GP11A-0813 0800h
Marine Controlled Source Electromagnetics for Gas Hydrate Evaluation on the Cascadia Margin: Correlation Between Resistivity Anomalies and Seismic Blank Zones
The gas hydrate deposits on the Cascadia Margin have been the focus of a vast number of projects and marine experiments to investigate the distribution and concentration of this potential future energy resource. Gas hydrate or frozen gas consists mainly of methane and water molecules. It is stable at low temperatures and high pressures and forms in pore space within the hydrate stability zone in marine sediments. Hydrate itself is electrical insulating and replaces conductive pore fluid, which subsequently increases the bulk resistivity of a hydrate formation. Accurate measurements of the seafloor resistivity can be a useful tool for hydrate estimation, which is essential for resource and environmental hazard evaluation. The instrument - a development of the University of Toronto - is basically an inline dipole-dipole configuration dragged along the seafloor. It has been successfully applied in previous experiments on the Cascadia Margin and on the Chilean Margin. Here we present a promising new data set collected in summer 2004. Measurements have been conducted along three profiles. For the first time data have been collected with the marine controlled source electromagnetic method in water depths shallower than required for hydrate stability (i.e. below 500m). These data are important as a reference site for hydrate assessment. The second profile covers the bullseye, a vent site in vicinity of ODP site 889B that correlates with a region of seismic blanking and recovered hydrate outcrops. The 7km long profile also covers another series of seismically identified vent sites. The average amplitudes of the measured electric fields and thus the related apparent resistivities along the first profile (no hydrate) are smaller than along the second profile (vent sites). This is consistent with the idea of an increased resistivity in hydrate rich zones. Two pronounced anomalies occur along the second profile in spatial agreement with the bullseye and the other series of vent sites. The third profile intersects the second profile at the bullseye. However, beside of a careful lowering procedure, the array was likely tangled on the seafloor for most parts of the deployment which complicates or even inhibits the interpretation, but points out the importance of a careful coordination of ships navigation and instrument deployment as well as weather and sea conditions for a successful experiment.
http://www.physics.utoronto.ca/~edwards/
GP11A-0814 0800h
New Magnetotelluric Measurements across the Magnetic Beattie Anomaly and the Southern Cape Conductive Belt in South Africa.
South Africa hosts two of the Earth's largest known geophysical anomalies, the Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB), that extend in South Africa for almost 1000 km in east-west direction, and possibly continue also into Antarctica and southern South America. In South Africa, the surface expressions of these anomalies appear to coincide approximately with the mapped boundary of the Cape Fold Belt and the Namaqua-Natal Mobile Belt. The nature of both anomalies remains enigmatic. However, they have been interpreted as a slice of paleao-oceanic lithosphere or alternatively as thrust zones, but the existence of a common source, their extent and internal structures are all unknown. We report on the first results of a high resolution magnetotelluric (MT) study conducted in March, 2004 along a profile between Prince Albert and Fraserburg crossing the BMA and the SCCB in their entirety. Within the framework of the multi-disciplinary integrated German-South African project "Inkaba yeAfrica", MT data were collected at 82 sites in the frequency range from 1000 Hz to 0.001 Hz with an average site spacing of 2 km. With the new data we are able to resolve conductivity structures associated with both geophysical anomalies, and an initial 2D inversion model resolves the conductivity distribution of the entire crust. A zone of very high electrical conductivity (~$1 \Omega m$), at a depth of approximately 5-10 km, seems to be associated with the BMA. Another conductivity anomaly is located beneath the northern boundary of the SCCB, extending from the shallow crust down to approx. 15 km depth. Both conductors are inclined towards the south, which coincides with a general southward dipping trend of mapped faults of the Cape Fold Belt. We therefore interpret these high conductivity anomalies as images of tectonic structures which may have evolved during the formation of the Karoo basin. The conductivity image furthermore reveals several sub-horizontal regions of high conductivity ($2 \Omega m$) in the upper 5 km of the crust, which may reflect sedimentary sequences of the Namaqua-Natal Mobile Belt.
