continental scale salt tectonics on mars and the origin of valles marineris and associated outflow channels a synthesis of deformation patterns within and around the thaumasia plateau mars points to a new interpretation for regional deformation and the origin of valles marineris and associated outflow channels the morphology of the thaumasia plateau is typical of thin skinned deformation akin to a mega slide in which extensional deformation in syria planum and noctis labyrinthus connects via lateral zones of transtension and strike slip claritas fossae and valles marineris to a broad zone of compressional uplift and shortening defined by truncated craters and thrust faults along the coprates rise and thaumasia highlands however the low regional slope 1 results in gravitational body forces that are too small to deform the basaltic lava flows conventionally thought to compose the flanks of the tharsis volcanic province instead we conclude that geothermal heating and topographic loading of extensive buried deposits of salts and or mixtures of salts ice and basaltic debris would allow for weak detachments and large scale gravity spreading we propose that the generally linear chasmata of valles marineris reflect extension collapse and excavation along fractures radial to tharsis either forming or reactivated as part of one lateral margin of the thaumasia gravity spreading system the other dextral lateral margin is a massive splay of extensional faults forming the claritas fossae which resembles a trailing extensional imbricate fan the compressional mountain belt defined by the coprates rise and thaumasia highlands forms the toe of the mega slide topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below syria planum that could have provided further thermal energy and topographic potential for initiating regional deformation either intrusively through inflation or extrusively through lava flow and or ash fall emplacement higher heat flow during noachian time or geothermal heating due to burial by tharsis derived volcanic rocks would have contributed to flow of salt deposits as well as formation of groundwater from melting ice and dewatering of hydrous salts we further propose that connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outflow channels of echus coprates and juventae chasmata at relatively uniform source elevations along the northern margin of the mega slide where regional groundwater flow would have been directed toward the surface our hypothesis provides a unifying framework to explain perplexing relationships between the rise of the tharsis volcanic province deformation of the thaumasia plateau and the formation of valles marineris and associated outflow channels

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

morphometric and geometric characterization of normal faults on mars abstract using three different approaches fault plane fitting 3d crater rim palinspatic restorations and fault scarps morphometric analysis we investigate the geometry and degradation history of martian normal faults in two distinct areas the three independent methods produce similar results indicating that the average dip angle of the normal faults on these two locations is probably below the value that is usually assumed for mars 60 our best estimate for this average dip angle is 46 8 9 8 which is a value comparable with the mean dip angle inferred on earth for seismically active normal faults this lower average dip angle implies that all the rift strain estimates performed until now might be underestimated from the comparative analysis of the two faulted regions phlegethon catena and claritas fossae we show that local and regional dip variabilities may exist on mars this reinforces the idea that the amount of extension associated with martian rifts must be reconsidered we also demonstrate the advantages of performing a comparative morphometric analysis of fault scarps this approach enables the reconstruction of the faults scarps degradation history and can be used to evaluate how environmental conditions changed through time after modeling the degradation of the fault scarps at the two sites we conclude that the observed morphometric variations are mainly due to the different faulting ages in an environment characterized by low scarp degradation rates 4 10 3 m 2 kyr over the last 3 ga

