FEASIBILITY STUDY ON THE USE OF A

MICROWAVE SYSTEM FOR THE NONDESTRUCTIVE

EVALUATION OF HISTORIC ADOBE STRUCTURES

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

• • •

. . .

• \

NATIONAL BUREAU

OF STANDABCS

LIBEART

OCT 5 '981

NBSIR 79-1610

FEASIBILITY STUDY ON THE USE OF A

MICROWAVE SYSTEM FOR THE NONDESTRUCTIVE

EVALUATION OF HISTORIC ADOBE STRUCTURES

D.R. Belsher

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

July 1979

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Table of Contents

Page

List of Figures iv

1. Introduction 1

2. FM-CW Radar Description 2

3. Measurement Approach 6

4. Experimental Results 7 4.1 Campo Santo Wall 7 4.2 West Sanctuary (Chancel) Wall 8 4.3 West Nave Wal 1 8 4.4 East Nave Wall 9 5. Conclusions 10

6. Acknowledgments 12 7. Bibliography 12

Abbreviations and Symbols

dBm Signal level with respect to 1 mW, decibels

E Electric field intensity, v/m

G s Average density of wall material

c Velocity of light in free space, m/sec

e Signal voltage

d-j Sample or layer thickness, m

f Frequency, Hz

i 0, 1, 2 for media 0, J, and 2 respectively

R Air path length, m

t time, sec.

r Reflection coefficient of a boundary

ct.j Attenuation constant, nepers/m

8-j Phase constant, radians/m

Yi Propagation constant

e Permittivity, farad/m, where e = e-j£ 0

e-j Permittivity relative to free space (dielectric constant), farad/m

y-j Permeability relative to free space, henry/m

a Conductivity, mho/m w Angular frequency, radians/sec

7j Intrinsic impedance, ohm

The subscript i is used in this paper to designate a special material to which the parameter

applies, i = 0 is reserved for the free-space value of the parameter.

i ii
List of Figures

Page

Figure 1. East interior nave wall showing rocks at the base 14

Figure 2. View of the church of San Jose de Tumacacori looking north. Photograph

courtesty of the National Park Service, Department of the Interior 15

Figure 3. Interior view of west sanctuary and nave walls showing the general

measurement locations by measurement number. Figure courtesy of the

National Park Service, Department of the Interior 16

Figure 4. Interior view of the east sanctuary and nave walls showing the general

measurement locations by measurement number. Figure courtesy of the

National Park Service, Department of the Interior 17

Figure 5. Block diagram of FM-CW electromagnetic system 18

Figure 6. Campo santo wall site with test equipment 19

Figure 7. The FM-CW response of a solid area of the campo santo wall compared to the

response obtained opposite a niche shown with the corresponding horizontal

wall section 20

Figure 8. Response from upper west sanctuary wall 21

Figure 9. West sanctuary wall positions of measurement 2 and series 3 22

Figure 10. Northwest nave wall measurement positions of measurement series 4 and series

5. See figure 3 for the general location of these measurements 22

Figure 11. Response at 1.17 m (46 in.) down from bead in measurement series 4 23

Figure 12. West nave wall position of measurement series 6. See figure 3 for the

general location of measurement series 6 24

Figure 13. Response at top step in measurement series 7 25

Figure 14. East nave wall position of measurement 8 and series 7. See figure 4 for

the general location of measurement 8 and measurement series 7 26

Figure 15. Cross section of the northeast nave wall at location 7. The amplitude of

response of the FM-CW system is shown as it corresponds to the position of

the antennas on the wall. As discussed in the text, the response amplitude

correlates to the estimated relative .near surface moisture 27

iv
Feasibility Study on the Use of a Microwave System

for the Nondestructive Evaluation of Historic Adobe Structures

D. R. Bel sher

A frequency-modulated continuous wave (FM-CW) radar system has been utilized

at Tumacacori National Monument to provide usable on site information and to evaluate its potential for nondestructi vely measuring certain parameters

associated with the soundness of historic adobe walls. The parameters of interest

considered were layer thickness, presence, position, and thickness of voids or other inhomogeneities, and moisture content and its distribution. The results were generally favorable. The results indicate that an FM-CW system can nondestructi vely provider major portion of the information needed to evaluate the

soundness of adobe structures at a relatively low cost and in less time than

present techniques. With some further work the FM-CW system can be developed into

a useful archeological exploration and evaluation tool that should operate in rock

and soils other than adobe.

Key Words: Adobe building materials; adobe soil; dielectric constant

measurements; electromagnetic moisture measurements; nondestructive

evaluation; radar soil measurements; soil moisture; underground radar;

void detection.

1. Introduction

This work was carried out by the National Bureau of Standards (NBS) under a contract

with the National Park Service, Department of the Interior (PX 8100 9 0003).