GP11A-0815 0800h
The Altiplano (Central Andes) High Conductivity Zone: Interpretation and Modeling
Magnetotelluric measurements in the South American Andes have revealed regions of highly conductive structures in the middle and lower crust of the Altiplano Plateau. Over wide areas the resistivity drops below 1 Ohm m and an overall conductance of more than 20,000 S is obtained, which represents one of the highest conductance observed on Earth. Several conduction mechanisms and combinations of them have been analyzed to explain the extreme conductivity, such as conduction by graphite layers, saline fluids, and partial melts. The combination of conduction mechanisms is critical, since processes being very efficient by themselves can become very inefficient (e.g. due to polarization effects) if jointed. The conductance measured with magnetotelluric methods represents integrated values for a large volume. To produce that unusual conductivity one or more very conductive materials with a high degree of interconnection have to be present over an enormous spatial range. The distribution of the liquid in the crystalline crust is a crucial constraint, since a lower degree of interconnectivity or of the amount of conductive material within a given volume requires a higher conductivity of the material itself. Magnetotelluric methods are especially sensitive for conductive structures with a pronounced horizontal extent. Assuming preferably saline fluids and partial melts being responsible for the high conductivity zone several adequate patterns of liquid distribution in the crust are modeled numerically and discussed. Concepts of large scale fluid transport (for example via dikes and diapirs) and of storage are analyzed for their geological and petrological relevance as well as for their geophysical impacts, and are compared with the field observations. In this context also the question has to be addressed how and how long fluids serving as conductors can be stored in the crust at a certain depth and how they are replaced when they are lost due to cooling/crystallization, mineral reactions or buoyant instability. The combination of these dynamic processes and their petrophysical signature will be used to distinguish different concepts and to better constrain the nature and evolution of this conductivity anomaly.
GP11A-0816 0800h
Three-dimensional Electromagnetic Modeling of the Hawaiian Swell
An anomalous behavior of the geomagnetic deep sounding (GDS) responses at the Honolulu geomagnetic observatory has been reported by many researchers. Kuvshinov et al (2004) found that the predicted GDS Dst C-response does not match the experimental data -- 10-20% disagreement occurs for all periods of 2 to 30 days, qualitatively implying a more resistive, rather than conductive, structure beneath the Hawaiian Islands. Simpson et al. (2000) found that the GDS Sq C-response at the Honolulu observatory is about 4 times larger than that at a Hawaii island site, again suggesting a more resistive (than elsewhere around) structure beneath the observatory. Constable and Heinson (2004, http://mahi.ucsd.edu/Steve/swell.pdf), presenting a 2-D interpretation of the magnetotelluric (MT) and GDS responses recently obtained at 7 seafloor sites to the south of the Hawaii Islands, concluded that the dataset require the presence of a narrow conducting plume just beneath the islands. The main motivation of our work is to reveal the reason of the anomalous behavior of the Honolulu response. Obviously, the cause may be due to heterogeneity of either the conductivity or the source field. We examine this problem in some detail with reference to the Constable and Heinson's seafloor dataset, as well as the available dataset from the Honolulu observatory. To address the problem we apply numerical modeling using the three-dimensional (3-D) forward modeling code of Avdeev et al. (1997, 2002). With this code we simulate various regional 3-D conductivity models that may produce EM responses that better fit the experimental datasets, at least qualitatively. Also, to explain some features of the experimental long-period GDS responses we numerically studied a possible effect in the responses caused by the equatorial electrojet. Our 3-D modeling results show that, in particular: (1) The GDS responses are better explained by models with a resistive lithosphere whereas the MT data are better fit by models without one; (2) A conductive plume under the Hawaiian Islands may not be required by the MT and GDS datasets considered; (3) An equatorial electrojet might affect the imaginary part of the GDS responses at periods of 2 h and more; (4) The anomalous large value of 0.4 observed in the real part of the seafloor GDS responses still cannot be explained by the 3-D models considered. It seems to require more complicated models.
GP11A-0817 0800h
Electrical Conductivity Across the Northern Ethiopian Rift
Audio-frequency magnetotelluric data were acquired along an approximately 200km long profile across the northern Ethiopian rift in early 2003. We present examples of the data collected, discuss their dimensionality, and show a two-dimensional model along the profile. The model has high conductivity close to the surface beneath the rift where there is volcanic activity and partial melt, as expected, but also in a mid-crustal layer on the northern side of the plateau. A sensitivity analysis demonstrates that this layer is required, and that there is a difference in plateau structure on the two sides of the rift (which are on different plates). There is no northern plateau mid-crustal layer corresponding to our conductor on seismic refraction and wide angle reflection data along the same profile, but they do also show a difference in structure on the two sides of the rift. The last volcanism on the northern plateau where our mid-crustal conductor is strongest was ~11 Ma when the mid-Ethiopian rift propagated north, but close to the rift, where we do not see a mid-crustal conductor, there was volcanism only 2 Ma. Thus although volcanism is the most likely explanation for our mid-crustal conductor, interpretation is not straight-forward.
