley-andridge terrane 

showing the Pine 

Mountain Thrust 

Sheet, southern 

Appalachians, U.S.A. 

Imaging azimuth is 

67.5 deg.

7 

8 

65
ORIGINAL PAGE

&.ACK AND WHITE PHOTOGRAPH 

. - 

••. ,. S..., 

P 4 - . 't' •fr 

-

...; . JF• I :' 

•; •• 

0 40 

I I I km Illumination 

Figure 38. Seasat 

SAR image 

corresponding to 

part of the valleyand-ridge terrane in 

Figure 37. Imaging 

azimuth is 292.5 deg. 

Al 

N/ 

66
- A B C I D E F G H Figure 39. 

1

A

structural 

ountaln !_.\TcI 

matic cross

^^^^Rocky Face section of 

2

Fault

the area 

- \\\\ It "+" Middlesboro - covered by 

Midd lesboro 

3

Jacksboro\ /L.\..._____./

I I 

I

Fault in Figure 37 

from Harris 

and Milici, 

I

:. p •

'In e 1977). 

Ow eli va\ 

-

j

\'\\ 

\\\\\\ //\\\I////\\\\\\\ - 

5 .. ...... Valley 

- WaI%' •.: :::. .. .. 

:..i1nCl,P0rt Fe - 

6

-

V. .4t11e Fault - 

qatley 

- .

......... vault 

Be a V eT

Al I 

Middlesboro Powell Valley 

A

Syncline Anticline A' 

Anticlinal Axis

0 Synclinal Axis t.

40 j km

67
(Figure 39) can be inferred from the geomorphology. In this terrane, the majority of erosional features that are responsible for extensive 

linear changes of tone and texture on the images 

are structurally controlled (Ford, 1980). 

Pine Mountain is a major linear feature that 

exposes a northwest facing scarp (Figure 37, C2 

to Hi). The extensive linear to curvilinear trace 

of Cumberland Mountain shows a southeast 

facing scarp (C4 to 1­13), the base of which marks 

the north limb of the Powell Valley Anticline. A 

broad synclinal structure is apparent from the 

pattern of small flatirons that face each other 

across the dip slopes of the Pine and Cumberland 

Mountains (Middlesboro Syncline). Additional 

structures evident within this block include the 

Rocky Face Fault (H 1 to 1 ­12) and the faultbounded Middlesboro Basin (G2/G3 to H2/H3). 

The basin structure (about 6 km in diameter) has 

been described as a probable meteorite scar or 

astrobleme (Englund and Roen, 1962) and later 

confirmed as such from the discovery of shatter 

cones in the central uplift portion of the structure 

(Dietz, 1966). 

Many extensive topographic lineaments correspond with major faults. The Jacksboro and 

Pine Mountain Faults are among the more 

pronounced examples that give a distinctive 

outline to the terrane in this scene. The highangle truncation of the Pine Mountain Thrust 

Sheet by the Jacksboro Fault suggests a wrench 

or tear origin for this fault. Repetition of 

distinctive linear topography in the successive 

parallel to subparallel ridges and valleys denotes 

repetition of the underlying lithologies by 

faulting (Figure 39). It also denotes the effects 

of differential erosion on beds of differing 

hardness. Corresponding repetition of distinctive 

image textures on the Seasat SAR images allows 

the faults to be located. The low-angle truncation of the Wallen Valley Thrust by the Clinchport Thrust is clearly evident in Figure 37 at the 

base of the junction between B6 and C6. The 

intersection of these thrust faults marks a hinge 

between regional directions of structural strike—

about N50E to the west and N60'E to the east. 

Offset ridges and abruptly truncated ridges 

typically indicate high-angle faulting (for

example, the termination of Beaver Ridge at the 

right side of D7). 

Minor geomorphic lineaments consisting of 

linear valley segments and stream channels less 

than 10 km long are widespread. Few of these 

features have been mapped as faults or fracture 

traces, though some are adjacent to or aligned 

with major fault structures. The Seasat SAR 

images (Figures 37 and 38) show short, aligned, 

and en echelon valley segments up to 3 km long 

that parallel the Rocky Face Fault and extend far 

north and south of its mapped trace. A linear 

north-trending magnetic gradient in the basement 

lies parallel to this fault and extends south across 

the Powell Valley Anticline. This gradient has 

been interpreted as reflecting a pre-Appalachian 

structural grain in the basement (Watkins, 1963 

and 1964). The close coincidence of the small 

topographic lineaments aligned with the Rocky 

Face Fault and observed on the Seasat SAR 

images probably reflects this subsurface structural grain. 

Discontinuous short lineaments mapped 

from the SAR images are aligned with the 

Jacksboro Fault and extend tens of kilometers 

southeast of its surface trace. Gravity and 

magnetic discontinuities that are also aligned in 

this direction have been cited as evidence for 

subsurface extension of the Jacksboro Fault in 

the basement (Watkins, 1964). The lineaments 

mapped from the SAR images may represent 

fracture traces associated with the compressive 

stresses that gave rise to the Jacksboro Fault. 

Strong local brightness variations occur 

within corresponding areas of the two images 

(Figures 37 and 38). Some surfaces that appear 

bright in one illumination azimuth are dark or 

obscure in the other. Examples are clearly 

evident along the ridge-and-valley terrane. 

Where the ridges and valleys are illuminated 

parallel to the strike (Figure 37) there is little 

image contrast between them. Correspondingly, 

there is pronounced highlighting of the ridges 

and clear discrimination of the intervening 

valleys where these features are illuminated 

across strike (Figure 38). This example illustrates one of many conditions under which radar 

response is strongly directional. The valleys 

have level to gently undulating surfaces with 

slopes that are mostly less than 7 deg. The 

ridges and mountains have steep slopes, commonly 20 to 35 deg and up to a maximum of 

about 40 deg. Bright image tones generally 

68
.:• .••• ••..... . . . .... . : :. .:

.: 1....•• ... .••• . ... .1•.•• 

00

1)

...... 

— - — — 

dop 

/ . 

.00

..1 . .••

. 

.....

,-...... .. 

1

...................... ...................... 

................ ............ 

.............. ...................... 

.............. ......... I ............ ....... 

...................... ..... ...................... ............... ...... 

...................... 

— — — Fault; dashed where located approximately from radar Images 

0 40 

I I 1 km 

Figure 40. Image texture map from the Seasat SAR image in Figure 37. Texture units 

correspond to major lithologic associations: (1) sandstone/shale/slltstone (massive), 

(2) clay-shale/limestone, (3) conglomerate/sandstone/shale, (4) sandstone/shale! 

slltstone (thin-bedded), (5) dolomite/chert/crystalline limestone (Ford, 1980).

69
Figure 41. j

A B 

SeasatSAR 

image of 4. 

folded

valley-andridge terrane 

in central

2

N\

CD E j

F G H

J 

0

Ire

IF 

i$;"4frY 

f r r#si7

Ir

4. yç4?' 

I At 5YJ, 

ot 

JI ; &t 

'1k \61 

J;d : '• IP4

OA.

Apo; '14w 

of 

IL 

; 

•f;A-, e 

'Jjty' I'n 

Illumination 

0 40 

I I 1 km

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGp.,AJ 

70
Figure 42. 

Simplified 

structural 

sketch map 

and schematic cross 

section of 

the area 

covered by 

Seasat SAR 

in Figure 41. 

Cc C C 

0 .Co 0

MN 

A -

U)2-

122 

___ ---------- A' 

Ridge-Forming Strata, including Tuscarora, 

Pocono, and Pottsville Formations 

Anticlinal Axis With Plunge Direction 

-k- Synclinal Axis

40 j km

71
Figure 43. Location 

and plate-tectonic 

setting of Sibi 

Trough, Pakistan. 

Bold lines denote 

plate boundaries. 

Arrows show 

relative motion 

between plates. 

Toothmarks indicate 

direction of 

thrusting. (Modified 

from Jacob and 

Quittmeyer, 1979.)

EURASIAN PLATE 

Chaman Fault 

00 Afghan

Sibi

- 

Makrar, ' Trough 

ARABIAN

PLATE

INDIAN 

PLATE 

represent strong radar backscatter from sloping 

surfaces or foldover of slopes that face toward 

the illumination vector (foreslopes). Some dark 

tones represent weak returns from backsiopes. 

Foreslopes in one illumination azimuth appear as 

backsiopes in the other. On a scale large enough 

to be independent of this effect, however, 

distinctive image textures can be discriminated 

in corresponding areas on the two images. 

In many areas of the Valley and Ridge 

Province, lithologic mapping is facilitated by 

studying the topography (Swingle, 1973). Image 

textures that are broadly representative of 

topography can be discriminated and mapped 

from the Seasat SAR images (Figure 40). The 

types of topography reflected in the image 

textures correlate reasonably well with distinct 

bedrock associations and bulk lithologic sequences, though they do not identify individual 

lithologies or stratigraphic units. 

Central Appalachian Valley and Ridge 

Province, U.S.A. In the central Appalachians, 

the Valley and Ridge Province exposes a 

sequence of long narrow ridges and wide open 

valleys that extend in broad arcuate form over 

600 km. A Seasat SAR image of this terrane in 

central Pennsylvania is shown in Figure 41. 

Major ridges are situated 5 to 15 km apart and 

extend for tens of kilometers in length. Structurally, the terrane consists of numerous elongate 

subparallel plunging anticlines and synclines

with fewer thrusts than those in the southern Appalachians (Figure 42). 

The exact nature of the structures can be 

inferred in the image (Figure 41) from the 

bedding attitudes in successive ridges and from 

the shape of the noses formed by the convergence of the paired limbs. Owing to the steep 

dips of the limbs, most anticlines appear as 

narrow slender features with long sharp noses 

(e.g., Shade Mountain, Figures 41 and 42, C3 

through A8). The noses denote axial plunge 

directions. With more gently dipping limbs, 

most synclines appear as wide, more open 

features that have shorter and more rounded 

noses (e.g., Mahantango-Buffalo and Berry 

Mountains, F2 and G to Hi). Major ridges are 

held up by resistant sandstones and conglomerates of the Tuscarora, Pocono, and Pottsville 

Formations. Minor ridge offsets indicate highangle cross-cutting faults. The ridges are 

homoclinal, each representing a single limb of a 

larger structure. Incipient breaching of an 

anticline beneath Tuscarora Mountain is evident 

in Figure 41 at D3. In one instance, a ridge 

(Third Mountain) is situated centrally in the core 

of a plunging syncline (H2). 

Sibi Trough, Pakistan. Profound and 

large-scale deformation of the lithosphere has 

occurred in localities where there is a history of 

collision and suturing of continental plates. This 

is exemplified in south Asia by the collision and 

suturing of India and Eurasia in the mid-Tertiary 

(Molnar and Tapponnier, 1975 and 1977; Peltzer 

and Tapponnier, 1988). Where the collision was 

oblique, such as occurred in western Pakistan 

adjacent to Afghanistan, it has given rise to great 

structural complexity. Major strike-slip faults 

terminate and interact with thrusts and reverse 

faults in zones of convergence. Such interactions are well marked in Pakistan, where the 

Chaman transform zone connects the Makran 

and Himalayan convergence zones (Figure 43). 