The preservation and restoration of historic adobe structures have become important in

many parts of the world [1,2,3]. The preservation process for adobe structures is largely

one of preventing deleterious effects of water, and the restoration process is often one of

repairing water damage. For the safe and effective preservation of historic adobe

structures, it quickly becomes necessary to evaluate the present condition of the

structure.

To evaluate the existing condition of an adobe wall in a structure, it is desirable to know several things. Among these are the homogeneity, the extent and location of high

moisture areas, and the thickness of plaster or other wall patching and covering materials.

An adobe wall may easily suffer voids and cracks due to water erosion that may not be

fully visible at the surface. Voids can be generated by water entering the wall in an upper

portion and following a porous path or a crack generated by the initial shrinkage of the

adobe bricks or mortar and washing the adobe to a lower region of the wall. In the lower

part of the wall, the adobe may merely be compacted by the settling of the washed material

or the washed material may exit through a relatively small hole. Thus some voids or cracks

may be quite hard to locate visually. The general homogeneity of an adobe wall can usually

be assessed only by drilling holes at the present time. It is not desirable to drill any more holes than is absolutely necessary in an historic structure. In addition, those holes

that are drilled may not detect an imperfection in the wall. A drill may easily miss a small but substantial crack lying in a plane approximately paralleling the hole. A thin

porous area or a thin crack approximately perpendicular to the drilled hole may go unnoticed

as drilling proceeds since only slight changes in resistance to the drilling may be evident.

Because high moisture areas of a wall can easily settle or fail from loss of strength,

it is important to locate them and prevent moisture from entering [4,5]. Moisture can easily enter a wall from the ground by capillary action or from any leak in the roof
drainage system. It is not uncommon in some areas for adobe walls to be given a base of

rock to inhibit the entrance of moisture by capillary action from the ground (fig. 1). At

Tumacacori this rock barrier was sometimes only on the outer edges of the wall. Perhaps

this was for splash protection of the wall.

An adobe wall may be covered or repaired with a mud, a lime and sand, or a cement

plaster. Additionally, repairs may be made with concrete. Problems can arise from two sources when an adobe wall is covered with one of these materials. First, the covering

material may be relatively impermeable and is then a barrier to the movement of moisture out

of the wall. A previously stable wall area with low moisture content can become very wet with trapped moisture and subject to failure. Second, a layer of the covering material may

have a gap in places where it was attached to the wall. In this case, failure of the

covering material may be imminent but may not always be visually obvious. Also to facilitate maintenance or repair work it would be useful to know the thickness of these

coverings and how the thickness varies over the wall.

A major goal of this work was to provide data that could be used during the

preservation effort as well as for evaluation of the potential of an electromagnetic

approach to obtain, nondestructi vely, information about the homogeneity, wet areas, and

layer thickness of adobe walls.

For the field work a frequency-modulated continuous wave (FM-CW) radar was used in an attempt to obtain this information. Although other systems are available, this system was

felt to have the most promise for field work of this type [6]. This type of electromagnetic

system has been used mainly for measurement of aircraft altitude, and snow, coal and

rock layer thickness.

Field work for this study was done at Tumacacori National Monument at Tumacacori, Santa

Cruz County, Arizona. Data were taken using walls of the campo santo (cemetery), sanctuary

(alter room), and nave (main assembly room) (see figs. 2, 3, and 4).

2. FM-CW Radar Description

The following equations outline the properties of a complex medium [7]. In general, waves propagating in a medium undergo attenuation and phase shift. For plane waves, the

complex propagation constant in terms of the basic electrical parameters of a medium is:

and can be written as

Y

= a(uj) + jB(io) ,

where a is the attenuation constant and 6 is the phase constant,

gives

( 2 )

Solving for a(u) and B(w)

( 3 )

2
B (oj) = 1/2

(4)

which for small losses (a/uie << 1) become, using a binomial expansion approach,

aM

tvr

The intrinsic impedance of a medium is given by

(5)

( 6 )

(7)

Initially, the adobe medium is assumed to be isotropic and homogeneous with parallel plane

boundaries (see fig. 5). Electrical properties of any material can be described by

permeability (p), permittivity (e), and conductivity (a). The adobe is assumed to be

nonmagnetic (p = p0 , where p 0= 4tt x 10"7 H/m). Additionally, e is assumed to be

independent of frequency within the transmitted frequency band. Ray paths for

electromagnetic energy are also assumed.

Due to the directivity of the ideal horns assumed in the model, only reflected waves

are received. In practice, some control over the placement and polarization strategy are required to reduce the direct coupling to an acceptable level. The horns used were not ideal; however, a single-ray, normal-incidence model was used and provided useful

resul ts.

The reflection coefficient for any planar boundary can be written as

for the case where the electric field is normal to the plane of incidence [8].