GP11A-0818 0800h
Electrical image of the crust below Deccan Volcanic Province of Indian peninsular shield: Is the lower crust anisotropic?
The Deccan Volcanic Province (DVP) of the Indian shield has witnessed major stable continental region (SCR) earthquakes in the last few decades. The deeper crust below the Deccan traps, where thick basaltic layers cover large areas of the Indian shield, remains little known until today. A magnetotelluric study covering a broad period range (T = 0.001 - 1000 s) was conducted to achieve more insights into the nature of the crustal electrical structure below the flood basalts. The results from isotropic 2D inversion suggests that the basalts are characterized by low electrical resistivities, while the crust is in general highly resistive. Enhanced conductivity zones were delineated in the middle to lower crust, which are explained as imaging hidden, partly reactivated faults/fractures of the Precambrian basement. The basement in the DVP is highly resistive (5000 - 10,000 Ohm-m) representing the top of the upper crust. It is also seen that the upper crustal resistivities though generally high, vary along the traverse. For example along the 300 km long Sangole (in the south)-Partur (in the north) traverse they tend to become relatively low (less than 5000 Ohm-m) in its central portion as compared to those on either side (greater than 10,000 Ohm-m) of this segment. The results indicate that the high resistive layer representing the upper crust though shows some variations in its thickness along the traverse, extends in general down to about 20 km; and the subsurface region further down, corresponding to lower crust is characterized by resistivities ranging from about 1000-3000 Ohm-m. This shows that the lower crustal region in the Deccan Volcanic Province is characterized by relatively higher resistivity values compared to some of the other continental lower crusts. Relatively higher resistivities for the lower crust indicate that the deeper horizons of the crust in this part of the Deccan Volcanic Province are dry and compact. Similarity of sounding curves and an almost constant phase split over large parts of the measuring area indicate an anisotropic (macro?) lower crust or mantle. Synthetic anisotropic model study was carried out to generate the phase split in the case of 1-D and 2-D environments. The influence of different model parameters like, anisotropic strike, geometry of the layer/block and anisortopic ratio was analysed. Preliminary results of these model studies will be presented here.
GP11A-0819 0800h
Project CMICMR: Long Period Magnetotellurics on Iceland
The method of long period magnetotellurics can be used to determine the electrical conductivity distribution within the earth for depths reaching into the upper mantle. Knowledge of this conductivity distribution can provide important constraints for geodynamical models, complementing constraints derived from other, especially seismic, data. Earlier magnetotelluric studies by other authors give detailed insight into the conductivity distribution within the Icelandic crust, revealing e.g. a good crustal conductor beneath most of Iceland. However, they don't reach the upper mantle due to a limitation of their period range to less than 2000~s and the shielding effects of the good crustal conductor. To surpass this limitation three long period magnetotelluric field stations were set up on Iceland by Beblo and Bj\"{o}rnsson in 1999. To ensure the continuation of these measurements and to analyze and interpret the resulting data, a joint project between the universities of Akureyri (Bj\"{o}rnsson), Munich (Beblo) and Frankfurt (Junge) was initiated in 2001, titled ``Continuous Monitoring of the Icelandic Crust and Mantle Resistivity'' (CMICMR). Since the beginning of the project a huge data set has accumulated, mostly from the following field stations: Akureyri (2000--present), H\'{u}safell (2000--present), Skrokkalda (2000--2002), Gr\'{\i}mstadir (2002--present) and St\'{i}flisdalsvatn (fall~2003). The recorded time series are sampled at periods around 1~s and contain long continuous sections, some of them covering several months. While most of the stations used a conventional land based magnetotelluric instrument setup, the station St\'{\i}flisdalsvatn was an experiment to apply the method of lake bottom magnetotellurics for the first time in Iceland: the station used a prototype of the new data logger Geolore (Geophysical longtime recorder), which was developed at Frankfurt University, optimized for shallow under water applications. Recording the electric field components in the very stable environment of a lake bottom results in nearly drift free time series, hardly achievable by land based measurements. A number of people, K\"{o}nig, Salat and Golden, have worked on the analysis of the collected data. The most important result so far is a conductivity maximum at periods of about 5000~s, which can be explained by a good conductor in about 200--300~km depth. Two possible explanations for this conductor are partial melt within a plume head or enhanced melting at the ridge, but further modeling efforts are necessary to differentiate between these or other interpretations. On the poster an overview on the current state of the CMICMR project is given, with special emphasis on new results, such as those from the lake bottom experiment.