Combined eastward thrusting and sinistral strikeslip along north-south oriented faults have been 

interpreted to indicate northwest motion of the 

Indian subcontinent relative to the Afghan Block 

(Lawrence et al., 1981). 

The SIR-A image (Figure 44) covers the 

Sibi Trough in western Pakistan. This area is 

located about 100 km east of the Chaman 

Transform. The trough is situated at the western 

extremity of the Indian subcontinent in a zone of 

72
I Sibi 

6I: 

- .... 1 Ha? 

7 

8

ORIGINAL PAGE

BLACK AIW WHITE PHOTOGRAPH 

N -' Illumination 

20 km

A t B I C I D I E 

I Mach 

1I 

H 

2

oe

Tor Ghar

]\ 

&is&\) 

• \ 4 

Madatl/ ,.:I •...,Z 

Khan Mirl 

Gidarl 

...,'-.. / Peak 

Tadri 

• Bedding, Strike, and Dip 

Anticlinal Fold, Showing Plunge 

* Synclinal Fold 

-- Fault 

Figure 44. SIR-A image of Sib! Trough, 

Pakistan, with simplified structural sketch map.

73
compression. The tectonic effects of collision 

are marked here by thrusts and by folds that 

show abrupt changes of orientation over short 

distances. Broad synclinal basins are separated 

by narrow, tightly folded anticlines. Bedding 

patterns on the image outline the strike and dip 

of the rocks. Moderate to gently inclined strata 

in the limbs of synclines show patterns of 

flatirons from which the dip directions are 

inferred (e.g., Figure 44, B4 to C7). Narrow 

outcrop patterns indicate steeply dipping to 

vertical or overturned beds. Bright radar returns 

from the antidip slopes of tightly folded anticlinal limbs are pronounced about 10 km northwest 

of Sibi and at Tor Ghar (A5 to B5 and B3 to C4). 

A few kilometers west of Spintangi, vertical to 

overturned beds that extend southeast for about 

20 km appear as a narrow saturated band on the 

image. 

North and east of the Sibi Trough, older 

rocks are exposed in the cores of doubly plunging folds. In contrast to the Sibi Trough, the 

folds to the north and east show broad anticlines 

with narrow, tightly folded synclines. Deformation was Late Tertiary to Recent. Separation of 

this area from the Sibi Trough is distinctly 

marked along a fault trace that extends on the 

image from D9 to E6. Examples of the broad 

anticlines are apparent from Galusha Hill, near 

Spiritangi, to Gidari Peak (E6 to E9). Faults 

oriented approximately north-south in the upper 

left of the image (A3 to Dl) mark the western 

limit of left slip in the Chaman transform zone. 

Accreted terranes 

The Paleozoic and Early Mesozoic rock 

sequences of much of western North America, 

including about 95% of Alaska, have widely 

divergent lithotectonic origins and, in many 

places, are juxtaposed along fault boundaries. 

They have been interpreted as slabs of crust 

added to the preexisting continent by piecemeal 

accretion during Late Mesozoic and Cenozoic 

time (Coney et al., 1980; Churkin et al., 1982;

Jones et al., 1983 and 1987). The crustal slabs 

are referred to as terranes. Most of the terrane 

boundaries are known or suspected faults, but the 

recognition of an accretionary origin resulted 

from the observation of pronounced discontinuities in lithology and paleobiology combined with 

striking contrasts in paleomagnetism across fault 

boundaries. 

The potential for discriminating accretionary 

tectonic landforms on images with Magellan 

characteristics and comparison with corresponding images having Venera characteristics has 

been investigated by Arvidson et al. (1988). 

Seasat SAR mosaics of an area of eastern interior 

Alaska with simulated Magellan and Venera 

image characteristics were evaluated and 

compared for relevant geologic information 

content. The simulated parameters included 

spatial resolution, number of looks, system 

signal-to-noise ratio, and incidence angle. The 

major contrast between the simulations is the 

spatial resolution, which is 10 to 15 times higher 

for Magellan than for Venera. 

Eastern interior Alaska, U.S.A. Seasat 

SAR mosaics of an area of accretionary terranes 

in eastern interior Alaska are shown with 

simulated Magellan resolution (-120 m) in 

Figure 45 and with simulated Venera resolution 

(-2000 m) in Figure 46. Tectonized highlands 

are located in four distinct regions from north to 

south. They include (1) the southern foothills 

and the Philip Smith Mountains of the Brooks 

Range (Figures 45 and 46, C1ID1 to C2/D2), (2) 

the Kokrines Hills-Ray Mountains-Hodzana 

Highland (A4 to C2), (3) the Yukon-Tanana 

Upland (B4/C4 to C5/D5), and (4) the northern 

foothills and mountains of the Alaska Range (B6 

to C5 and C6). The highlands are underlain by a 

wide range of complexly deformed, predominantly Precambrian through Lower Paleozoic 

metasedimentary and metaigneous rocks 

(Beikman, 1980, and sources quoted therein). 

The generalized lithology and the distribution of 

lithotectonic terranes in the area covered by the 

SAR image simulations are shown in Figure 47. 

Both the Magellan and the Venera image 

simulations (Figures 45 and 46) show the 

dominance of steep slopes up to 30 deg and local 

relief up to 1000 m in the highland areas. The 

radar sensitivity to slope distorts the topography 

in a strongly directional fashion and obscures 

most of the other surface characteristics in the 

74
highland areas. Though it is possible on both 

simulations to observe broad tectonic fabrics 

imprinted on the highlands on a scale of tens of 

kilometers, only the Magellan simulation shows 

sufficient detail to distinguish structures and 

drainage networks on a scale of hundreds of 

meters. For example, the mountains in the north 

have a blocky structure with dominantly easttrending facets and smooth valleys up to 10 km 

wide that widen southward in the direction of 

drainage. They follow linear and broadly arcuate 

traces with a symmetry that appears to be structurally controlled. The outline suggests a broad 

structural antiform with an east-west axis, accompanied by plutonic intrusion. The U-shaped 

form of the wide valleys seen at the simulated 

Magellan resolution strongly suggests glacial 

erosion. These valley forms are not evident at 

the simulated Veneta resolution. 

The structural relations among major 

accretionary terranes recognized in the area are 

shown in Figure 48. Though the Angayucham 

and the Tozitna Terranes are bounded by major 

faults and separated by the intervening Ruby 

Terrane, they contain grossly similar oceanic 

rock associations that give rise to broadly 

comparable landforms. Thus, discrimination 

between the Angayucham and Tozitna Terranes 

is not possible on either of the image simulations 

(Figures 45 and 46) owing to the absence of 

well-defined topographic characteristics or 

unambiguous boundaries. Numerous radar 

lineaments that correspond with previously 

mapped faults, some of which extend for over 

100 km, are seen in the Magellan simulation. 

However, the lineaments in many cases do not 

correspond with terrane boundaries. 

In addition to accretionary terranes, the area 

includes postaccretionary Cretaceous plutons, 

which are widely distributed from the Ray 

Mountains through the Hodzana Highland to the 

Brooks Range, and younger relatively undeformed sedimentary basins, especially in the 

Yukon Flats and Tanana Lowlands (Figure 47, 

C3 to E3 and B5 to CS). These postaccretionary 

features cannot be discriminated from the 

accretionary terranes based only on their 

topography in images. 

The basic observations for recognizing 

accretionary terranes are mostly not susceptible 

to measurement or verification by radar or other 

forms of remote sensing. Specifically, confusion

arises in identifying accretionary terranes on the 

images because (1) different terranes that have 

grossly similar lithologies may appear to be 

texturally similar (compare Angayucham and 

Tozitna Terranes), (2) the traces of low-angle 

thrust faults that mark some terrane boundaries 

are commonly not distinguishable in images, (3) 

the fault traces that are distinguishable do not 

necessarily represent terrane boundaries, and (4) 

the distinction between lithotectonic terranes of 

accretionary origin and younger nonaccretionary 

plutons or sedimentary rock bodies of postaccretionary origin cannot be made from image data 

alone. In view of the above observations, there 

is little possibility of detecting and correctly 

identifying lithotectonic accretionary terranes on 

radar images of Venus. 

Continental boundary transforms 

Plate tectonics involves major strike-slip 

faulting. Some of the longest strike-slip faults 

occur as boundary transforms between plates that 

move parallel to the boundary. A remarkable 

feature of such faults is their disregard of gross 

physiographic features such as mountains and 

plains and their pronounced linearity across such 

features over extended distances. A review of 

the principles, concepts, and tectonic implications of strike-slip faults has been given by 

Sylvester (1988). Because the San Andreas 

Fault is the most extensively studied strike-slip 

fault, it has been the source of many ideas 

relating to strike-slip tectonics. The San Andreas 

Fault is a ridge/ridge transform that extends for a 

distance of about 1000 km and marks the 

boundary between the North American and the 

Pacific Plates. It represents a deep discontinuity 

in Earth's crust that extends down to the lithosphere. At the surface, it forms a nearly continuous extension of roughly parallel fractures 

distributed in a band that varies in width from a 

few tens of meters up to 10 km (Crowell, 1975). 

Strike-slip faulting produces distinctive geomorphic features at regional and local scales that 

stand out on spaceborne radar images.

75
ORIGINAL PAGE 

SLACK AND WHtTE EHOIOGRARIi 

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Figure 45. Seasat 

SAR mosaic of 

eastern interior 

Alaska at simulated 

Magellan resolution.

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76
77 

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ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

200 -j km \ Illumination 

Figure 46. Seasat 

SAR mosaic of 

eastern interior 

Alaska at simulated 

Venera resolution. 

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Lithology

Cenozoic and Upper Cretaceous 

Sedimentary Deposits

-

+ ± - I

Cretaceous Plutonic Rocks, 

Felsic 

I A A A A

Tertiary and Cretaceous 

Volcanic Rocks, Felsic _______

Paleozoic Sedimentary and 

Metasedimentary Rocks 

V V V V 

V V V V

Jurassic to Permian 

Volcanic Rocks, Mafic

Paleozoic and Precambrian 

Metasedimentary and 

Metaigneous Rocks

Terranes 

AA Arctic Alaska 

AM Angayucham 

RB Ruby 

TZ Tozltna 

VT Yukon-Tanana 

U Undifferentiated

0 200 

I 1 m 

Figure 47. Generalized lithology and distribution of lithotectonic terranes in the area covered by Seasat 

SAR simulations (Figures 45 and 46). Bold lines denote terrane boundaries. Nonaccretionary rock 

bodies are shown in blue. Complex and fragmentary terranes are not differentiated. (Modified from 

78 Arvidson etal., 1988.) A schematic cross section is given in Figure 48. 
Cache 

Mountain 

Pluton

TZ YT 

Hodzana 

Highland 

Multiple Terranes 

Undifferentiated 

Yukon River 

1•2 

RB 

0 100 I 1km

Figure 48. Schematic cross section 

(from Figure 47) 

showing 

Angayucham (AM) 

and Tozitna 

Terranes structurally 

above Ruby Terrane 

(RB) (modified from 

Churkin et al., 1982; 

Jones et al., 1986). 