For the special case where the first medium is air, ai = 0, where normal incidence

is assumed such that 9

i - 9? ~ 0, and where for the second medium,

o^/u)^e<<! and p^ cs 1, eq (8) becomes

3
In operation a microwave signal generator (see fig. 5) is swept in frequency over its

bandwidth so that an output signal is produced that has a linear frequency change with

respect to time. The microwave signal travels from the generator to the mixer by more than

one path. If the electrical lengths of the paths were identical, the reference and test

signals going into the mixer would arrive at the same time and the instantaneous rf frequencies would be identical at all times. But the signal that travels through the

antennas arrives at a later time because that signal goes through the antennas and a path

which may include the wall. A portion of the test signal is reflected at the near-side

interface, and another portion is reflected at the far-side interface. These reflected

signals arrive at the mixer at times t

2 and tj, respectively, as shown above the mixer

in figure 5.

Because all the reflected signals that are input to the mixer arrive at different

times, the instantaneous microwave frequencies differ. The mixer is a product demodulator

and has an output that is a function of the product of the inputs. Only the lowest

difference frequency components in the output are preserved; the higher frequencies are filtered out.

The lower frequency components are displayed on a spectrum analyzer. The location of

the first peak is a function of the distance from the antennas to the near-side surface.

The location of the next peak is a function of that distance plus the wall thickness and its

dielectric constant.

Assume for the moment that the microwave signal is incident upon an infinitely thick

layer of adobe. Let the reference signal arriving at the mixer be represented as

e = E cos w(t)t, (10)

and the test signal reflected from the near-side surface be represented as

S

2 = ri E 2 C ° S [

U(t) U + ’ (11)

where 2R/c is the time required for the signal to travel a distance R from the

transmitting antenna to the near-side surface and back to the receiving antenna. It is

assumed that both antennas are equidistant from the wall surface and that the angle of

incidence is zero degrees.

Going through the product demodulation process and neglecting the harmonic microwave

frequency component, the mixer output signal is,

r

i E

! E2 . . 2R

e - — cos w(t) — , o 2 c

where R is the distance from the antennas to the wall. Letting

0 ,

and

, = u(t) ^

= 2irf(t) ~

(t) c c

d0 _doj 2R

( df(t) \ 2R

dt dt c \ dt / c

( 12 )

(13)

4

(13a)
the frequency of the signal out of the mixer is

- 1 d0(t)

f ol

= to dt

5 ° r (14)

f

ol

2R df ( t)

c dt

(15)

If the frequency of the microwave signal is changed at a constant rate, the frequency

of the signal out of the mixer is proportional to the distance from the antennas to the

near-side wall surface.

Extending the analysis to include a finite wall thickness bounded by the near-side wall

surface and a far-side wall surface, the total received test signal is [9]

r + r e

1 2

-2yd

l + r r e

1 2

-2yd

9 D -jw(t) (t +

e f-) c (16)

Recognizing that adobe is generally a lossy microwave material and utilizing typical

relative dielectric constants of soil or adobe and air, the amplitude of the denominator in

eq (16) is found to be approximately unity. Thus, neglecting the multiple reflection terms

contributed by the higher order reflection term in the denominator, eq (16) can be written

as

e ~ T E cos

r 12

’ / 2R \" " (t)

(

t + r) + r

2

E 3 cos

. . . 2R 2d/F

w(t) t + — +

c c 2ad

. (17)

Mixing this received signal with eq (10) will provide a mixer output signal

e -

o

r

! E

l E 2 COS

r , % 2Ri

F

2

E

1 E 3

_

w(t) —

c 2 cos w(t)

-2ad

e (13)

when the harmonic microwave signal is neglected.

The frequency of the first component is given by eq (15) and the frequency of the

second component is given by

f

02

df (t)

c / dt

' (19)

The difference between eqs (19) and (15) is a frequency proportional to the thickness

and the dielectric constant of the wall.

o c dt

(20)

The procedure is to measure the dielectric constant, e-j , on site and reduce the

working equation to one unknown, d, the layer thickness or range.

5
d 2/FT df(t)/dt

AV ( 21 )

The analysis given here for one layer may be extended for multiple layers [10,11].

3. Measurement Approach

Equation (20) of the previous section is a basic working relation for the FM-CW

system.

The dielectric constant, e-j, is a function of frequency for most materials. However,

to use eq (20) with the FM-CW system, a constant value of the dielectric constant is assumed. For adobe walls, this value is usually and most easily measured by using eq (20).

The output frequency difference between the responses of the front surface and the rear surface of a known thickness of adobe wall is measured, and this value is used to provide

a value of dielectric constant, e-j, for subsequent measurements. Usually the change in dielectric constant is small over the transmitted frequency range, and the resulting error

in range, d, due to this source is negligible.

The dielectric constant can also be affected by changes in moisture content or density

with respect to the wall area initially used to determine e-j [12]. This means a measurement of the change in moisture content of a wall can be made as the surface is

scanned. The dielectric constant can also be measured by using eq (9) and measuring a relative amplitude of reflection of the wall compared to a metal plate placed on the front

surface of the wall. There are several common sources of error in this technique, but it is satisfactory for initial rough estimates of e-j.