stratigraphy of aeolis dorsa mars stratigraphic context of the great river deposits unraveling the stratigraphic record is the key to understanding ancient climate and past climate changes on mars grotzinger j et al 2011 astrobiology 11 77 87 stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers therefore river deposits when placed in stratigraphic order could constrain the number magnitudes and durations of the wettest and presumably most habitable climates in mars history we use crosscutting relationships to establish the stratigraphic context of river and alluvial fan deposits in the aeolis dorsa sedimentary basin 10 e of gale crater at aeolis dorsa wind erosion has exhumed a stratigraphic section of sedimentary rocks consisting of at least four unconformity bounded rock packages recording three or more distinct episodes of surface runoff early deposits 700 m thick are embayed by river deposits 400 m thick which are in turn unconformably draped by fan shaped deposits 900 m thick unconformably drape all previous deposits river deposits embay a dissected landscape formed of sedimentary rock the river deposits are eroding out of at least two distinguishable units there is evidence for pulses of erosion during the interval of river deposition the total interval spanned by river deposits is 1 10 6 2 10 7 yr and this is extended if we include alluvial fan deposits alluvial fan deposits unconformably postdate thrust faults which crosscut the river deposits this relationship suggests a relatively dry interval of 4 10 7 yr after the river deposits formed and before the fan shaped deposits formed based on probability arguments yardang forming layered deposits unconformably postdate all of the earlier deposits they contain rhythmite and their induration suggests a damp or wet near surface environment the time gap between the end of river deposition and the onset of yardang forming layered deposits is constrained to 1 10 8 yr by the high density of impact craters embedded at the unconformity the time gap between the end of alluvial fan deposition and the onset of yardang forming layered deposits was at least long enough for wind induced saltation abrasion to erode 20 30 m into the alluvial fan deposits we correlate the yardang forming layered deposits to the upper layers of gale crater s mound mt sharp aeolis mons and the fan shaped deposits to peace vallis fan in gale crater alternations between periods of low mean obliquity and periods of high mean obliquity may have modulated erosion deposition cycling in aeolis this is consistent with the results from an ensemble of simulations of solar system orbital evolution and the resulting history of the obliquity of mars 57 of our 61 simulations produce one or more intervals of continuously low mean mars obliquity that are long enough to match our aeolis dorsa unconformity data

the timing of martian valley network activity constraints from buffered crater counting abstract valley networks concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity have been cited as indicators that the climate of mars differed significantly in the past from the present hyperarid cold desert conditions some researchers suggest that the change in climate was abrupt while others favor a much more gradual transition thus the precise timing of valley network formation is critical to understanding the climate history on mars we examine thirty valley network incised regions on mars including both cratered upland valley networks and those outside the uplands and apply a buffered crater counting technique to directly constrain when valley network formation occurred the crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units from these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age 1 valley network activity in the cratered highlands has an average cessation age at the noachian hesperian boundary and all valleys that we crater counted are early hesperian or older no evidence is found for valley networks in the cratered uplands of late hesperian or amazonian age the timing of the cessation of cratered upland valley network activity at the noachian hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate type weathering 2 a few valley network incised regions formed outside of the cratered uplands on volcanic edifices in association with younger impact craters and on the rim of valles marineris we applied our buffered crater counting technique to four such valleys on the volcanoes ceraunius tholus hecates tholus and alba patera and on the rim of echus chasma and find that each has distinctive and different late hesperian or early amazonian ages indicating that valley networks formed from time to time in the post noachian period unlike the cratered upland valley networks these isolated occurrences are very local and have been interpreted to represent local conditions e g snowpack melted during periods of intrusive volcanic activity in contrast to a gradual cessation in the formation of valley networks proposed by some workers our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the noachian followed only by episodic and very localized valley network formation in later mars history very likely due to specific conditions e g local magmatic heating these valley network ages and correlations are thus consistent with a major change in the near surface aqueous environment on mars at approximately the noachian hesperian boundary the noachian environment supported surface running water and fluvial erosion across mars in the cratered uplands enhanced crater degradation and a weathering environment favoring the formation of phyllosilicates the hesperian amazonian environment was more similar to the hyperarid cold desert of today with valley networks forming only extremely rarely and confined to localized special conditions sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin axis orbital parameters and to periodic catastrophic emergence of groundwater

kinematic gps as a source for airborne gravity reduction in the airborne gravity survey of switzerland an airborne gravity survey of switzerland was performed in a joint project between the swiss federal institute of technology zurich and lacoste and romberg gravity meters inc the survey was flown in a twin otter aircraft equipped by the swiss federal directorate of cadastral surveying the aircraft was outfitted with three global positioning system receivers one for navigational purpose and two as sources of positioning velocity and vertical acceleration for airborne gravity reduction four receivers were installed on the ground as reference stations the gravity data were recorded with a modified lacoste and romberg marine gravimeter at a sampling rate of 1 s modifications included anti alias filters and an absolute encoder for the measuring screw the flights were performed at a barometric altitude of 5100 m above sea level an airborne bouguer anomaly map computed at flight altitude with topographic corrections of up to 167 km and a density of 2670 kg m3 is presented bouguer anomaly values at crossing points between lines flown at comparable altitudes h 10 m show differences varying between 1 1 and 2 7 mgal 10 5 m s2 comparison of the ground bouguer anomaly map upward continued to 5100 m and the one computed from airborne data shows a strong similarity both in amplitude and wavelength