Southern California, U.S.A. The Seasat 

SAR image mosaic in Figure 49 shows segments 

of the major crustal blocks and structural features 

that make up the landscape in southern California. Most of the landforms in this area are the 

direct result of tectonic processes. Major fault 

structures that give rise to prominent topographic 

features on the image are shown in Figure 50. 

The dominant structure is the San Andreas Fault. 

In this scene, it extends for about 320 km from 

the junction of the Garlock Fault (Figure 49, 

A2), across the east-west trending Transverse 

Ranges (A3 to C4), to its termination east of the 

Salton Sea (136). The fault separates the Los 

Angeles Basin, Coastal Ranges, and Salton 

Trough to the west and south from the Mojave 

Desert Block to the east. 

The Oarlock Fault separates the Mojave 

Desert Block from the Sierra Nevada and the 

Basin Ranges (including Death Valley and 

Panamint Valley) to the north. This major fault, 

which extends from the San Andreas Fault to the 

south end of Death Valley (A2 to 132), has been 

described as an intracontinental transform (Davis 

and Burchfiel, 1973). Seismic studies suggest 

that it is a relatively shallow structure that transforms to a horizontal shear zone at a depth of 

about 10 km (Serpa, 1987). 

The linear to curvilinear traces of the San 

Andreas and the Garlock Faults form large-scale 

topography that is strongly enhanced on the radar 

mosaic (Figure 49). The San Andreas Fault trace 

bends notably at localities where other faults 

intersect it at a high angle (e.g., A2 and CS). 

Extensive northwest-southeast oriented linear 

features in the central Mojave area denote the 

traces of strike-slip faults within the Mojave 

Desert Block. These faults are part of a complex 

regional network of right shear that connects the

Death Valley region with the San Andreas Fault 

system (Dokka and Travis, 1989). 

West of the San Andreas Fault, the image 

shows extensive linear topography and alignment 

of landforms over distances upward of 20 km. 

These features mark the traces of faults in the 

San Jacinto and the Elsinore Fault zones (Figure 

SO). The San Jacinto Fault zone is thought to 

represent a major component of braided subparallel faults that occur toward the termination of 

the San Andreas Fault system. The fault system 

is transformed at its south end to a spreading rift 

as the structure passes southeast into the Salton 

Trough and the Gulf of California spreading 

center (Crowell, 1979; Elders et al., 1972; 

Robinson et al., 1976). Further west, the 

Elsinore Fault zone displays a narrow linear 

trough that extends for about 40 km toward Lake 

Elsinore. The right-stepping fault strands at this 

locality have given rise to a pull-apart basin 

(Mann et al., 1983) in which the lake is situated 

(Figure 49, box 1). 

When opposing blocks in a fault system 

move in strike-slip, long thin slivers of rock may 

subside between parallel or en echelon fault 

strands to form closed depressions, or they may 

rise to produce pressure ridges. Thus active 

strike-slip faulting is expressed by the presence 

of comparatively small-scale youthful geomorphic features aligned along a fault zone and by 

offset drainage channels across the fault. 

Conclusive evidence for strike-slip faulting is 

provided by offset scarps that face in opposite 

directions across a fault plane. Associated smallscale geomorphic features such as closed 

depressions, troughs, sag ponds, and various 

types of linear ridges and valleys have been 

described and defined by many workers (e.g., 

Sharp, 1954; Wesson et al., 1975; Crowell, 1979;

79
ORIGINAL PAGE

BLACK AND WHIT PHOTOGRAPH 

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Figure 49. Seasat SAR mosaic of southern California. Areas within boxes I through 4 

illustrate comparatively small-scale, fault-related features described in the text.
A I B 

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3 

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Los Angeles

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CR

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Figure 50. Sketch map of major crustal blocks and strike-slip faults in southern California. Faults and fault systems 

include Blackwater (B), Blue Cut (BC), Calico (C), Camp Rock (CR), Death Valley (DV), Elsinore (E), Helendale (H), 

Lenwood (L), Pinto Mountain (PM), San Gabriel (SG), San Jacinto (SJ). 81
,. , I 

Figure 51. Segment 

of Garlock Fault 

(Figure 49, box 2) 

showing small-scale, 

youthful geomorphic 

features aligned 

along the fault zone. 

Offset drainages 

indicate left-lateral 

displacement on the 

fault.

Shutterridge Linear 

Formed By Ridge 

Offset Alluvial 

Fan

Offset 

Depression Drainage

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGR,Ap 

Linear 

Valley 

N/ o 10 I I I km -- Illumination 

Hester and Hallinger, 1987). Notable examples 

of closed depressions, shutterridges, and other 

linear ridges and valleys along a portion of the 

Garlock Fault are evident in Figure 51 from their 

spatial relations and morphotectonic outlines. In 

addition, the direction of horizontal displacement 

can be determined from the direction of stream 

valley offsets across the fault, which is left 

lateral in this instance. 

From the above discussion, it is clear that a 

variety of different tectonic landforms are 

enhanced on radar images because radar systems 

are very sensitive to slope characteristics. 

However, where relief is low and slopes are 

negligible, imaging radars are sensitive both to 

the small-scale roughness of a surface and to its 

complex dielectric constant. Small-scale 

roughness is a measure of average microrelief at 

a scale of centimeters. Rough surfaces yield

diffuse backscatter that appears bright on images. 

Smooth surfaces produce dominant forward 

scatter and appear dark on images. Nevertheless, 

in this context "rough" and "smooth" are relative 

terms and they vary with the observing wavelength. A surface that is "rough" at a short 

wavelength is "smooth" at a longer wavelength 

(see Synthetic-Aperture Radar Imaging). 

The dielectric constant of most rocks lies 

within a narrow range; however, it varies as a 

function of moisture content in porous rocks, 

sediments, and soils. Porous surface materials 

with a comparatively low dielectric constant 

when dry have a much higher dielectric constant 

when they contain moisture; this results in 

considerably brighter radar returns. Radar 

sensitivity both to surface roughness and to 

moisture content have enabled the perception of 

nonrelief features of tectonic origin. Compari82
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Figure 52. Durmid 

Hills east of the 

Salton Sea (Figure 

49, box 3): (a) 

Seasat SAR image 

obtained at 23.5-cm 

wavelength in 

August, 1978; (b) 

corresponding 

airborne radar image 

obtained at 2.4-cm 

wavelength in 

November 1985 by 

Aero Service for the 

U.S. Geological 

Survey. The tonal 

lineament marking 

the trace of the San 

Andreas Fault on the 

Seasat SAR image is 

not perceptible at 

the shorter 

wavelength of the 

airborne radar. 

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0 4 I I km OR?C-rNIA[ PAGE 

Sons of Seasat SAR images obtained at 23.5-cm 

wavelength (L-band) with corresponding 

airborne radar images obtained at wavelengths of 

2.8 cm (X-band) and 0.8 cm (Ka-band) provide 

examples. 

The Seasat SAR image of the Durmid Hills 

east of the Salton Sea (Figure 49, box 3) shows a 

pronounced tonal lineament marked by bright 

radar returns to the southwest and radar darkness 

to the northeast (Figure 52(a)). Slopes in this 

area are 2 deg or less. The tonal lineament is not 

apparent on a corresponding airborne radar 

image obtained with higher spatial resolution at a 

shorter wavelength (Figure 52(b)). Sabins et al. 

(1980) reported that the Seasat lineament marks 

the trace of the San Andreas Fault and coneBLACK AIW WHITE PHOTOGRAPH 

sponds to a fault boundary between outcrops of 

the Borrego Formation to the southwest and 

Lake Cahuila deposits to the northeast. The 

Borrego surface is highly dissected by numerous 

closely spaced gullies and littered with graveland boulder-size fragments of sandstone. The 

surface is rough at both the long wavelength of 

Seasat SAR and the shorter airborne radar 

wavelength. This results in diffuse backscatter 

and bright image tones in each image in Figure 

52 (cf. Sabins et al., 1980, Figure 10, which 

shows corresponding bright image tones at 0.8-

cm wavelength (Ka-band)). 

The Lake Cahuila deposits overlie and 

mostly conceal the Borrego Formation. Because 

the surface of these deposits is a smooth layer of

83 
iA 

ORIGINAL PAGE

BLACK AMI W-HUZ

 FHOIOGRAF1I 

Figure 53. Bristol 

Dry Lake (Figure 49, 

box 4): (a) airborne 

radar image 

obtained at 2.4-cm 

wavelength in July 

1975 by Goodyear 

Aerospace (courtesy 

of Ray Sugiura); (b) 

corresponding 

Seasat SAR image 

obtained at 23.5-cm 

wavelength in 

August 1978. The 

tonal lineament 

extending 

northwest-southeast 

across the lake bed 

in the 2.4-cmwavelength image is 

not perceptible in 

the 23.5-cmwavelength image.

Illumination N

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 W. 

..

lit 

 -

.." 

LL ___ 

is

'. Illumination 

0 10 1km 

unconsolidated sand and silt, gullies are comparatively scarce. The small-scale relief of the 

surface is smooth at the Seasat SAR wavelength. 

This produces low backscatter and darkness on 

the image. However, at shorter airborne radar 

wavelengths of 2.8 cm or less, the Cahuila 

surface is rough and appears radar-bright on 

images (e.g., Figure 52(b), and Figure 10 of 

Sabins et al., 1980). 

In this example, the 23.5-cm radar wavelength (Seasat SAR, L-band) discriminates 

juxtaposed surfaces that have different smallscale roughness characteristics, thereby revealing 

the trace of the San Andreas Fault in an area 

where there are no clear scarps, depresssions, 

linear ridges, or offset drainages. This type of 

surface discrimination is clearly wavelength 

dependent and not feasible at the 2.4- or 0.8-cm 

wavelengths of the airborne radar systems (X- or 

Ka-band). In the field, the fault trace is relatively obscure. The absence of Cahuila deposits 

southwest of the fault has not been explained; it 

may represent nondeposition, or it may be the 

result of selective postdepositional erosion. 

An airborne radar image obtained at 2.4-cm 

wavelength south of the Bristol Mountains 

(Figure 49, box 4) shows a pronounced northwest-southeast tonal lineament across the playa 

deposits in Bristol Dry Lake (Figure 53(a)). This 

tonal lineament does not appear on a corresponding 23.5-cm-wavelength Seasat SAR image 

(Figure 53(b)). The sediments consist of silt, 

clay, salt, and minor amounts of fine sand. The 

playa surface is covered by a dry crust less than 

1 cm thick that has a puffy appearance. Sediments beneath the crust are moist to locally wet. 

Sugiura and Sabins (1980) reported no significant differences in the sediment characteristics on either side of the lineament on the 

airborne radar image, and they found no evi84
consists of a series of en echelon north-trending 

grabens and half-grabens (Woodward, 1977). 