The effect on e-j of changes in wall density may cause errors in range, d, if the

change is very gradual. These errors can usually be detected and allowed for. If the

density change is not very gradual or is random in a wall volume, scattering of the

transmitted wave will result. The detection of voids or soft spots in the walls is

dependent upon this.

To gain an idea of the sensitivity of the FM-CW system, note that eq-.( 9 ) gives a reflection coefficient T = 0.000624, which corresponds to a return signal voltage loss of

64 dB, given a medium with a dielectric constant of 4.00 containing a boundary with a dielectric constant of 4.01. This should be within the dynamic range of an FM-CW system as used in this work. This change in dielectric constant corresponds to a density change in adobe of approximately 0.2 percent or a moisture change of about 0.1 percent [12,13].

The attenuation of an electromagnetic wave propagating through a material is described

by eq (3). For adobe it can be assumed generally that the magnetic loss associated with y

will be negligible. Thus the observed loss will be due to the conductivity, a, and the

dielectric loss. It can be seen in eq (3) that the attenuation is frequency dependent and

that by lowering the frequency of operation the loss of transmitted signal will be less for

a given situation. It is important to choose a frequency band which is low enough to provide a reasonable amplitude of transmitted signal so that any reflections will be large

enough to be detected.

The resolution is basically limited by the wavelength in the material being

investigated. If one assumes an ideal FM-CW system and uses the Rayleigh criteria for

resolution of the waveform on the spectrum analyzer, then the limiting size of a layer that

is completely resolvable is [14]

Ad =

c

( 22 )

6
where e-j is the dielectric constant of the layer and Ad is the thickness of the layer.

In practice, this limit is seldom achieved since the system is not ideal. It should be

noted that a layer may be easily detected, but its thickness may not be resolvable. To gain

resolution capability, the average frequency in the swept frequency band must be

increased; at the same time, there is a competing requirement to lower the average

frequency of the swept frequency band to increase range. It becomes necessary to make the

best choice of these system parameters for an FM-CW system to be effective in a given

situation.

4. Experimental Results

4.1 Campo Santo Wall

The west wall of the campo santo (cemetery) was chosen for the beginning work since it

appeared visually homogeneous and moderately dry and access to both sides was relatively

convenient (see fig. 6). This wall is located north and west of the church.

Initially, an FM-CW system operating at 2 to 4 gigahertz was used, but the excessive

scattering due to the rough brick surfaces appeared to prevent a good response. A dielectric constant of e = 3.63 and an approximate loss of 19.2 dB per meter were measured.

At this time it was felt that a better response would be obtained from a system configured

to operate at 1 to 2 GHz. This was tried, and the response was improved. Initial

measurements gave a dielectric constant of e = 3.61 and a loss of approximately 19.3 dB per

meter. Oue to the better response of the system using this frequency, the latter values were considered to be more accurate. Variations in dielectric constant measured along

3 meters of wall went from a low of 3.29 to a high of 4.10. Thus, if a central fixed value

is assumed, a variation in range can be expected of approximately +6 percent due to this source. For the purpose of locating voids or cracks, this is an acceptable error.

A typical set of FM-CW responses along with a section of the campo santo wall is shown

in figure 7. One curve shows the response directly over a niche approximately 29.8 cm deep

on the back side of the campo santo wall. The second curve shows the response with the

antennas moved 45 cm to the left where the wall is 58.2 cm thick. The peaks on the solid curve correspond to the front and back surface of the wall. The peaks on the dashed curve correspond to the front surface of the wall and the inner surface of the niche. The

response shown in figure 7 shows a peak near the center that appears to result from a slight

change in material parameters near the center of the wall. This appears to correspond to the mud mortar joint between bricks.

The resolution capability of an FM-CW system operating over the frequency range of 1 to

2 GHz is given by eq (22) as 15 cm for an air-filled layer. With this system operating from

1 to 2 GHz, a void must be approximately 15 cm in thickness before the thickness can be

resolved accurately from the return signal. However, as shown in figure 7, detection (but not resolution) of a very thin void or boundary can easily be accomplished. To be

detectable, a crack or void must have a component that lies at least partially in a plane

approximately parallel to the wall surface. However, the voids lying perpendicular to the

wall surface are much more likely to be seen by visual inspection. Thus the FM-CW system

has a definite value in checking the homogeneity of adobe walls.