timing of the faulting on the wispy terrain of dione based on stratigraphic relationships with impact craters the trailing hemisphere of dione is characterized by the wispy terrain where it exhibits a hemispheric scale network of extensional tectonic faults superposed on the moon s cratered surface the faults likely reflect past endogenic activity and dione s interior thermal history although fresh exposures of pristine scarps indicate that the timing of the faulting is relatively recent the absolute age of the faulting remains uncertain to estimate the timing of the faulting we investigated stratigraphic relationships between impact craters and faults using high resolution images obtained by iss cameras onboard the cassini spacecraft we investigated craters with diameters exceeding or equal to 10 km that coincide spatially with the faults and classified the craters as crosscut craters or superposed craters as a result at least 82 of the craters were interpreted as clear examples of crosscut craters and 12 of the craters were interpreted to be candidates of superposed craters although stratigraphic relationships are often ambiguous the paucity of superposed craters and a predicted cratering rate indicate that the faulting of the wispy terrain is 0 30 0 79 ga if 12 18 of the craters are assumed to be superposed the timing of the faulting could be in the range 0 30 0 79 ga however it is possible that the faulting of the wispy terrain is still ongoing

tectonic history of the tharsis region mars the faults of the tharsis region of mars have been mapped and delineated on the basis of age the orientation of each group of faults was analyzed to determine if they were radial to a point the results indicate that there are at least four discrete centers of faulting within the tharsis area which are of significantly different age the four centers proposed are from oldest to youngest 1 in the thaumasia highlands 2 in northern syria planum and finally 3 and 4 near pavonis mons while episodes 3 and 4 occupy the same geographic position they are separated in time by the development of the tharsis lava plains and shields each episode of tectonism was separated from the next by a period of major basaltic volcanism the tectonic features of the tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting rather than the result of one episode about a single point

crater fault interactions a metric for dating fault zones on planetary surfaces abstract constructing a tectonic history of a planetary surface requires determining precise fault ages a task not always possible with current analytical methods here we introduce a new method to constrain the ages of faults improving upon earlier methods that used cross cutting relationships with crater dated host surfaces and apply it to faulted terrains by counting all craters and categorizing them into faulted unfaulted and unclear types to date deformation directly additionally we construct a simple probabilistic model to account for regions of low fault density this new technique is applied to the tectonically complex thaumasia plateau mars to assess the timing of regional faulting and demonstrate its usefulness and ease of application

azimuthal velocity variations caused by borehole stress concentrations in laboratory experiments borehole stress concentrations caused by far field uniaxial stress produce measurable variations in compressional wave velocities near the borehole uniaxial stress was applied perpendicular to the axis of a borehole drilled through a sandstone block inside the fluid filled borehole refracted compressional waves were propagated in the rock parallel to the borehole axis at discrete azimuths producing a scan of velocity versus azimuth as uniaxial stress increased significant variations of compressional wave velocity with azimuth developed of the order of 10 variation the azimuth of velocity minima provides an excellent indicator of the direction of maximum far field uniaxial stress accurate to 5 the detailed variation of velocity with azimuth while approximately sinusoidal has sufficient character to provide new information about the rock one interpretation yields the curvature of the velocity versus stress curve of the rock