The grabens are referred to as basins that 

separate the Great Plains from the Colorado 

Plateau, the Rocky Mountains, and the Basin and 

Range physiographic provinces (Figure 54). The 

regional outline of the structure is marked by 

offset segments along northeast-oriented 

lineaments. The rift has been divided into 

northern, central, and southern segments, each of 

which has its own structural style and history 

(Chapin, 1979). 

A portion of the central segment that 

includes the Jemez Volcanic Field is shown in 

the Seasat SAR image, Figure 30(a) of Volcanic 

Landforms. Most of the marginal faults that 

bound the rift are poorly exposed. The image 

shows the best exposures of marginal faults on 

the west side of the rift, in the form of prominent 

linear east-facing escarpments. Extensive 

northeast- and east-northeast-oriented lineaments 

in the image transect the rift and intersect at the 

1080 1070 1060 1050 

I SOUTHERN ROCKY 

- MOUNTAINS

Colorado

New Mexico 7 

- J(1j 

COLORADO

PLATEAU 

I GREAT 

PLAINS 

I IF BASIN\

AND 

RANGEL

I

New Mexico 

- - - I Texas1

Figure 54. Relations 

of Rio Grande Rift 

(stippled) to 

surrounding 

physiographic 

provinces. Faults 

are shown by lines 

with hachures that 

denote the 

370 downthrown side. 

The central portion 

of the rift marked "J" 

denotes the Jemez 

Volcanic Field and 

adjacent area shown 

in Figure 30(a) of 

Volcanic Landforms 

(sketch from 

Golombek et al., 

1983). 

38° 

36° 

34°

200 j km

0

dence for faulting of the sediments. They 

observed, however, that the sediments had a 

comparatively higher moisture content on the 

south side of the lineament and suggested that 

the lineament expresses a buried fault that 

controls ground water movement beneath the 

playa. Previous mapping has indicated the 

presence of a buried fault in the locality that 

coincides with the airborne radar lineament 

across the playa (Kupfer and Bassett, 1962). 

The presence of the lineament on the 2.4-

cm-wavelength image (Figure 53(a)) may denote 

a higher sensitivity of the shorter-wavelength 

radar in discriminating subsurface moisture 

conditions (hence dielectric constant). However, 

the two images in Figure 53 were obtained over 

an interval of 3 years, and the specific moisture 

content of the playa deposits at the instant of 

image acquisition in each case is not known. 

Consequently, any attempt to compare relative 

moisture sensing capabilities using these data 

will remain inconclusive. 

Continental interiors 

On the basis of plate tectonic theory, major 

deformation of Earth's crust takes place at active 

plate boundaries. However, crustal deformation 

has also occurred well into the interior of 

continental plates. Such deformation takes a 

variety of forms including continental rifting and 

associated volcanic eruption, magma inflation, 

plutonic intrusion, and widespread crustal 

upwarping or downwarping called epeirogeny. 

Each of these types of tectonic deformation is 

characterized by distinctive regional and local 

landforms. 

Continental rifts. Rifts occur where the 

entire thickness of the lithosphere has ruptured 

under extension (Burke, 1980). Rifts are 

common in the oceanic crust. To a lesser extent, 

they occur in the continental crust, but only there 

can they be observed by radar imaging techniques. Active continental rifts take the form of 

long narrow troughs of regional extent. Such 

troughs are bounded by normal faults and are 

commonly associated with volcanic constructs. 

Linear fault-bounded valleys are characteristic 

landforms in zones of rifting. As an example, 

the Rio Grande Rift is a zone of active extension 

about 850 km long that stretches from central 

Colorado to southern New Mexico and widens 

from about 10 km in the north to over 150 km in 

the south. It is a major structural feature that

85
Valles Caldera. These lineaments provided the 

structural control for emplacement of the Jemez 

magma system. 

Continental shields. Earth's major continental plates each display one or more shields 

that consist of extensive, ancient, crystalline 

rocks. Such shields form continental nuclei to 

which other slabs of continental crust have 

accreted episodically. Though shield areas vary 

in their geologic details, the rocks in all shield 

areas are very ancient (synonymous with 

Precambrian), deeply beveled by erosion, and 

widely separated in space and time from those in 

present-day mountain chains. Large areas of the 

ancient continental shields, known generally as 

basement, are overlain unconformably by 

sequences of younger rocks that reveal numerous 

episodes of epeirogenic uplift or subsidence, 

accompanied by corresponding episodes of 

erosion or sedimentation, respectively. Structural deformation and plutonic intrusion provide 

further evidence of intracontinental tectonism. 

The morphotectonic relations displayed by rock 

bodies in such continental interior areas serve in 

part to allow investigators to infer the geologic 

history. In this tectonic setting, however, crustal 

rocks are neither as thick nor as complexly deformed as they are in continental collision zones. 

The SIR-A image in Figure 55 covers an 

area of basement terrane in the Guiana Shield of 

northern South America. The scene is located in 

Amazonas, Venezuela. Structures are indicated 

by the erosional characteristics of the land, the 

drainage, and the outcrop patterns. In this 

context, four radar map units are evident in the

In areas of prominent upland (map unit 2), 

the basement is overlain by younger rocks that 

have a thick tabular form. These rocks are 

probable equivalents of arenites in the Roraima 

Formation. The erosional remnants of the 

Roraima Plateau appear in the image as small 

mesas (1321134 to A6 and E6) and as residual 

inselbergs distributed throughout the lower third 

of the image. Joints and fracture traces in the 

basement are pronounced in the vicinity of the 

inselbergs, and locally they govern the erosional 

outlines of the mesas. The mesa scarps and the 

slopes of the inselbergs show very bright radar 

outlines where they face toward the radar 

illumination and radar darkness where they face 

away. This indicates that the slopes exceed the 

complement of the SIR-A nominal incidence 

angle (40 deg). Resistant ridges in the area from 

Cl to D2 reveal the outlines of a doubly plunging synclinal structure. Ridge offsets along the 

structure denote normal faults that produced a 

horst across the southern end. 

Cross-cutting relations are evident from the 

circular to elliptical outlines of an intrusive rock 

body in the basement (map unit 3, B5 to CS). 

This feature has a diameter from 7 to 10 km and 

is of unknown composition (Martin-Kaye and 

Lawrence, 1983). Other intrusive features with a 

comparable size and outline that have been 

mapped in Brazil within 150 km of the circular 

feature (Projeto Radambrasil, 1975 and 1976) 

include bodies of basic igneous rock and one 

occurrence of carbonatite (Seis Lagos). 

Lowland areas that are topographically

featureless (map unit 4) include weakly resistant 

image. Because of continuous forest cover, these alluvial deposits or basins with poorly consoliunits do not provide any direct information about 

their lithologic identity. The four units mapped 

in Figure 55 consist of (1) topographically 

subdued basement, (2) prominent upland, (3) 

cross-cutting intrusive rock, and (4) topographically featureless lowland. 

Basement rocks crop out at low elevations 

(map unit 1). The erosional topography in 

Figure 55 shows conjugate sets of linear features 

(e.g., at A7 to A8 and 137). These features 

represent joints, fractures, or faults whose 

orientation, spacing, and linear extent denote the 

structural trends in the basement. In this area of 

the Guiana Shield, the basement consists 

predominantly of granite, granite gneiss, and 

migmatite.

dated sediments overlying the basement (e.g., Al 

to El to E4). 

Canyons. Continental lowland areas that 

have been subjected to comparatively rapid 

epeirogenic uplift become incised by streams 

that cut canyons in the landscape as it rises. As 

stream downcutting tends to follow paths of 

structural and lithological weakness, a drainage 

network develops that outlines the local and 

regional patterns of fracturing and faulting. 

Canyons often expose layered rocks that have 

markedly differing resistance to erosion. Consequently, the profile across a canyon may alternate abruptly between precipitous to vertical 

slopes (held up by strong resistant rocks such as 

massive limestone or sandstone) and gentle 

slopes (underlain by weak, thin-bedded rocks 

86
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ORIGINAL PAGE

SLACK AND WHITE PHOTOGRAPH 

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2 Upland 

1 Basement 

Figure 55. SIR-A image of Guiana Shield in the vicinity of Rio Siapa, Amazonas, Venezuela, 

with simplified structural sketch map. Map units 1 through 4 are described in the text.

Fault 

Topographic 

Lineament 

Synclinal Fold

87
such as shale and siltstone). While the individual 

layered rock units exposed in a canyon wall 

rarely exceed 100 m in thickness, local relief of 

1000 m or more from the base to the rim of a 

canyon is not uncommon. 

The vertical succession of abruptly contrasted slopes that is characteristic of profiles 

across canyons produces strong differences in 

radar backscatter under certain circumstances 

(Elachi and Farr, 1980). The differences in 

backscatter vary with the incidence angle and the 

direction of illumination as illustrated here by 

images obtained with the Seasat SAR and SIR 

systems. 

A Seasat SAR image of a portion of the 

Grand Canyon, Arizona, is shown in Figure 56. 

In this locality, the canyon is about 16.2 km wide 

and 1.3 km deep. With the low nominal incidence angle of Seasat SAR (23 deg), foreslopes 

are compressed to thin lines of saturation. 

However, backslopes along the canyon wall 

show contrasting bands of bright and dark. 

These bands correspond to alternating gentle and 

steep slopes, respectively (Berlin et al., 1982). 

Each band denotes a sequence of layered rocks 

that has strongly contrasting erosional characteristics relative to those in the adjacent bands. The 

bands provide generalized surface-slope information, but they cannot be used to measure 

stratigraphic thickness or to identify lithology. 

Individual bands can be traced and matched in 

the image along segments of the canyon wall for 

about 20 kin (C2 to Dl) and across the canyon 

(133). Different levels of the plateau surface 

above the canyon are evident on each side of the 

Coconino Rim (AS to C2). 

Shuttle radar images of the Pakaraima 

Mountains, Guyana, South America, and the 

Central Kalimantan Plateau, Borneo, Australasia 

(Figure 57(a) and (b)) show the drainage of the 

Mazaruni and the Mahakam Rivers and their 

tributary canyons, respectively. In each of the 

two localities, relief in the canyons is upward of 

400 m, and dissection has exposed rock layers 

that have differing resistances to erosion. For 

purposes of comparison, image (a) was obtained 

at a nominal incidence angle of 50 deg by SIR-A 

and image (b) at a nominal incidence of 42 deg 

by SIR-B. With this moderate to high range of 

nominal incidence angles, foreslopes appear 

bright and backsiopes are partly in radar shadow, 

depending on their orientation relative to the 

illumination vector. Appropriately oriented

backslopes of canyons in the center of Figure 

57(b) show alternating light and dark bands 

along the canyon walls; these bands denote corresponding alternations of gentle and steep 

slopes (compare Figure 56). More extensive 

landform degradation is evidenced in Figure 

57(a), with three successive erosional levels of 

the plateau above the canyon at the bottom 

center of the figure. In both images, the canyons 

appear to be regularly spaced and to follow 

predominantly linear traces. Abrupt changes in 

the orientation of a canyon coincide with the 

dominant fracture pattern in each case. Abrupt 

bends in the major drainage channels indicate 

control of these features by the same sets of 

intersecting fracture patterns. 