7
4.2 West Sanctuary (Chancel) Wall

When plaster or concrete overlay the adobe wall surface, it was not possible to resolve

adequately the plaster thickness due to the frequency limitations of the available

equipment. If the water content of the plaster or concrete is low, going to a larger

bandwidth will probably give some usable results. In any case, it will be quite difficult

to measure accurately a void thickness just behind the plaster. Although with an optimized

system, the fact that a void exists may be noted. Since water has a loss of about 90 dB per

meter at 1 GHz, the loss due to the moisture content of the wall material will quickly

determine the usable frequency range and the resultant resolution possible from a reasonable

system. Measurements were attempted in frequency ranges of 1 to 2 GHz, 2 to 4 GHz, and 8 to

12 GHz. Both the plaster and adobe had too great a loss for the 8 to 12 GHz system to give

usable results. The 2 to 4 GHz system as configured with the available equipment lacked

some sensitivity, and the results were not as good as with the 1 to 2 GHz system. Figure 8

shows an incompletely resolved plaster layer on the upper west wall of the sanctuary taken

at position 2 (see figs. 3 and 4 for measurement location). Also shown in figure 8 are responses which appear to be mortar joints between brick courses and a return which

originates with the outer wall surface. Since no borehole data are available for this

location, it is assumed from the FM-CW returns that the wall is fairly solid. However, the

combination of small responses on either side of the first, large, mud mortar joint return

and the relatively large main return for this joint may indicate small cracks or soft spots

on the sides of the mortar joint. These data were taken using a 1 to 2 GHz frequency band

to get the best system results and reasonable penetration; thus, resolution is degraded

from what may be possible with a higher frequency band. Total wall thickness was measured

to be 0.95 m (37.5 inches) in this area.

Series 3 measurements were also run along the path shown in figure 9. These

measurements were made at closely spaced intervals. They show local variations but are generally very similar to the sample measurement shown in figure 8.

4.3 West Nave Wall Areas

Several series of measurements on the west nave walls were made in the areas whose

locations are shown in figure 3. Series 4 and 5 measurements were made as shown in

figure 10. A sample measurement from series 4 is shown in figure 11. This graph shows some

of the greatest inhomogeneities found in this area, but it is felt that the pertinent

response peaks are not great enough in amplitude or width to suggest a serious flaw in the

wal 1

.

The series 4 measurements were made at 5.1-cm (2.0-inch) intervals going down the wall

as shown in figure 10. A steadily decreasing, average dielectric constant was observed for

the first 20 measurements. The values ranged from 7.4 at the top to 5.7 toward the bottom.

In the absence of any observable changes in wall roughness, thickness, or density, it is assumed the moisture content is steadily decreasing over this vertical distance.

Measurements made at NBS with sandy loam indicate that this change in dielectric constant

corresponds roughly to a 3 percent change in moisture content [12,13]. This contrasts with

the horizontal series 5 measurements that show extremely constant average dielectric

constant values of about 6.7 for the complete distance.

In measurement series 5 there were some slight anomalous variations in response near the top of the window. These appeared to be associated with the upper wood supports for the

window and were not considered notable.

Figure 11 represents results using a spectrum analyzer bandwidth of 15 hertz and shows

the frequency spikes due to the repetition rate of the sweep frequency generator. Figure 7 represents results taken with a wider bandwidth on the spectrum analyzer and shows

8
considerable smoothing of these spikes and is perhaps easier to interpret visually. These

figures represent different areas and are mentioned only to compare the output format which

tends to be limited by the available equipment.

Also, series 6 measurements were made on the west nave wall as shown in figures 3 and

12. This was an area where a section of original plaster was pegged to tne adobe wall with

steel lag bolts to keep it from falling since the bond between the plaster and the adobe is failing. This series of measurements was directly opposite a vertical drainage canal on the

outer wall. This canal area is about 0.5 meters wide and has a metal mesh under cement

plaster which has been removed from adjacent wall areas. As a result of this, it would be

expected that the outer wall response would be slightly enhanced in amplitude but also

slightly spread out because responses from the canal area and the wall surface are at slightly different ranges.

The wall measured 1.7 meters (67 inches) thick, and the dielectric constant varied from

3.53 to 3.79 over this series of measurements. Comparing this to the other measurements

implied the wall was relatively and consistently dry here.

As can be seen from the response shown in figure 13, the wall is quite homogeneous in

this area. Also it can be seen that the plaster layer is not well resolved. Since the

resolution of the front and rear surfaces of the plaster is not good, the first surface

response contains the response of the back of the plaster as well as the response of the air

to the adobe boundary if a gap exists behind the plaster. Thus the first surface response

contains additional energy representing those areas where a gap exists, but the resultant

difference in amplitude could only be significant under conditions where the wall surface

topography and moisture are quite constant. Although a correlation exists between the first

surface amplitudes of this series of measurements and the visually estimated gap behind the

plaster, it is felt that it would be better to use an FM-CW system with antennas modified

for this job and to operate at a wider bandwidth and somewhat higher frequency. If one is

concerned about inhomogeneities in a wall of this thickness, it will be necessary to operate

at a lower frequency and suffer some loss of potential resolution to get improved signal

transmission to the far side of the wall and back.

The responses identified in figure 13 were originally correlated to the wall surfaces

using reflecting metal plates while making other graphs of this series.