http://www.geophysik.uni-frankfurt.de/em/icelmt/
GP11A-0820 0800h
Two-dimensional Electrical Section Beneath the Backarc Region of Northeast Japan
According to Iwamori (2002), water transported by a cold plate such as the Pacific plate beneath Northeast Japan can subduct deep into the Earth to reach the backarc side and dehydrates at depths of 150-200 km, which is one of the main supply phases of water to the wedge mantle. However, seismic tomography conducted so far has not resolved the region where the proposed dehydration occurs. In this paper, we aim at resolving the water supply to the wedge mantle by constructing an east-west electrical section of the arc-backarc region beneath Northeast Japan since electrical conductivity is sensitive to presence of water. A seafloor magnetotelluric (MT) profile had been occupied by six ocean bottom elecrtomagnetometers from OCT/'02 through JUL/'03 at 39.5 N in order to clarify upper mantle dynamics beneath Japan Sea. High quality MT data were obtained at four sites from the 10-month long geoelectromagnetic time-series. Two-dimensional (2D) preliminary inversions were conducted by Toh et al. (2003) for TE mode, which successfully resolved a resistive lithosphere beneath the Sado ridge, the very eastern margin of Japan Sea where large earthquakes occur. Baba et al. (2004) constructed a new MT dataset by adding four land sites and removing bathymetry/topography effects from all the sites to yield a 2D electrical section for both modes. However, the new dataset was found still affected by severe surface distortions and TM mode data have not been fitted by our 2D inversions. In this paper, we report a new 2D electrical section that explains vertical gradient sounding (VGS) responses belonging to TM mode. Since the VGS responses are the ratios of horizontal geomagnetic components between land and the seafloor, they are free from surface distortions for the electric field. Three-component geomagnetic data observed at Mizusawa Geodetic Observatory was used to evaluate the VGS responses for northward geomagnetic components, which corresponds to TM mode in magnetotellurics. The VGS-TM responses were obtained at 22 periods ranging from 480s to about 7.5 day whose coherences were higher than 0.9. The high quality TM mode data will be inverted by a modified version of Uchida's (1993) 2D ABIC inversion, in which 2D bathymetry can be included.
GP11A-0821 0800h
A Network Induction Study Using Magnetometer Data
Planetary exploration follows in general a strategy starting with spacecraft flyby, followed by orbiter missions and in-situ exploration. It is well accepted that network of landers with in-situ science payload play an important role in the in-situ exploration phase. Among other techniques, electromagnetic soundings provide a relevant contribution to the study of the deep interior of the planet. They are based upon the determination of the impedance Z for frequencies down to 10-4 and even less. In the case of the Earth, impedances are estimated from simultaneous variations of the horizontal magnetic and electric fields recorded at a station. In the case of Mars, only magnetic variations can be recorded at such low frequencies. We present here a new method for impedance derivation from a network of at least three 3-components magnetometers. Consider a network of three stations arranged in a triangular configuration with a separation distance allowing the description of magnetic variations associated to sources of regional or hemispheric extent. Assume further that the primary source field at the surface of Mars can be approximated as a superposition of independent plane waves. The magnetic data series will be analysed using the method proposed by Pinton and Lefevre (1991, 1992), and by Pinton et al (2000). In this approach the determination of horizontal gradients of the magnetic field components is achieved by wave-vector identification. The resulting frequency wave vector spectrum of the magnetic field over the 3 stations network will be used to estimate the variation of the Mars inductive response as a function of frequency. In order to assess the performances with field data of the method we propose, we use minute values from Earth geomagnetic observatories (INTERMAGNET data). We always consider data from only three stations. We present results from studies done for three different regions of the Earth, namely Indian ocean, Central Europe and North America.