Summary and Discussion 

Seasat SAR and Shuttle Imaging Radar have 

demonstrated the ability of spaceborne radar 

systems to obtain high-resolution image coverage of a wide range of landforms on Earth. From 

these images it is possible to interpret regional 

and local tectonic relations. With similar system 

capabilities, Magellan radar images are expected 

to provide evidence for tectonic or other origins 

of the landforms on Venus. 

Tectonic landforms occur notably in linear 

to curvilinear belts that stretch for hundreds of 

kilometers and include major crustal elements 

such as ridges and trenches in the oceanic crust 

and mountain chains and rifts in the continental 

crust. Examples of tectonic landforms in the 

Appalachian Mountain fold belt (Figures 37, 38, 

and 41), the North American—Pacific plate 

boundary transform (Figure 49), and the Rio 

Grande Rift (Figure 30(a), Volcanic Landforms) 

show that tectonically deformed features are 

predominantly linear in outline at both regional 

and local scales. 

The high sensitivity of imaging radars to 

slope at moderate to low incidence angles 

enhances the perception of linear topography on 

images. It reveals broad spatial patterns that are 

essential to structural and tectonic interpretation. 

As radar responses are strongly directional, the 

ability to discriminate linear features on images 

varies with the orientation of the features relative 

to the illumination vector. Landforms that 

appear prominent on images where they are 

transverse to the illumination (Figure 38) may be 

88
Figure 56. Seasat 

SAR image of Grand 

Canyon, Arizona, 

shows contrasted 

bands of bright and 

dark radar 

responses from 

backsiopes along 

the canyon walls. 

0 20 -J km Illumination 

A B 

Nt 

1

— 1⁄4

4 

J2

'-

:

.

WC 

ii '*-

4 

5

obscure to indistinguishable on images where 

they are parallel to the illumination (Figure 37). 

Variations in nominal incidence angle can make 

the difference between enhancement (Figure 57) 

or obliteration (Figure 56) of sloping surfaces on 

images. Linear fault traces should be examined

carefully because other linear features such as 

dipping strata, dikes, or linear jointing patterns 

may be expressed in a similar manner on images. 

In the absence of relief, nontopographic lineaments may appear in response to local contrasts 

of small-scale surface roughness or dielectric 

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

89
Figure 57. Canyons 

imaged by (a) SIR-A 

in the Pakaraima 

Mountains, Guyana, 

South America, and 

(b) SIR-B in the 

Central Kalimantan 

Plateau, Borneo, 

Australasia, appear 

regularly spaced and 

show preferred 

linear orientations 

that reflect regional 

conjugate fracture 

patterns in the 

underlying rocks.

ORIGINAL PAGE 

SLACK AND WHITE PHOTOGRAPH 

- - - - -- ---• 

-. -

N 1

. 

/

\I 

I k --• 

_

/ 

N / N Illumination 

0 40 I I km 

constant (Figures 52 and 53). The possibilities 

for alternative interpretations of image data can 

significantly influence the image analyst. Such 

possibilities are to be anticipated in the interpretation of Magellan radar images. 

Landform interpretation from radar images 

is governed at a local scale by the spatial 

resolution of the images and at a regional scale 

by the areal coverage available. Contrasts in 

feature perception at different simulated resolutions were demonstrated for accreted terranes in 

eastern interior Alaska (Figures 45 and 46). 

Magellan radar is expected to discriminate 

surface features at a scale of hundreds of meters. 

To obtain the desired regional coverage of 

Alaska (Figure 45) and Southern California 

(Figure 49), it was necessary to mosaic adjacent 

Seasat SAR image swaths. The Magellan 

coverage of Venus will be mosaicked in a similar 

fashion. This will allow the preparation of maps 

showing regional and local landform patterns 

and global tectonic networks, if they exist. 

However, as on Earth, it is not possible to erect 

an unambiguous model for the tectonic evolution 

of Venus based only on detection and interpretation of landforms at the surface. 

Magellan radar images are expected to

provide substantial advances in detecting and

interpreting the Venusian landscape. This will 

be accompanied by radar altimetry measurements of topographic relief. Details of the 

landforni patterns derived from these complementary data sets will provide a basis for 

modeling the tectonic evolution of Venus on a 

global scale. Age measurements and precise 

timing of tectonic events are essential elements 

of such modeling. Impact craters may be used in 

estimating an average surface age for Venus, 

though the validity of applying either terrestrial 

or lunar cratering rates to estimates of cratering 

rates on Venus is not known and may be 

questionable. 

On Earth, the precise timing needed to 

document plate tectonic events was determined 

from geophysical measurements, independent of 

surface imaging or altimetry data and unrelated 

to rates of cratering. Reversals of the magnetic 

field have provided a sequence of intervals, 

resulting in time markers that vary in duration 

from tens to hundreds of thousands of years. By 

comparison, impact cratering may yield a few 

intervals and time markers on the order of 

hundreds of millions of years duration. Thus, for 

Venus, we probably should not expect to develop 

a definitive model of tectonic evolution from the 

analysis of images and altimetry data alone. 

90
Image Anomalies and System Artifacts 

a S S S S S S S S I a • a a a a a S S a a 

Introduction 

In this chapter, characteristics of SAR 

images that distinguish them from images taken 

with other sensors are presented first. Analysts 

trained in conventional photointerpretation must 

be aware of them to avoid misinterpretation. 

Next, artifacts contributed to a SAR image by the 

limitations or incorrect operation of a sensor or 

processing system are considered. These 

artifacts may create image anomalies that can 

lead to misinterpretation of the data. Many of 

the examples given are images of cultural 

targets; this is due partly to an observational bias 

in the existing data caused by the prevalence of 

such targets on Earth and partly to the fact that 

the simple geometries of cultural targets make 

artifacts easier to see. Where possible, similar 

effects in images of natural targets are shown. 

Unique Characteristics of SAR Images 

The three basic characteristics of the 

returned radar echo measured by a typical SAR 

sensor are backscatter (usually represented as 

normalized radar backscattering cross section), 

time delay, and Doppler shift. The first of these 

characterizes the target radiometrically, while the 

latter two allow the returned radar signal to be 

positioned properly in an image. 

In the SAR processing system, a relationship 

is assumed between the Doppler-shift/time-delay 

coordinates and the along-track/cross-track 

coordinates. The latter correspond to linear 

dimensions along the target, the so-called 

"ground range." The resulting displayed perspective is that of a photographic camera located in a 

plane roughly perpendicular to the flight track 

and containing both target and SAR, but in a 

direction 90 deg from the SAR as seen from the 

target. 

However, there are limits to this analogy. 

First, radar backscatter is controlled by properties of the target that are fundamentally different 

from those that control visible-light reflectance.

Second, some target characteristics can make the 

geometric conversion to ground range incorrect. 

The result is misplacement of the target's image 

when the conversion is made to ground range, 

which can create difficulties in interpretation. 

Radiometric effects 

The obvious radiometric difference between 

SAR images and most other images is the grainy 

texture known as speckle (Dainty, 1984). Most 

digital camera systems are designed to be radiometrically quantization-noise limited. This 

means that the quantization level in the analogto-digital conversion is set to encompass all or 

most of the analog noise. Therefore, the dominant noise in the resulting image is digital quantization, and any individual pixel represents a 

deterministic estimate of the reflectance of the 

target, limited in accuracy only by the digitization interval. On the other hand, speckle is generally the dominant noise in SAR images. Pixel 

intensities within such images represent only a 

statistical measure of the target backscatter. 

Speckle appears in any coherent imaging 

system when an extended (i.e., nonpoint) target 

is imaged. It is a result of the coherent addition 

at any given time of returns from many individual scatterers within a resolution element. The 

coherent sum of these returns is a function of the 

sensor position. If the sensor is moved a small 

distance, the composite amplitude will be an 

independent sample of the target backscatter. In 

principle, each of these independent samples 

gives additional data about the target, but their 

differences are generally regarded as an artifact 

or treated as a noise source. Speckle is Rayleigh-distributed when the receiver is linear (as 

for Magellan), and the distribution can be wide. 

Interpretability can often be increased by 

narrowing this distribution at the expense of 

spatial resolution. To this end, SAR images are 

usually presented as pixels made up of an 

incoherent sum of several of these independent 

samples, or "looks," of the target. A four-look 

image, for example, will have a 3-dB variation 

among pixels that observe the same target. This

91
noise can be reduced by incoherently averaging 

those pixels (i.e., increasing the number of 

looks), but if the target is not spatially homogeneous, the process of spatial averaging leads to a 

loss in resolution of whatever inhomogeneity 

exists. For example, averaging 10 resolution 

cells in a four-look image would reduce speckle 

noise to about 0.5 dB, but would reduce the 

resolution by an order of magnitude. Whether 

this loss is worth the corresponding reduction in 

speckle depends on the specific image and the 

intent of the investigation. 

To illustrate this tradeoff, Figure 58 shows a 

Seasat SAR image of the SP Lava Field, a 

basaltic flow area that is part of the San Francisco Volcanic Field in Arizona. The image 

includes SP Mountain, a 1.2-km-diameter cinder 

cone, at its center (marked as "1" on the sketch 

map). The SP flow (2) extends from it about 7 

km. Several smaller cinder cones lie to the east 

and south of SP Mountain, most of which are 

breached. In particular, cinder cone (3) is topped 

by a large crater, (5) contains a smaller round

crater, and (4), (6), and (7) have linear or arcuate 

vents at their peaks. The feature (8) has a 

roughly concentric ridge surrounding it to the 

north (Schaber et al., 1980). 

The image is displayed at one, two, four, 

eight, and sixteen looks in parts (a) through (e) 

of the figure, respectively. Note that the first 

visual effect produced by increasing the number 

of looks is a blurring of the speckle; central 

craters, especially those at (3), (4), and (7), are 

more distinct. As the number of looks further 

increases to eight, however, loss of resolution 

begins to dominate, and central craters are less 

well resolved. The vent at (7) disappears 

completely at greater than eight looks. In this 

example, the two- and four-look images seem to 

offer the best presentation of the targets, but the 

choice clearly depends on both the target and the 

purpose of the investigation. Ford (1982) 

conducted a survey of geologic analysts, the 

results of which showed that although the 

optimal number of looks depends on the target 

type and resolution, a majority of the analysts 

Figure 58. Seasat 

image of SP Lava 

Field, Arizona. 

Images (a), (b), (c), 

(d), and (e) have 1, 2, 

4, 8, and 16 looks 

with resolution of 86, 

120, 172, 240 and 

344 m, respectively. 

The sketch map (f) 

shows features 

described in the text. 

92

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ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH
preferred two-look images. Seasat SAR, SIR-A, 

and SIR-13 standard-product images were processed at four looks. 