airborne gravimetry from a small twin engine aircraft over the long island sound in january 1990 a test of the feasibility of airborne gravimetry from a small geophysical survey aircraft a cessna 404 was conducted over the long island sound using a bell aerospace bgm 3 sea gravity meter gravity has been measured from large aircraft and specially modified de havilland twin otters but never from small standard survey aircraft the gravity field of the long island sound is dominated by an asymmetric positive 30 rngal anomaly which is well constrained by both marine and land gravity measurements using a trimble 4000 gps receiver to record the aircraft s horizontal position and radar altimeter elevations to recover the vertical accelerations gravity anomalies along a total of 65 km were successfully measured the root mean square rms difference between the airborne results and marine measurements within 2 km of the flight path was 2 6 mgai for 15 measured values the anomalies recovered from airborne gravimetry can also be compared with the gridded regional free air gravity field calculated using all available marine and land gravity measurements the rms difference between 458 airborne gravity measurements and the regional gravity field is 2 7 mgai this preliminary experiment demonstrates that gravity anomalies with wavelengthsas short as 5 km can be measured from small aircraft with accuracies of 2 7 mgal or better the gravity measurements could be improved by higher quality vertical and horizontal positioning and tuning the gravimeter s stabilized platform for aircraft use

an airborne gravity study of eastern north carolina the naval research laboratory nrl has developed a prototype airborne gravity measurement system the core of the system is a lacoste and romberg air sea gravity meter mounted on a three axis stable platform corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters the original prototype system was designed for use over oceanic areas we recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained the radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces vertical accelerations due to horizontal velocity over a curved rotating earth the eotvos correction and precise two dimensional positions are determined from a texas instrument p code global positioning system the updated system was tested

thick shell tectonics on one plate planets applications to mars a theory for stress distributions in thick lithospheric shells on one plate planets is developed based on the zero frequency equations of a self gravitating elastic spherical shell overlying a strengthless fluid stress distributions in lithospheres are reviewed for both the compensated and flexural modes in the former case surface stresses only depend on surface topography while for the latter case it is shown for long wavelengths that stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties computational analyses are carried out for mars and it is found that isostatically compensated models correctly predict the graben structure in the immediate tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding tharsis a three stage model is hypothesized for the evolution of tharsis isostasy with north south graben formation on tharsis followed by flexural loading and radial graben formation on the perimeter of tharsis followed by a last stage of loading with little or no regional deformation this model is consistent with the martian lithosphere monotonically thickening over geologic time

interferometric global positioning system three dimensional positioning source for airborne gravimetry the naval research laboratory nrl and the national geodetic survey recently collaborated on an experiment to test interferometric mode global positioning system gps tracking as a source of altitude and vertical acceleration for airborne gravity measurement reduction a texas instruments ti 4100 gps receiver and an extremely precise radar altimeter were installed in an nrl p 3 orion aircraft a second ti 4100 was located at a fixed base station of known location carrier phase data from four gps satellites were recorded from the fixed and airborne receivers and postprocessed to produce three dimensional displacements of the airborne station relative to the base station differences in gps and radar altimeter coordinate systems were modeled and removed the resulting gps and radar altitudes were compared the two unfiltered 1 hz data sets differed by an rms of 20 cm when aircraft dynamics were controlled by autopilot in straight and level flight during hand flown maneuvers such as turns climbs and descents rms differences varied from 40 to 80 cm a significant portion of the differences can be attributed to remaining coordinate system discrepancies and to uncompensated geometric effects due to aircraft motion we have also examined vertical acceleration time series produced by doubly differencing both data sets by applying a low pass filter which allows resolution of features with wavelengths of 20 km and greater at nominal aircraft speeds of 100 m s the rms difference between the radar and gps derived vertical accelerations was less than 2 mgal 2 10 5 m s2 during straight and level flight it should be possible to improve this accuracy and wavelength resolution with future improvements in measurement and processing techniques the results of this experiment show that the incorporation of interferometric mode gps tracking into the airborne gravity measurement system will make possible rapid and relatively inexpensive gravity surveys over any type of terrain while simultaneously providing accurate measurements of ground height profiles beneath the aircraft