4.4 East Nave Wall

Series 7 measurements were made as shown in figure 14. The measurements were made

15.24 cm (6 in) apart in a vertical line that crosses a boundary between a new adobe

replacement area and the original adobe. In figure 15 the relative amplitude of the surface

reflection response is plotted with respect to the wall cross section. This amplitude

corresponds roughly to the relative near-surface moisture content as taken from data

provided by the National Park Service for the area of the old bricks. It does appear that

this method will provide relative moisture values for the near surface. However, it is important to realize that the surface response amplitude is influenced by the surface

roughness and conductivity of the material. In this case the area of new brick is relatively smooth, making the response slightly higher in this region than it would

otherwise be. While the moisture content of the new natural soil adobe bricks was judged to

be lower than the old brick at the surface at the time of these measurements, previously

measured Park Service data showed them to be relatively wet. Just above the new brick the

old bricks are relatively rough; however, the response goes up and then falls in a space

of a few inches corresponding closely to the moisture content as taken from National Park

Service data for the remainder of the measurements.

9
By going to a lower frequency range, the effect of surface roughness can be minimized

so it is not a serious problem. These measurements were made using a frequency range of 1 to 2 GHz and could easily be made at lower frequencies where the variations in surface

height are very small compared to a wavelength. In all these measurements , the surface

roughness varied from quite smooth on some plaster surfaces to a value of about +2 cm for some of the roughest adobe walls.

The effect of conductivity (due to soluble salt content) can be appreciable, especially

at lower frequencies. It is felt that this problem can best be handled by making a series

of measurements in which the conductivities and the water contents are known to calibrate

the system. This was not done in this study due to a lack of time.

Additional measurements were made in this area at position 8 to evaluate alternate

antennas and to check on the average moisture content of the total thickness of wall. When

the effect of any soluble salt was ignored, these transmission measurements gave a dielectric constant value of 4.62 that corresponds to an average moisture content of about

5 percent for a sandy loam [10]. This corresponds well to the Park Service estimate of the

present value for this position. These last measurements were made using a metal plate to enhance the return signal from the outside wall. This extra effort could be avoided in most cases by operating at a lower frequency band. This method also requires a preliminary set of calibration data on a known thickness of wall before relative moisture measurements can

be made, but should provide good results for the total relative wall moisture. These

measurements can then be made when the search for voids is carried out.

5. Conclusions

A frequency modulated continuous wave (FM-CW) radar system has been utilized on site at Tumacacori National Monument to evaluate its potential for nondestructi vely measuring

certain parameters associated with the soundness of historic adobe walls. At the same time,

information was gathered to provide useful on site information. The parameters of interest

considered were layer thickness, presence, position and thickness of voids or other

inhomogeneities, and moisture content and its distribution. The results were generally

favorable. It appears that an FM-CW system can nondestructi vely provide a major portion of

the information needed to evaluate the soundness of adobe structures at a relatively low

cost and in less time than present techniques when done in conjunction with a visual check.

During the field work, the ability to detect voids was demonstrated initially on the

campo santo wall area. Here a niche approximately 30 cm by 40 cm by 20 cm deep was easily

detected, and in addition the mud mortar joint in the center of the wall was detected (see sec. 4.1 and fig. 7). Although some inhomogeneities were found in many other spots, none were judged to be serious voids requiring immediate attention. The most notable internal

inhomogeneity was found in the west sanctuary wall and is described in section 4.2 and shown

in figure 8. The potential sensitivity of the system is quite good. As noted in section 3,

the system can respond to a boundary difference in dielectric constant under reasonable

conditions of 0.01. This sensitivity is usually degraded by increasing range, increasing

electromagnetic loss (increasing water content), decreasing the size of the discontinuity as "seen" by the system, and increasing surface roughness of a boundary. These problems are

all somewhat relative, and altering system operating parameters can commonly reduce their

effects so that usable results are obtained. Based on these results, it is probable that an FM-CW system can be very useful for void detection.

The results of attempts to measure the thickness of wall covering materials (usually

plaster) indicate some immediate success and that improved results can be expected by

providing a system with greater bandwidth. As noted in section 3, given suitable operating

parameters, the limit for complete resolution of two close signal returns from both sides of

a layer can be described by eq (22). This relation shows that the resolution is essentially

10
a function of the bandwidth transmitted by the FM-CW system. Thus an improved system could

be assembled to operate from 1 to 4 GHz. (System components are commercially available to cover this range but were not available for this work.) This should greatly improve the

ability to measure the thickness of wall covering materials and to detect any voids behind

them. In any case, an initial indication of completeness of the bond of the covering

material to the adobe wall often may be obtained as described in section 4.3 and figure 13

and in section 4.2 and figure 8. This may be quite sufficient along with visual observation

to effectively evaluate the status of a wall covering bond.

One of the most desirable pieces of information about adobe walls is the amount and

distribution of any water contained in them. To obtain good values of absolute moisture in the general case will take an additional research effort, but the measurement of relative

values of moisture can be effectively accomplished. These relative values can be converted

to approximate absolute values' by calibrating the system for a known set of conditions.