An exceptionally strong return is generated 

by any target that directs radiation preferentially 

toward the sensor (Figure 59). Examples of such 

targets are (1) those whose orientation is such 

that they specularly reflect the radar signal back 

to its source, and (2) dihedral and trihedral

corner reflectors. Corner reflectors are formed 

by alignments of two or three mutually perpendicular reflective planes. They will return 

radiation preferentially in the direction from 

which it came over a broad range of incidence 

angles. Trihedral (three-plane) corner reflectors 

are often used terrestrially as fiducial marks 

because they are exceptionally bright in SAR 

images. Dihedral (two-plane) corner reflectors 

Figure 59. Seasat 

image of the Los 

Angeles Basin. The 

pronounced 

brightness in some 

areas is the result of 

the chance parallel 

alignment of streets 

and adjacent 

buildings with the 

flight path. 

IJ 

I 

r1⁄4

N 9

10 km / Illumination 

ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH
function in a similar way but over a narrower 

angular range. Strong radar returns are commonly generated by natural approximations to 

dihedral corner reflectors formed by two 

perpendicular planar (or near-planar) surfaces. 

These include segments of crater rims, scarp 

faces, embankments and river channels, and 

cultural structures such as buildings, fences, and 

roadbed berms. They must be oriented with both 

planes perpendicular to the plane of zero Doppler 

shift. The effect in the image is to emphasize 

such features; if the features are curved, those 

portions that lie along-track are emphasized. In 

the Seasat image (Figure 57), streets aligned 

parallel to the flight track form corner reflectors 

with the adjacent building fronts to produce a 

return that is much brighter than that from areas 

where this alignment does not occur. 

Conversely, care must be taken to anticipate 

possible causes of data saturation—a common 

artifact in most types of imaging systems. 

Natural targets have a wider variation in 

backscatter than the dynamic range of most SAR 

systems, at least when operating at a single gain 

state. Targets whose backscatter exceeds the

available dynamic range (either before or after 

compression) will not be faithfully represented in 

the resulting image. Slopes with preferential orientation and foreshortened mountains are 

examples of targets that may saturate the system 

and thus create images that are difficult to 

analyze. Crater rims, for example, would be 

expected to show a brightening at those parts of 

the rim normal to the illumination, as discussed 

above. The strength and form of this brightening 

can be used to infer details about the roughness 

or slope of the crater rim, if the brightening is not 

limited by saturation. In Figure 60(a), taken by 

SIR-13 of a crater on the Iraqi–Jordanian border, 

brightening on the far rim is evident despite the 

low signal-to-noise ratio of the image. The same 

crater as imaged by SIR-A, which had a significantly smaller dynamic range, is shown in Figure 

60(b). In the latter case, saturation has obscured 

the effect and a different conclusion might be 

reached about the nature of the rim if the artifact 

were not recognized. 

Geometric artifacts 

Geometric artifacts can be introduced by 

Figure 60. Images of 

a crater on the 

Iraqi–Jordanian 

border: (a) SIR-Bnote rim brightening 

in far range; (b) SIRA—data saturation 

masks the effect 

observed in (a).

Illumination 

N X --Illumination 

0 2 

I I 1 km

failure of the geometric conversion algorithm to 

account properly for target characteristics when 

converting the Doppler/range data into groundrange coordinates. The conversion from time 

delay into a cross-track ground coordinate 

assumes that targets farther away from the SAR 

antenna are also farther away from nadir. The 

assumption is generally true for larger look 

angles and moderate topography. Failure to 

meet this assumption can result in an effect 

known as layover or foldover, which occurs 

where foreslopes exceed the nominal (i.e., 

horizontal-terrain) incidence angle. 

The geometry for layover is illustrated by 

the sketches in Figure 61. Two positions of a 

transmitted wavefront are shown, with radii 

R 1 <R2 . Note that in Figure 61 (a) the point 

labeled (b) on the top of a mountain is farther 

both from the SAR nadir and from the SAR itself 

than point (a) at the base of the mountain. The 

simple mapping of time delay into ground range 

will properly place the mountain in the image. 

Note also that cameralike perspective is created 

as the distance between points (a) and (b) is foreshortened. In Figure 61(b), however, the 

mapping is not performed correctly because of 

the smaller look angle. Point (a) is now closer to 

the SAR than point (b). In the related ground94

ORIGINAL PAGE 

SLACK AND WHITE PHOTOGRAPH
Figure 61. 

Transformation from 

slant range to 

ground range: (a) 

the assumed 

transformation is 

accurate and 

features are properly 

placed; (b) the 

incidence angle is 

smaller, the 

transformation is 

inaccurate, and 

layover results. 

(a)

SAR

R1 R2

I iII'ar_;

SAR 

(b)

R1 R2 

\\ II \ II \

\

\ \\ \ \\ 

\\

C 

7 a 

range image, point (a) will be misplaced with 

respect to point (b), and the mountain slope will 

be displayed in reverse, relative to the illumination vector. Note also that points (a) and (c) will 

be overlaid in the image. Without additional 

data, this misplacement is not only irrecoverable, 

but not identifiable except from local knowledge. 

Clearly, layover is more likely to occur when 

using low-incidence angle SAR in rugged 

terrain. Figure 62 is a Seasat image of a section 

of the Alaska Range near Mount McKinley. 

This 23-deg-incidence-angle image shows a 

ridge and a peak laid over into the glaciercovered valley below. Only from knowledge of 

the topography can the misrepresentation shown 

here be corrected (Kwok et al., 1987).

Figure 63(a) is another example of pixel 

misplacement in range. It shows an image of 

Golden Gate Bridge in San Francisco, California, 

taken by the JPL airborne SAR aboard the 

NASA CV-990. The bridge lies almost parallel 

to the flight direction. Not one but three bridges 

can be seen in the image. These are not ambiguities (which will be discussed later), but three 

separate returns from the bridge: one direct, one 

bounced from the water surface, and one 

bounced from the water twice. Figure 63(b) 

illustrates the three return paths. Since the three 

time delays were different, the mapping into 

ground range placed the three returns in three 

different range positions. With Magellan, 

multiple bounces may occur from targets similar

95
ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH 

Figure 62. Seasat 

image of a portion of 

the Alaska Range 

showing layover of a 

ridge and peak 

adjacent to a glacierfilled valley.

FF 

N"

0 2 I I J km Illumination 

in morphology to the cultural target described 

here. Such targets will be misplaced in groundrange images. Although these effects can be 

removed by using dual antennas and interferometric techniques (Zebker and Goldstein, 1986), 

Magellan will not have such capabilities. 

The second geometric dimension measured 

by SAR is Doppler shift. The Doppler shift of 

the target is used together with target range delay 

to properly locate the target image. Again, a 

simple geometric relationship and constant 

relative motion are assumed. However, since 

both target and sensor have, in general, motions 

of their own, the transformation requires 

knowledge of their relative velocities. Imperfect 

knowledge of the relative velocity or nonuniform 

motion of the imaged field will result in misplacement of targets in the image. In Figure 64, 

which is an enlargement taken from another JPL 

aircraft SAR image, Golden Gate Bridge lies 

approximately parallel to the flight track along a 

line of constant azimuth. The bridge is correctly 

placed assuming that its Doppler shift is identical 

to that of the Earth. Cars on the bridge (indicated by arrows), however, are misplaced

because their velocities relative to the sensor are 

different from that of the bridge. 

SAR Processing Errors 

Ambiguities 

In the azimuth (along-track) direction, a 

target's return is sampled at the pulse-repetition 

frequency (PRF). Since the target return is not a 

band-limited signal, it is not possible to sufficiently sample it (and even if it were, it is not 

generally efficient to do so). The resulting signal 

thus may contain high-frequency data that 

masquerade as data at lower frequencies. Such 

data are called "aliased" and appear in the 

correlated image as ghosts of the target repeated 

at some distance in azimuth. These are referred 

to as "azimuth ambiguities." In normal processing, azimuth ambiguities are suppressed by 

limiting the processing bandwidth to an appropriate fraction of the PRF so that the signal-toambiguity ratio is maximized. In Magellan, for 

example, total ambiguities are specified to be at 

least 15 dB lower than the properly placed 

96
ORIGINAL PAGE

BLACK AND WHITE PHOTOGRAPH

Figure 63. Pixel 

misplacement in 

range: (a) JPL 

aircraft SAR image 

of Golden Gate 

Bridge, San 

Francisco (note the 

tripled image of the 

I)' ,• bridge); (b) the three 

possible paths for 

signal return 

(modified from 

Zebker and 

Goldstein, 1986.) 

"S 

N ' 'N I 1 km Illumination 

(b)

Direct Path

Intermediate Path 

Longest Path Golden Gate 

Bridge, End View 

Ocean

return. Therefore, they should not be observable 

unless the target is exceptionally bright against a 

low background or the processing bandwidth is 

improperly set. However, in such naturally 

repetitive scenes as mountain ranges or drainage 

systems, azimuth ambiguities can easily be 

misinterpreted. Figure 65 is a SIR-13 image 

taken southeast of Juneau, Alaska; improper 

processing has decreased the signal-to-ambiguity 

ratio in a mountainous area. Note that in the left

half of the image the ambiguities blend in with 

the rugged terrain and are difficult to distinguish 

from the natural scene, but in the right half they 

are obvious at the coastline. 

Range ambiguities result from echoes from 

the extreme far range that have a round-trip time 

long enough to place them in the receive window 

intended for the succeeding pulse (or, conversely, echoes from near range with a round-trip 

short enough to place them in the receive

97
Figure 64. JPL 

aircraft SAR image 

of Golden Gate 

Bridge, San 

Francisco. Traffic 

going across the 

bridge is 

mispositioned 

because of its 

movement with 

respect to the 

bridge.

ORC-N.L PAGE

SLACK AND WHITE PHOTOGRAPH 

N __________ km Illumination 

Figure 65. SIR-B 

image of a 

mountainous area 

near Juneau, Alaska, 

shows azimuth 

ambiguities because 

of improper 

processing.

N

0 20

I 1 1 km Illumination 

98
Figure 66. Seasat 

image of the 

Appalachian 

Mountains in 

Kentucky. A rangegate shift caused 

features in the image 

to double. 

. .. 

.. -- - 

window intended for the preceding pulse). 

These echoes are also incorrectly placed in the 

image. The effect in the image is similar to that 

shown in Figure 63, but the additional extra time 

delay is not due to a characteristic of the target. 

Sidelobes in the antenna gain pattern, incorrect 

swath illumination, uncompensated roll of the 

sensor, and terrain effects all increase the 

possibility of range ambiguities. 

Range-gate misalignment effects 

One principal advantage of SAR is that it 

provides its own illumination source. There is, 

however, the associated complexity of integrating target illumination and echo reception: the 

receiver must be timed to acquire the echo from 

the same surface area illuminated by the antenna 

radiation pattern; this period is the receiver's 

"range gate." Since the latter alignment is one of 

geometry and the former one of timing, the relationship between the two changes with sensor 

altitude. Often the range gate must be changed 

during data acquisition to accommodate altitude 

changes. Near periapsis, the Magellan radar will 

change range gates many times per second. In 

situations where the sensor altitude is imperfectly known, the alignment of the antenna pattern and range gate may be likewise imperfect. 