what can airborne gravimetry contribute to geoid determination this paper evaluates the suitability of airborne gravimetry for geoid determination spectral analysis of gravity data is used to derive the high frequency geoid spectrum and to compare it to the error spectrum of the geoid as derived from an airborne gravity system gravity values in various areas with typical topographic features are used for the spectral analysis the achievable accuracy of airborne gravimetry in geoid determination is studied through an assessment of inertial navigation system ins and global positioning system gps errors in the spectral ranges of interest the results of these computations are presented in a number of graphs which allow the following conclusions to determine geoid undulations with an accuracy of 1 cm rms the minimum wavelength to be resolved is 14 km in flat terrain and 5 km in mountainous terrain using a flight speed of 300 km h at an altitude of 2 5 km or less above ground current hardware sensitivity is sufficient to resolve the geoid spectrum for such wavelengths the total gps ins induced geoid undulation error rms in this case is less than 1 cm for wavelengths between 5 km and 100 km and less than 10 cm for wavelengths between 5 km and 500 km the cumulative rms geoid error is less than 30 cm for wavelengths up to 1000 km the accuracy for relative geoid determination is 0 1 ppm for wavelengths up to 100 km and 0 2 ppm for 500 km the results were verified by analyzing the data collected during an airborne test the results indicate that airborne gravimetry has the potential to determine geoid undulation with centimeter level accuracy and to fill the gaps in the global gravity map in an efficient manner and with an accuracy superior to current terrestrial methods

lithospheric scale buckling and thrust structures on mars the coprates rise and south tharsis ridge belt detailed photogeologic mapping documents for the first time the deformation sequence of the coprates rise materials of middle noachian through perhaps early hesperian age were involved in the horizontal shortening of crustal and lithospheric rocks that produced uplift and asymmetric east vergent folding of these layers during the late noachian and perhaps early hesperian previous suggestions of normal faulting as the deforming mechanism are not supported by the new mapping or by dislocation models of rise topography the most likely mechanism for producing the coprates rise is folding due to buckling instability and perhaps thrust faulting an extensive ridge belt concentric to south tharsis is defined by a newly recognized system of approximately coeval long wavelength topographic ridges and swales that include the coprates rise horizontal shortening of crust and lithosphere is manifested by periodically spaced structures at two different scales wrinkle ridges spaced tens of kilometers apart and the south tharsis system of ridges spaced hundreds of kilometers apart both types of structures may be associated with early volcanotectonic activity and crustal thickening in south tharsis these and other large ridges indicate that lithospheric buckling and thrust faulting may have been common on early mars

effect of obliteration on crater count chronologies for martian surfaces 1 the density of impact craters calibrated against lunar data is currently the only quantitative measure of surface age for terrestrial planetary surfaces unlike the moon however mars has been weathered and eroded obliterating some small 10 km diameter craters a phenomenon addressed in the mariner viking days of mars exploration but commonly overlooked in recent studies we present a quantitative model that extends this earlier work to assess the effect of erosion and infilling on surface ages inferred from crater frequency distributions our work affirms that small cratersizedistributionscanbeinterpretedquantitatively in terms of effects of erosion and crater infilling at rates comparable to those reported for mars a reanalysis of prior studies indicates that low to moderate long term rates of erosion and crater infilling can mask an ancient age and result in small crater populations similar to those offered as evidence for young and geologically significant surface activity citation smith m r a r gillespie and d r montgomery 2008 effect of obliteration on crater count chronologies for martian surfaces geophys res lett 35 l10202 doi 10 1029 2008gl033538

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

airborne measurement of absolute sea surface heights topographic profiles may be measured from an aircraft by differencing precise radar or laser altitudes with ellipsoidal heights obtained by means of kinematic interferometric mode gps over the ocean the difference between the two altitudes is the geoid height plus dynamic oceanographic effects such as tides currents eddies etc aircraft might provide on a local scale altimetry measurements similar to those from geosat and ers 1 such airborne measurements could be applied to the determination of a local high resolution geoid or for oceanographic studies several long over water altimetry profiles were obtained during the 1991 field campaign of the greenland aerogeophysics project gap91 kinematic interferometric gps between multiple short or zero baseline airborne receivers and several sets of stationary gps receivers on the ground provided precise three dimensional positioning for airborne gravity magnetic and topographic profiling sensor systems mounted aboard a p 3 orion aircraft the logistics of surveying a large region long gps baselines and poor satellite coverage limit the accuracy of the gps positioning but the geoid signal is clearly visible in the over water profiles in spite of gps baseline lengths of up to 1000 km differences between modeled geoid heights and measured sea surface heights ranged from about 1 2 meters r m s although the differences are primarily due to errors in the gps positions some portion is due to errors in the model geoid uncompensated tides sea ice contamination of the radar data and other errors in the radar we believe that this represents the first airborne measurement of absolute sea surface heights