GP11A-0822 0800h
Electronic Charge Carriers in Igneous Rocks and their Activation by Stress
The electrical conductivity of the crust does not only depend on saline fluids in interconnected pores, conductive mineral phases such as sulfides, graphite, and/or intergranular carbon films. Igneous and high-grade metamorphic rocks also possess an intrinsic electrical conductivity, which can make an important contribution. This conductivity is electronic in nature and p-type due to the presence of defect electrons (holes) as mobile charge carriers. To study this conductivity, we subject the inner portion (7.5 cm diameter) of thin slabs of air-dry granite and anorthosite (30 x 30 x 0.95 cm$^{3}$) to uniaxial stress with and without applying dc voltages. With a dc voltage applied, we measure currents through the rocks that indicate a substantial intrinsic conductivity. Upon applying a load we measure additional currents that flow (i) through the stressed rock and (ii) through the surrounding unstressed rock. Igneous and high-grade metamorphic rocks contain positive hole pairs (PHP), O$_{3}$X-OO-XO$_{3}$ with X = Si$^{4+}$, Al$^{3+}$ etc. According to our data stress activates these defects, thereby generating positive holes. These represent defect electrons in the valence bands of the otherwise insulating materials. Similar currents or conductivity changes are expected to be activated in the crust during the build-up of tectonic stresses, for instance before earthquakes.
GP11A-0823 0800h
Joint Magnetotelluric-Transient Electromagnetic Imaging of Basin-Bounding Faulting in the Rio Grande Rift, New Mexico
During the summers of 2001-2004 100-m spaced magnetotelluric (MT) recordings, supported by transient electromagnetic (TEM) soundings, were collected during the SAGE program (Summer of Applied Geophysical Experience) in the Rio Grande rift of New Mexico. The profiles of less than 10-km length were aimed at imaging the upper 4 km in the vicinity of the La Bajada fault, a major basin bounding feature. Two different stages of inversion were applied to the data. First, smoothest inversion approaches were applied to derive two-dimensional (2-D) images from the MT data and one-dimensional (1-D) models from the TEM results. The 1-D TEM results were then `stitched' together to produce pseudo-2-D images. The second stage of imaging involved combined constrained inversion and hypothesis testing. Here, a priori knowledge was used to position discrete interfaces between conductive basin fill and resistive basement and intrusives detected by the smooth inversion results. The inversions were then rerun. The results indicate a basin-ward thickening of sediments with two normal faults, the La Bajada fault and a previously unknown fault. Depth to resistive (over 1000 ohm-m), crystalline basement is approximately 2500 m on the downthrown (western) end of the lines, 1500 m between the two faults, and 700 m on the upthrown (eastern) side of the La Bajada fault. Using the joint TEM and MT images the conductive basin fill is subdivided into: 1) an unsaturated surface layer, 50 to 100-m thick, of high resistivity (90 to 150 ohm-m), 2) a 200 to 500-m thick section containing the freshest water and/or least amount of conductive clay with moderately high resistivity (about 10 to 40 ohm-m), and 3) a deeper, more conductive zone of about 5 to 10 ohm-m with salty water and/or high clay content.
http://www.sage.lanl.gov
GP11A-0824 0800h
Seismic and Magnetotelluric Interpretation Near Parkfield, California: Testing a Joint Inversion of Independent Data Sets.
We present a simultaneous interpretation of coincident magnetotelluric (MT) and seismic refraction lines near the Parkfield, CA, San Andreas Fault Observatory at Depth site. The MT data set was acquired with Phoenix 4-channel systems with average station spacing of about 250 m over a profile length of ($\sim$5000) m. The time-domain recording scheme was designed to yield data in the 0.01-200 Hz frequency band. The seismic travel-times were obtained from a SAFOD-related refraction/reflection line that was shot in the fall of 2003. The station and shot spacing for the line section coincident with the MT data were 50~m and 500~m, respectively. An initial, first-pass model of the seismic data suggests a simple structure lies beneath the study site: a 1.0-1.5 km thick, gradational, sedimentary layer with surface topography over a more uniform igneous basement. Modeling of the MT data alone shows a significant trade off exists in these data between the sediment thickness and the basement resistivity. To integrate the separate seismic and MT interpretations we combine both data sets into a joint inversion scheme by assuming that rapid velocity changes are accompanied with correspondingly rapid resistivity changes. We expect the combination will yield a mutually consistent estimate of sediment thickness and basement topography beneath the studied line.