Two different kinds of artifacts can result

from this procedure. First, a fade to black at one

edge of the image can occur if the range gate and 

antenna pattern are not properly aligned, because 

the edge of the image will be determined by the 

edge of the range gate. Second, and much more 

likely to be misinterpreted, a repetition of data 

can result from a cross-track shift in groundrange coordinates; this shift occurs if the range 

gate is shifted and proper account is not taken of 

that fact when the data are correlated. Figure 66 

illustrates this latter effect in a Seasat image of 

the Appalachian Mountains in Kentucky. A 

range-gate shift occurred in this image, and both 

the unshifted and shifted data occupied the same 

synthetic aperture. The feature indicated by 

arrows is prominent, but all data in the same 

azimuth band on the image are repeated. 

Many of the artifacts illustrated in this 

chapter can be traced to characteristics unique to 

a particular system. The Magellan SAR has 

many unique design features made necessary by 

the elliptical orbit, the shared high-gain antenna, 

the limited data rate, and other aspects of the 

mission. Likewise, the Magellan data processing 

system has its unique aspects, such as multilook 

registration using spacecraft navigation and 

mosaicking. The SAR principles that underlie 

the system are familiar, but these new features 

will without doubt create their own brands of 

artifacts.

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References 

Anderson, C.A., 1941, "Volcanoes of the 

Medicine Lake Highland, California," Univ, 

Calif Pub. Geol. Science, V. 25, p. 344-422 

Arvidson, R.E., M. Schulte, R. Kwok, J. Curlander, C. Elachi, John P. Ford, and R.S. 

Saunders, 1988, "Construction and analysis 

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Venus," Icarus, v. 75, p. 163-181. 

Baloga, S., and D. Pieri, 1986, "Time-dependent 

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v. 288, p. 735-738. 

Zotkin, 1.1., and V.I. Tsvetkov, 1970, "Searches 

for meteorite craters on Earth," Astron. 

Vestnik., v. 4, p. 55-65. 

.ri
Glossary

a a a a a a a a a a a a a a a a a a I a a 

aa basaltic lava flow with rough jagged surfaces 

agglutinate welded pyroclastic fragments characterized by vitric bonding material 

alluvial fan fan-shaped deposit of loose rock debris formed where a stream drains from 

mountains onto lowlands 

altimetry measurement of altitude 

andesite fine-grained extrusive igneous rock composed mostly of plagioclase and 

one or more mafic minerals 

angle of repose slope at which unconsolidated material remains stable 

anticline folded rock structure having stratigraphically older rocks in the core and 

limbs that dip away from the fold axis 

antidip slope inclined across the strike of layered rocks in a direction opposite to 

the dip 

antiform folded rock structure where the relative age of rocks in the core is not 

known and the limbs dip away from the fold axis 

apoapsis the greatest distance between a planet and an orbiting object (e.g., spacecraft) 

arachnoids (Venus) spiderlike features 

arenite general term for sedimentary rocks composed of sand-sized fragments 

regardless of their mineral composition or origin 

azimuth (imaging) along-track direction of image acquisition 

azimuth resolution separation of points in the along-track direction 

backscatter portion of microwave energy returned to the radar antenna to create a radar 

image 

band interval in the electromagnetic spectrum whose boundaries are marked by a 

lower and an upper limiting wavelength or frequency. 

band range of wavelengths, cm frequencies, 0Hz 

L 30-15 1-2 

S 15-7.5 2-4 

C 7.5-3.75 4-8 

X 3.75-2.4 8-12 

Ka 1.18-0.75 26.5-40

115
basalt fine-grained extrusive igneous rock rich in mafic minerals 

block lava lava with angular blocklike surfaces 

bolide exploding meteorite 

breccia coarse-grained clastic rock composed of angular rock fragments 

butte conspicuous isolated flattop hill with steep slopes (see mesa) 

caldera large volcanic depression caused by collapse or explosion at the summit of a 

"("cnn,' 

cinder cone conical hill formed of volcanic cinders 

clastic refers to rock composed of fragments from the erosion of preexisting rocks 

composite volcano volcano composed of interbedded lava and pyroclastic material commonly 

with steep slopes 

dacite fine-grained extrusive igneous rock similar to andesite but having less 

plagioclase and more quartz 

ddcollement detachment structure associated with folding and overthrusting that results 

in independent styles of deformation above and below 

dielectric constant ratio of electric flux density to electric field 

dip angle that a structural surface makes with the horizontal, measured perpendicular to strike 

discontinuity physical interruption in the sequence or distribution of strata 

dome (volcanic) rounded formation of viscous lava extruded from a volcano 

Doppler shift change in the observed frequency of electromagnetic waves caused by 

relative motion between source and detector 

en echelon steplike arrangement of features 

endogenic refers to geologic processes originating within the Earth 

eolian related to wind deposits and associated effects 

fault fracture or zone of fractures in Earth's crust accompanied by displacement 

of the opposing sides. Faults are classified according to the direction of 

relative movement: 

(a) normal—where the hanging wall moved down relative to the footwall 

(b) reverse—where the hanging wall moved up relative to the footwall 

(c) thrust—low-angle reverse fault where the dip of the fault plane is 

mostly below 45 deg 

(d) strike-slip--where movement is parallel to the strike of the fault 

(e) transform—variety of strike-slip fault where displacement abruptly terminates and crustal structure changes

116
flatiron on remotely sensed images, a triangular pattern from which dip and strike of 

layered rocks may be inferred 

flood basalt extensive high-volume basaltic lava flow erupted from fissures 

fold product of the deformation of tabular rock bodies (e.g., anticline, antiform, 

homocline, and syncline) 

foldover (see layover) 

geometric rectification removal of geometric distortions from an image 

geosyncline downwarping of Earth's crust measured in lengths of hundreds of kilometers, where layered rocks have accumulated in thicknesses of tens of 

kilometers 

graben depressed elongate crustal block bounded by faults along its sides 

granite coarse-grained plutonic igneous rock composed mostly of quartz and feldspar 

granite gneiss coarse-grained banded metamorphic rock having the mineral composition of 

granite 

grus fragmented products of in-situ granular disintegration of granite and granitic 

rocks 

hawaiite type of olivine-rich basalt 

homocline general term for a series of strata that have the same dip 

horst uplifted elongate crustal block bounded by faults along its sides 

hot spot center of persistent volcanism, thought to be the surface expression of a 

rising hot plume in Earth's mantle 

ignimbrite igneous rock formed by widespread deposition and welding of ash flows 

inselberg prominent erosional hill or mountain rising abruptly above an extensive 

lowland surface 

lahar mudflow composed mostly of volcaniclastic debris on the flank of a 

volcano 

lapilli pyroclastic fragments with diameters between 2 and 64 mm 

lava general term for extrusive igneous material and rocks that form from it 

lava tube tunnel formed underneath the surface of a solidifying lava flow 

layover extreme case of foreshortening, where mountain tops are projected nearer 

the radar ground track than their bases (syn.,foldover)

117
lineament linear topographic feature that may depict crustal structure 

lithosphere rigid outer layer of Earth that includes the crust and part of the upper 

mantle, about 100 km thick 

loess deposit of wind-blown silt and clay 

looks number of independent radar observations summed for each resolution 

element 

mafic refers to rock or magma comparatively rich in magnesium and iron si1i't 

magma molten rock material, including gases and liquids 

mesa broad flattop erosional hill or mountain, commonly bounded by steep slopes 

(see butte) 

migmatite composite rock of igneous and metamorphic materials 

morphotectonic refers to a tectonic interpretation of Earth's topographic features 

orogeny process of mountain building 

pahoehoe basaltic lava flow with smooth undulating surfaces 

palagonite - - altered volcanic glass 

periapsis the least distance between a planet and an orbiting object (e.g., spacecraft) 

phenocryst large crystal in a porphyritic igneous rock 

pit crater (volcanic) small circular or elliptical collapse depression on or adjacent to a volcano 

pluton 

porphyritic 

phreatic 

pressure ridge (volcanic) 

pseudocrater

massive igneous intrusion 

refers to an igneous rock texture with two distinct sizes of crystals 

refers to a volcanic explosion of nonincandescent steam 

ridge formed by the uplift of a lava flow crust due to pressure of the flowing 

lava 

crater formed probably by a steam explosion as lava is emplaced over 

water-saturated sediments 

pyroclast fragmental material ejected by volcanic eruption 

pumice a light vesicular form of volcanic glass 

range (imaging) cross-track direction of image acquisition 

range resolution separation of points in the cross-track direction 

118
rheology study of the physical properties that govern the flow characteristics of solid 

material over time 

rhyodacite group of porphyritic extrusive igneous rocks intermediate in composition 

between dacite and rhyolite 

rhyolite fine-grained extrusive igneous rock, commonly with phenocrysts of quartz 

and feldspar in a glassy groundmass 

rms (root mean square) statistical measure of magnitude (e.g., small-scale roughness of a surface) 

sag pond small body of water in an enclosed depression where faulting has impounded the drainage 

scarp cliff or steep slope of some extent that may form a marked topographic 

boundary 

shatter cone striated conical fragment of rock with fractures 

shield volcano broad volcanic cone with gentle slopes constructed of successive nonviscous, mostly basaltic lava flows 

shutterridge ridge formed by displacement on a fault that transects ridge-and-valley 

topography 

silicic refers to rock or magma comparatively rich in aluminum and potassium 

silicates 

spatter cone low steep-sided cone built up from fluid pyroclasts coating the surface 

around a vent 

speckle noise scintillation of return from individual scattering elements caused by 

coherent illumination 

specular reflection mirrorlike reflections of a radar signal 

strike horizontal direction of a structural surface 

subduction process of one lithospheric plate descending beneath another 

syncline folded rock structure having stratigraphically younger rocks in the core and 

limbs that dip toward the fold axis 

synthetic-aperture radar side-looking imaging system that uses the Doppler effect to sharpen the

effective resolution in the cross-track direction 

tectonic refers to structural and deformational features in Earth's crust and to forces 

that produce such features 

tephra general term for pyroclasts 

terrain physical region or feature of Earth's surface

119
terrane region where a particular rock or rock group predominates 

tholeiite subalkaline type of basalt 

tuff general term for consolidated pyroclastic debris 

tumuli (plural of tumulus) domes or small mounds on the crust of a lava flow 

vent opening in Earth's crust through which volcanic material erupts 

volcanic rock igneous rock formed by eruption at P rth ' urfc 

120
Appendix A

I I I I I I I I I I I I I I I I I I I I I 

Orbital and Physical Properties of Venus and Eartha 

Property Venus Earth 

Mean distance from sun, AU 0.723 1 

Mean distance from sun, 106 km 108.2 149.6 

Orbital period, days 224.7 365.25 

Mean orbital velocity, km/s 35.05 29.79 

Eccentricity 0.0067 0.0167 

Inclination to ecliptic, deg 3.39 - 

Reciprocal mass 408,523.9 328,900.0 

Mass (Earth = 1) 0.815 1.0 

Mass, kg 4.871 x 1024 5.976 x 1024 

Equatorial radius, km 6051.31 6378 

Equatorial radius (Earth = 1) 0.949 1.0 

Ellipticity (oblateness) 0.0 0.0034 

Mean density, g/cm 3 5.24 5.52 

Equatorial surface gravity, m s 2 8.87 9.78 

Equatorial escape velocity, km s' 10.4 11.2 

Sidereal rotation period 243.01 days 23.9345 hours 

Obliquity, inclination of equator to orbit, deg -2.6 23.45

"Data from Beatty et al., 1981; and Hunt and Moore, 1981. 