That is, a preliminary set of measurements can be made on a given wall having known values

of surface roughness and texture, soluble salt content, and moisture. Then, given similar

walls, reasonable absolute values can be obtained.

For most work, relative values of moisture content will be sufficient and relatively

easy to obtain. The average dielectric constant measurements can be directly correlated

with the average moisture content if some reasonable assumptions are made [10]. The average

density must be assumed to be constant, and the concentration of soluble salts in the

moisture present must be assumed to be approximately constant. The dielectric constant can

be obtained from the FM-CW returns using eq (20) as described in section 3.

The series 4 and 5 measurements described in section 4.3 give a set of dielectric

constants that indicate the wall is wetter at the top and has a fairly constant moisture

content in a horizontal direction over the distance covered. A transmission measurement was made at location 8 as described in section 4.4 to get an average total moisture estimate.

Any sudden change in moisture content within the wall can be expected to give a reflection

and thus be detected, although no changes were found on this site.

The FM-CW system can also be used as described in section 4.4 to plot changes in near-surface moisture content of adobe surfaces as was done at location 7. This can be done

even though the range of the system might be insufficient to penetrate a wall or the back

surface might be inaccessible. This requires essentially no computation and can be read in

real time from the normal output of the system.

Another item which was considered was to use the FM-CW system to look for water table

height. This was not done due to limited time, increased rainfall just prior to this work,

and the belief that other methods might be more useful at this site. A stand pipe can be

installed at most sites to provide continuously at least one good measurement point of water

table position. Some work is being carried forward at NBS, Boulder, toward providing a high-frequency electromagnetic means of mapping water table positions as well as moisture

gradients in the ground.

The value of an FM-CW radar system for evaluating adobe walls seems established.

However, any further work to optimize the FM-CW system or to use it in the field would best serve all users if the data obtained could be tied closely to immediate and complete

measurements of the soil type, amount and type of salts, moisture content, and void size and

position. This information was not generally available for this work because of funding

limits and the historical nature of the walls but would contribute greatly to future work.

It would be desirable to pursue this work to develop accuracy limits for the moisture

measurements and attempt to put absolute moisture measurements on a well -documented basis.

Any future evaluation should also include the measurement of test walls carefully

characterized and especially constructed for the purpose. These should include various

11
types and shapes of test inhomogeneities that can be accurately described and directly

sampled.

Since layering, inhomogeneities, and voids are detectable in adobe walls, it seems reasonable that similar dectections can be made for adobe soils and other soils as well as rocks. As a consequence, it is felt that the FM-CW system should be compared to other

archeological survey systems, such as the magnetometer, which are presently in use. The

FM-CW system will respond to different parameters but can be expected to be very useful

either as the primary survey instrument or as a secondary instrument used to provide

additional information in selected areas. The FM-CW system can also be expected to provide

information in volcanic soils, which often inhibit the effective operation of the

magnetometer.

It is felt that the FM-CW system shows considerable promise as an archaeological

analysis tool. It may, with some additional effort, be developed as an exploration and

mapping tool to provide some unique capabilities.

6. Acknowledgments

The author wishes to thank Mr. Joseph Sewell and the staff at Tumacacori National

Monument in addition to Mr. Anthony Crosby of the U.S. Park Service for their cooperation

and assistance in this study. Also the extensive efforts, both in the field and in the

laboratory, of Mr. Robert H. McLaughlin are greatly appreciated. The consultation and

advice of Mr. Doyle A. Ellerbruch were especially helpful.

7. Bibliography

[1] Torraca, G., An International Project for the Study of Mud-Brick Preservation, ref. 7, pp. 47-57.

[2] Conservation of mud-brick monuments, Yazd, Iran, March 6-11, 1976, Information Sheet on Stone Conservation no. 5 (International Centre for Conservation, Rome, Italy).

[3] Steen, C., An archaeologist's summary of above, El Palacio, 77_, No. 4, 29-38 (1971).

[4] Webb, T. L., Cilliers, T. F., and Stutterheim, N., The properties of compacted soil and

soil-cement mixtures for use in building. National Building Research Institute Series

DR 2 (South African Council for Scientific and Industrial Research, 1950).

[5] Eyre, T. A., The physical properties of adobe used as a building material. The

University of New Mexico Bulletin, Engineering Series, 1, No. 3 (Albuquerque, New

Mexico, 1935)

.

[6] Luck, D. C., Frequency Modulated Radar (Mc-Graw Hill, 1949).

[7] Kraichman, M. B., Handbook of Electromagnetic Propagation in Conducting Media (U.S.

Government Printing Office, 1970).

[8] Ramo, S., Whinnery, J. R., Van Duzer, T., Fields and Waves in Communication Electronics

(John Wiley and Sons, 1965).

[9] David, Mansour, Brown, Marshall, P., Field Analysis and Electromagnetics (McGraw-Hill,

1963).