borehole damage indicator from stress induced velocity variations i develop a simple technique to indicate the onset of stress induced mechanical damage near a wellbore stress concentrations near a wellbore can cause the yield strength of the rock to be exceeded the resulting mechanical damage causes a decrease in acoustic velocity that precedes the development of breakouts with laboratory experiments on stressed boreholes i show that high resolution shallow acoustic measurements made inside the borehole can detect these stress induced velocity changes the onset of damage can be predicted by comparing these shallow measurements to a deep velocity measurement in undamaged rock when the maximum shallow velocity is decreased by damage and approaches the deep velocity i infer that damage has occurred and breakouts and or borehole failure are imminent if implemented in the oilfield this technique could form the basis for a real time damage warning for drillers that allows for preventive action

airborne gravity and precise positioning for geologic applications airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade airborne gravity is especially useful when measured in conjunction with other geophysical data such as magnetics radar and laser altimetry the aerogeophysical survey over the west antarctic ice sheet described in this paper is one such interdisciplinary study this paper outlines in detail the instrumentation survey and data processing methodology employed to perform airborne gravimetry from the multi instrumented twin otter aircraft precise positioning from carrier phase global positioning sys tem gps observations are combined with measurements of acceleration made by the gravity me ter in the aircraft to obtain the free air gravity anomaly measurement at aircraft altitude gps data are processed using the kinematic and rapid static kars software program and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the gps position solu tion accuracies for the free air anomaly are determined from crossover analysis after significant editing 2 98 mgal rms and from a repeat track 1 39 mgal rms the aerogeophysical survey covered a 300 000 km 2 region in west antarctica over the course of five field seasons the grav ity data from the west antarctic survey reveal the major geologic structures of the west antarctic rift system including the whitmore mountains the byrd subglacial basin the sinuous ridge the ross embayment and siple dome these measurements in conjunction with magnetics and ice penetrating radar provide the information required to reveal the tectonic fabric and history of this important region

tectonic history of the syria planum province of mars we attribute most of the development of extensive fractures in the tharsis region to discrete tectonic provinces within the region rather than to tharsis as a single entity one of these provinces is in syria planum faults and collapse structures in the syria planum tectonic province on mars are grouped into 13 sets based on relative age areal distribution and morphology according to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order stage i is an early to late noachian deformation forming mostly east west structures fault set ia some large volcanoes also formed faults were produced possibly by flexural uplift also arcuate north trending grabens set ib indicate that faulting is transitional to the next stage stage ii is late noachian to early hesperian radial faulting centered in syria planum possibly due to isostatic uplift in late noachian set iia to early hesperian set iib time stage iii is early to late hesperian faulting tangential to syria planum that was related to local centers of uplift sets iii1 iii3 on the periphery of syria planum stage iv is a late hesperian graben formation that was circumferential to syria planum set iva caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading in association with volcanism minor faulting occurred producing an oval pattern of faults in southwest syria planum set ivbl and a radial pattern south of the planum set ivb2 that apparently rejuvenated buried stage ii faults stage v is a late hesperian to early amazonian development of grabens and troughs of noctis labyrinthus and western valles marineris set v that was probably instigated by local uplift exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement stage vi is early amazonian northwest trending faulting in noctis fossae set vil perhaps due to tharsis monies centered tectonism and north northwest normal faulting along the eastern side of the claritas rise set vi2 that was due to tectonic subsidence the duration of tectonic activity in the syria planum province was perhaps 2 3 b y photoclinometric topographic profiles across 132 grabens and fault scarps show that syria planum grabens have widths average of 2 5 km and most range from 1 to 6 km similar to lunar grabens but the martian grabens have slightly higher side walls average about 132 m and gentler wall slopes average of 9 and range of 2 25 than lunar grabens 93 m high and 18 slopes scarp degradation on mars has progressed through quakes impact shaking and dry slope processes the lower slopes may be due to mars higher gravity estimates of the amount of extension for individual grabens range from 20 to 350 m most estimates of the thickness of the faulted layer range from 0 5 to 4 5 km average is 1 5 km this thickness range corresponds closely to the 0 8 to 3 6 km range in depth for pits troughs and canyons in noctis labyrinthus and along the walls of valles marineris we propose that the predominant 1 to 1 5 km values obtained for both the thickness of the faulted layer and the depths of the pits troughs and theater heads of the canyons reflect the initial depth to the water table in this region as governed by the depth to the base of ground ice maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