'Revised research indicates that the mean radius value for Venus is 6051.92 (Ford, 1986).

121 
Appendix B

N . I N I N I I I I I I N I I I I N I I 

Magellan Standard Data Products 

AEDR Archive Engineering Data Record: all spacecraft engineering data, Deep Space Network monitor data, conversion tables, and decommutation maps. For archive only. 

ALT-EDR Altimeter Experiment Data Record: altimeter pulse echo data, radiometer data, and 

ancillary data. 

ATDFDR Archival Tracking Data File Data Record: raw Doppler count and residuals edited 

and in time-ordered sequence. For archive only. 

BADR Basic Altimeter Data Record: a compilation of the backscatter coefficient, range 

measurements to the planet surface, and terrain elevation measurements from each 

altimeter burst. 

BRDR Basic Radiometer Data Record: contains brightness temperature and effective 

blackbody source temperature information derived from the radiometer data samples 

contained in the radar data stream, along with ancillary, calibration, ephemeris, and 

spacecraft engineering data corresponding to radiometer data. 

C-BIDR Compressed Basic Image Data Record: all F-BIDR data compressed from 75-rn 

pixel spacing to 225-rn pixel spacing by taking the average of 3 x 3 F-BIDR pixel 

arrays. 

C1-MIDR Compressed-Once Mosaicked Image Data Record: unfiltered and seam-corrected 

rendition of C-BIDR data. Pixel spacing is 225 m. Seam registration error not to 

exceed 600 m. 

C1-MIDRP Compressed-Once Mosaicked Image Data Record Photoproduct. 

C2-MIDR Compressed-Twice Mosaicked Image Data Record: unfiltered and seam-corrected 

Cl-MLDR data that have been compressed by averaging 3 x 3 C1-MIDR pixel arrays 

into a single pixel. Pixel spacing is 675 m. Seam registration error not to exceed 

800m. 

C2-MIDRP Compressed-Twice Mosaicked Image Data Record Photoproduct. 

C3-MIDR Compressed-Thrice Mosaicked Image Data Record: unfiltered and seam-corrected 

C2-MTDR data compressed by averaging 3 x 3 C2-MIDR pixel arrays into a single 

pixel. Pixel spacing is 2025 m. Seam registration error not to exceed 1500 m. 

C3-MIDRP Compressed-Thrice Mosaicked Image Data Record Photoproduct. 

F-BIDR Full-Resolution Basic Image Data Record: processed SAR image swath containing 

75-rn pixel spacing with map projection dependent on latitude. Also includes 

ancillary data.

123 

PRECEDING PAGE BLANK NOT FILMED MGE,INTE.NTIOJIAbLY DLANK
F-MIDR Full-Resolution Mosaicked Image Data Record: mosaics of SAR image strips at full 

resolution, produced from F-BIDRs. Seam registration error shall not exceed 500 m. 

F-MIDRP Full-Resolution Mosaicked Image Data Record Photoproduct. 

F-PIDR Full-Resolution Polar Image Data Record: full-resolution SAR image strips from 

several orbits assembled side by side and not mosaicked. 

F-PIDRP Full-Resolution Polar Image Data Record Photoproduct. 

GADR Global Altimeter Data Rptnrd: bal map of topographic and backscatter information derived from altimetry data. 

GADRP Global Altimeter Data Record Photoproduct. 

GRDR Global Radiometer Data Record: BRDR data compiled into global maps of brightness temperatures with 10-km pixel spacing. A quarter of the planet per tape. 

GRDRP Global Radiometer Data Record Photoproduct. 

MCDR Media Calibration File Data Record: corrections for the effects of transmission 

media (tropospheric, ionospheric, and solar plasma) on the Doppler and very-longbaseline-interferometry (VLBI) data. 

MPL Maneuver Profile Listing: maneuver delta-Vs, timing, and associated information 

for midcourse and trim maneuvers. 

NCDR Navigation Constants Data Record: navigation constants file, station/quasar locations, and solar pressure parameters. 

ODFDR Orbit Data File Data Record: time-compressed Doppler and VLBI observations. 

ODR Original Data Record: contains all raw Magellan data transmitted to Earth. 

PADR Preprocssed Altimeter Data Record: produced from each ALT-EDR input; the 

input consists of pulse-compressed altimeter pulse echo. 

RDR Residuals without Spherical Harmonic Data Record. 

SAR-EDR SAR Experiment Data Record: a complete nonredundant set of radar, radiometry, 

and ancillary data. 

SFDFSL Small Forces Data File Summary Listing: impulses applied to the spacecraft during 

reaction wheel unloading. 

TPMFDR Timing and Polar Motion File Data Record. 

124
Appendix C

• a • IN IN a a a a a a IT a a a a a a a a a 

Magellan Special Data Productsa 

JET PROPULSION LABOPL4 TORY 

BOUGDR Bouguer Map Data Record: digital data necessary to contour a Bouguer map on a 

plotting system. 

BOUMLAP Bouguer Map: hardcopy of Bouguer map overlaid on Pioneer Venus Orbiter (PVO) 

topography. 

GEOIDR Geoid Map Data Record: digital data necessary to contour a geoid map on a plotting 

system. 

GEOIMAP Geoid Map: hardcopy of geoid map overlaid on PVO topography. 

GRAVMAP Gravity Map: hard copy of vertical gravity from spherical harmonic fits. 

ISOMAP Isostatic Map: hardcopy of variations in depth of compensation from an isostatic 

model. 

LOSAPDR Line-of-Sight Acceleration Data Record: digital data of line-of-sight acceleration on 

an orbit-by-orbit basis. 

P-MTDR Polar Mosaicked Image Data Record: a 16-frame data set that covers the area north 

of 80°N latitude. Mosaicking is by cut and paste, with no pixel averaging. Pixel 

spacing is 75 m at 80°N. 

P-MIDRP Polar Mosaicked Image Data Record Photomosaic. 

RSHDR Residuals with Spherical Harmonics Data Record. 

SHDR Spherical Harmonics Data Record: digital tabulation of the spherical harmonic 

coefficients estimated from the Doppler data. 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY 

ARCDR Altimeter/Radiometer Composite Data Record: altimeter and radiometer data in 

time sequence, orbit by orbit. 

GEDR Global Emissivity Data Record: resampled emissivity data from the ARCDR. 5-km

pixel spacing. Also, digital version of GEDRP on same medium. 

GEDRP Global Emissivity Data Record Photomosaic: cartographic reprojection of GEDR in 

Mercator and polar stereographic coordinate systems. 

a Listed by producing institution.

125
GREDR Global Reflectivity Experiment Data Record: resampled reflectivity data from the 

ARCDR. 5-km pixel spacing. Also, digital version of GREDRP on same medium. 

GREDRP Global Reflectivity Experiment Data Record Photomosaic: cartographic reprojection

of GREDR in Mercator and polar stereographic coordinate systems. 

GSDR Global Slope Data Record: resampled slope data from the ARCDR. 5-km pixel 

spacing. Also, digital version of GSDRP on same medium. 

GSDRP Global Slope Data Record Photomosaic: cartographic renrjticn cf CSDR ii - 

Mrcatcr and puiu' stereographic coordinate systems. 

GTDR Global Topographic Data Record: resampled topography data from the ARCDR. 

5-km pixel spacing. Also, digital version of GTDRP on same medium. 

GTDRP Global Topographic Data Record Photomosaic: cartographic reprojection of GTDR 

in Mercator and polar stereographic coordinate systems. 

STANFORD 

SCVDR Surface Characteristics Vector Data Record: sorted scattering function segments 

from altimetry and SAR analysis, electrical properties estimates from same, and 

radiometer and confidence estimates. Represents averages over surface elements of 

approximately 20 x 20 km. 

UNITED STATES GEOLOGICAL SURVEY 

CARTDR Cartographic Maps Data Record: digital record of CARTMAP. 

CARTMAP Cartographic Maps: 62 map sheets, scale 1:5,000,000. 

126
TECHNICAL REPORT STANDARD TITLE PAGE 

1. Report 2. Government Accession No. 3. Recipient's Catalog No. ii ication 89-41 

4. Title and Subtitle 5. Report Date 

Spaceborne Radar Observations: A Guide for December 1989 

Magellan Radar-Image Analysis 6. Performing Organization Code 

7. Author(s) J.P. Ford, R.G. Blom, J.A. Crisp, C. Elachi, 8. Performing Organization Report No. 

T.G.._Farr._R.S._ Saunders. _E.E._Theilig,_S.D._Wall

9. Performing Organization Name and Address 10. Work Unit No. 

JET PROPULSION LABORATORY

California Institute of Technology 11. Contract or Grant No. 

4800 Oak Grove Drive NAS7-918 

Pasadena, California 91109 13. Type of Report and Period Covered 

JPL Publication 12. Sponsoring Agency Name and Address 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Washington, D.C. 20546 02 

15. Supplementary Notes 

16. Abstract 

Geologic analyses of spaceborne radar images of Earth are reviewed and summarized 

with respect to detecting, mapping, and interpreting impact craters, volcanic 

landforms, eolian and subsurface features, and tectonic landforms. Interpretations are illustrated mostly with Seasat Synthetic-Aperture-Radar and shuttleimaging-radar images. Analogies are drawn for the potential interpretation of 

radar images of Venus, with emphasis on the effects of variation in Magellan look 

angle with Venusian latitude. In each landform category, differences in feature 

perception and interpretive capability are related to variations in imaging 

geometry, spatial resolution, and wavelength of the imaging radar systems. 

Impact craters and other radially symmetrical features may show apparent bilateral symmetry parallel to the illumination vector at low look angles. The 

styles of eruption and the emplacement of major and minor volcanic constructs can 

be interpreted from morphologic features observed in images. Radar responses 

that are governed by small-scale surface toughness may serve to distinguish flow 

types, but do no provide unambiguous information.. Imaging of sand dunes is 

rigorously constrained by specific angular relations between the illumination 

vector and the orientation and angle of repose of the dune faces, but is independent of radar wavelength. With a single look angle, conditions that enable 

shallow subsurface imaging to occur do not provide the information necessary to 

determine whether the radar has recorded surface or subsurface features. The 

topographic linearity of many tectonic landforms is enchanced on images as 

regional and local scales, but the detection of strutural detail is a strong 

17. Key Work (Selected by Author(s)) 18. Distribution Statement 

Geosciences and Oceanography (General) 

Geology and Mineralogy Unclassified - Unlimited 

Geophysics 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 126

JPL 0184 R 9183 
NASA 

National Aeronautics and 

Space Administration 

Jet Propulsion Laboratory 

California Institute of Technology 

Pasadena, California 

JPL Publication 89-41 12/89