[10] Ellerbruch, D. A., Belsher, D. R., FM-CW technique of measuring coal layer thickness,

NBSIR 76-840 (May 1976).

12
[11] Wait, J. R., Electromagnetic Waves in Stratified Media (McMillan, 1962).

[12] Jesch, R. L., Dielectric measurements of five different soil textural types as functions of frequency and moisture content, NBSIR 78-896 (October 1978).

[13] Bttttcher, C. J. F., Theory of Electric Polarization (Elsevier, 1952).

[14] Royer, G. M., The dielectric properties of ice, snow and water at microwave frequencies

and the measurement of the thicknesses of ice and snow layers with radar,

Communications Research Center, Canada, Report No. 1242 (June 1973).

13
14

Figure

1.

East

interior

nave

wall

showing

rocks

at

the

base.
0

Figure 2. View of the church of San Jose de Tumacacori looking north.

of the National Park Service, Department of the Interior.

Photograph courtesy

15
o

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CD S- E 03 Q03 r—

S- 03

CD

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(S) CO

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if- <3 -C O CD 4-> E 5 '+- CD >> O

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16
17

Figure

4.

Interior

view

of

the

east

sanctuary

and

nave

walls

showing

the

general

measurement

locations

by

measurement

number.

Figure

courtesy

of

the

National

Park

Service,

Department

of

the

Interior.
Figure 5. Block diagram of FM-CW electromagnetic system.

18
19

Figure

6.

Camposanto

wall

site

with

test

equipment.
30 cm

response response

obtained obtained

here. opposite niche.

frequency (range)

Figure 7. The. FM-CW response of a solid area of the campo santo wall compared to the

response obtained opposite a niche shown with the corresponding horizontal wall section.

20
21

Figure

8-,

Response

frtm

upper

west

sanctuary

wall.
Pilaster

edge

©

Measurement

Seri es 3 o

Measurement

Position 2

1.13 m

(44.5 in)

©-

0. 36 m

(14.0 in ) 2^©-=*-

r

1.44m

(56.5 in )

West Sanctuary

window opening at

inside plaster surface

0. 025m (1.0 in )

1.52m (60.0 in )

Figure 9. West sanctuary wall positions of measurement 2 and series 3.

Base of plaster bead.

Figure 10. Northwest nave wall measurement positrons of measurement series 4 and series 5. See figure 3 for the general location of these measurements.

22
23

Figure

11.

Response

at

1.17 m

(46

in.)

down

from

bead

in

measurement

series

4.
4.85 m

(191. in)

I

2.72 m (107. in)

V V rl

EH

Floor

Figure 12. West nave wall position of measurement series 6. See figure 3 for the general

location of measurement series 6.

24
25

Figure

13.

Response

at

top

step

in

measurement

series
Northeast

nave

w indow.

Figure 14. East nave wall position of measurement 8 and series 7. See figure 4 for the

general location of measurement 8 and measurement series 7. 26
Antenna

Position

on

Wall

J

*

- I L L 15 10 5 0

Relative Amplitude of Response

(Correlates with near surface

moisture content of wall )

Crossection of

Northeast Nave Wal

I

Location 7

*—— Section of

New Adobes

Floor

Figure 15. Cross section of the northeast nave wall at location 7. The amplitude of

response of the FM-CW system is shown as it corresponds to the position of the

antennas on the wall. As discussed in the text, the response amplitude

correlates to the estimated relative near surface moisture.

27
MBS-114A (REV. 9-76)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. GovH. Accession No. 3. Reelpteat's Accession No. BIBLIOGRAPHIC DATA

SHEET NBSIR 79-1610

4. TITLE AND SUBTITLE

Feasibility Study on the Use of a Microwave System for the

5. Publication Date July 1979

Nondestructive Evaluation of Historic Adobe Structures

lipal

* ^Igt

7. AUTHOR(S)

D. R. Belsher

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. PX 8100 9 0003

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, state, ZIP)

National Park Service

Department of the Interior

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas tactual summary of moat aighificant information. If document includea a significant bibliography or

literature survey, mention it here.)

A frequency-modulated continuous wave (FM-CW) radar system has been utilized

at Tumacacori National Monument to provide usable on site information and to evaluate its potential for nondestructi vely measuring certain parameters

associated with the soundness of historic adobe walls. The parameters of interest

considered were layer thickness, presence, position, and thickness of voids or other inhomogeneities, and moisture content and its distribution. The results were generally favorable. The results indicate that an FM-CW system can nondestructi vely provide a major portion of the information needed to evaluate the soundness of adobe structures at a relatively low cost and in less time than present techniques. With some further work the FM-CW system can be developed into

a useful archeological exploration and evaluation tool that should operate in rock

and soils other than adobe.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word un leaa a proper name;

a operated bvaemicolone) Adobe building materials; adobe soil; dielectric constant measurements;

electromagnetic moisture measurements; nondestructive evaluation; radar soil

measurements; soil moisture; underground radar; void detection.

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