GP11A-0825 0800h
The Application of a Combined Geophysical Survey (Ground-Penetrating Radar and Seismic Refraction) for mapping Sinkholes in Ghor Al-Haditha Area, Jordan
Earth subsidence and sinkholes are natural phenomena that occur in certain geological environments and are associated with surface, subsurface, local and regional processes. The natural solution cavities are common in carbonate rocks and other geological environments, which characterized by the presence of large volumes of evaporates. Subsidence and or sinkholes follow the creation of subsurface caverns, especially if the overlying material consists of soft or poorly cemented sediments. The water circulation and fluctuations play a major role in the creation of caverns as well as causing the triggering of the failure of the overlying earth material. Tectonic and seism tectonic processes have a major role in forming the subsidence and sinkholes particularly in tectonic-active zones such as the Dead Sea region that characterized by the presence of many local and regional subsurface salt features. The study area, Ghor Al Haditha form a part of the southern Dead Sea basin, where subsidences and sinkholes are representing natural phenomena. The occurrence of these phenomena in the area dates back to many years. But since the transfer of the Jordan River in the 1960s and the subsequent lowering of the Dead Sea level, the problem started to occur in a rather serious manner with a noticeable correlation with the fluctuations in the level of the Dead Sea. . Geophysical surveys were carried out in the study area. The aim is to perform a geophysical map of the subsurface layers and to infer its physical properties. Deployed geophysical methods are Ground Penetrating Radar (GPR) and Seismic Refraction technique. The results of the ground penetrating radar (GPR) indicate the presence of cavities and cracks, which may verify drainage of water at surface and actually affirms the formation of sinkholes. The seismic refraction survey support the idea of a possible regional subsurface fault in a NNE direction. The highest recorded P-velocity for subsurface layers is 3948 m/sec which may represent salt intrusions at 70m depth.
GP11A-0826 0800h
Electrical Conductivity of the Bishop Tuff, Bishop, CA: Implications for Ground-Penetrating Radar Performance
Ideal terrestrial analogues to Mars combine known features such as an arid environment, cold climate, deep water table, saline pore waters, and bedrock dominated by igneous or clastic sedimentary units. Terrestrial analogues best suited for calibrating a suite of planetary geophysical instruments, especially radar sounders, need to be sufficiently characterized to provide an accurate understanding of the local geologic context. The Bishop Tuff, Bishop, California is one of a number of recommended Mars analogue sites (National Research Council Decadal Study report on Terrestrial Analogues to Mars, 2001). While not cold, the Volcanic Tableland is situated in an arid environment, and is underlain by a relatively deep water table (100 to 180 m). These factors, combined with availability of detailed characterization data, made this a potentially appealing location in east-central California for testing the performance of existing and planned radar sounders for future Mars exploration. To take advantage of potential synergies that support improved subsurface resolution when applying multiple geophysical techniques, we developed a collaboration to mutually benefit from coordination of field and laboratory activities. Transient electromagnetic (TEM) soundings at several locations on the Volcanic Tableland were performed in preparation for potential ground-penetrating radar (GPR) investigations, and are documented in this paper. Laboratory data from Bishop Tuff samples, determined using capacitive cells in the frequency range of 1 to 1000 MHz, are also presented to assess the dielectric behavior of the local geologic units. Interpretation of geophysical data resulting from this field study is aided significantly by the wide range of geological, structural, and hydrogeological data collected by our team over the last 8 years. Knowledge about the subsurface electrical conductivity structure, determined through application of TEM, is used to quantify the expected magnitude of GPR signal loss due to absorption. One-dimensional TEM inversions suggest a relatively resistive (approximately 1000 to 5000 ohm-m) near-surface, overlying a conductive (approximately 15 to 40 ohm-m) region at depths ranging from approximately 100 to 180 m, depending on sounding location. This conductive region is preliminarily interpreted to be the saturated zone because its depth correlates with measured average local surface water elevations. Low to mid-frequency ground-penetrating radar soundings should be able to image the water table below the Volcanic Tableland because of the relatively resistive nature of the subsurface. We discuss implications of these results on radar performance in similar Martian environments.