assessing rock brittleness and fracability from radial variation of elastic wave velocities from borehole acoustic logging rock brittleness and fracability of subsurface formations are two important parameters for hydraulic fracturing in hydrocarbon reservoir production this paper presents an effective technique to assess these parameters using the radial variation of compressional and shear velocities from borehole acoustic logging our technique is based on a rock mechanic phenomenon that a brittle rock with high fracability tends to leave a significant amount of drilling induced cracks at the borehole wall resulting in radial elastic wave velocity variation away from borehole by determining the velocity variation the combined effects of brittleness and fracability of formation rocks can be assessed the compressional wave travel time tomography and flexural shear wave inversion methods are respectively used to obtain compressional and shear velocity variations well log data analysis examples demonstrate the practicability and effectiveness of this technique

advances in aerogeophysics and precise positioning gravity topography and high resolution applications airborne magnetic surveys have long been a primary tool for geologic mapping resource exploration hazard evaluation and tectonic studies recent advances in precise positioning have enabled other geophysical tools such as laser and radar altimeters and the gravity meter to be mounted on airplanes and contribute to geologic and environmental studies the principal advantages of airborne geophysical surveys over ground based studies result from enhanced experiment design including improved access to remote and contaminated areas optimized sampling rapid sampling rate and tremendous potential for interdisciplinary studies

airborne gravity measurements over mountainous areas by using a lacoste romberg air sea gravity meter this paper introduces a new approach to airborne gravity data reduction well suited for surveys flown at high altitude with respect to gravity sources mountainous areas classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western french alps by using a lacoste romberg air sea gravity meter the part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft this finding is theoretically argued by solving the equation of the gravimetric system gravity meter and stabilized platform the transfer function of the system is derived and a new formulation of airborne gravity data reduction which takes care of the sensitive response of spring tension to observable gravity field wavelengths is given the resulting gravity signal exhibits a residual noise caused by electronic devices and short wavelength eotvos effects the use of dedicated exponential filters gives us a way to eliminate these high frequency effects examples of the resulting free air anomaly at 5100 m altitude along one particular profile are given and compared with free air anomaly deduced from the classical method for processing airborne gravity data and with upward continued ground gravity data the well known trade off between accuracy and resolution is discussed in the context of a mountainous area

acoustic evidence of mechanical damage surrounding stressed boreholes laboratory experiments demonstrate that acoustic waveforms recorded in a borehole provide evidence of stress induced mechanical damage in surrounding rock in the experiments external uniaxial stress was applied perpendicular to the borehole stress concentrations near the borehole wall caused velocities of refracted compressional wave to vary with azimuth low velocities occurred in zones of tensile stress and high velocities occurred in zones of compressive stress velocity variations are on the order of 10 at high values of externally applied uniaxial stress rock exceeded its yield strength and permanent damage developed this damage decreased the measured velocities by approximately 10 especially in the zones of compressive stress concentration the heterogeneous nature of the velocities surrounding the borehole resulted in low velocity channels parallel to the borehole wall caused either by tensile stress concentrations or by mechanical damage these low velocity channels may be responsible for high amplitude bright spots that appear on variable density plots of azimuthal waveform scans the amplitude increases can be on the order of 500 and are associated with low velocity zones not with decreased attenuation the hypothesized mechanism is acoustic focusing whereby velocity gradients refract acoustic waves back towards the borehole