Ground-Penetrating Radar Investigations of Terrestrial Analogs
Inclusive Dates: 04/01/04 04/01/05
Background - The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is acquiring the first data set of Martian subsurface radar properties. The SHAllow RADar (SHARAD) instrument is also in Mars orbit and will begin to acquire data in November 2006. Because only limited ground-penetrating radar investigations of the Earth have been made within the low frequency range of MARSIS, accurate interpretation of its forthcoming data is uncertain. Terrestrial analogs to Mars provide the opportunity to collect low frequency radar data from well-known, well-characterized sites so that uncertainties may be minimized when interpreting Mars radar data.
Approach - Staff from SwRI's Geosciences and Engineering Division and Space Science and Engineering Division, together with the Lunar and Planetary Institute, conducted low frequency ground-penetrating radar investigations of terrestrial analogs to Mars to develop benchmarks against which we may interpret MARSIS and SHARAD data. We used commercial ground-penetrating radar systems operating at frequencies similar to SHARAD to sound geophysical environments with complexity analogous to that expected on Mars. Multiple ground-penetrating radar antennas were used to extend the field-tested frequency range up to 100 MHz, thereby essentially covering the low frequency range for planetary radars that may be used during the next decade. Transient electromagnetic and vertical electrical soundings measured the electrical conductivity of the subsurface, from which absorptive losses in the radar signal were estimated.
Accomplishments - We characterized several field sites, including the Volcanic Tableland, California, and Craters of the Moon National Monument, Idaho, using geophysical methods and obtained an improved understanding of factors controlling radar signal attenuation and scattering at low frequencies. We made three principal findings. First, the combination of ground-penetrating radar with transient electromagnetic soundings, or vertical electrical soundings enabled assessment of the extent to which conductive losses affected the penetration depth achieved by radar. Second, the first few meters of the subsurface strongly influence the ultimate penetration depth achieved by radar. Results indicated that the presence of conductive material in the first few meters, which may be below the radar resolution, can dramatically decrease the dynamic range of the radar-backscattered echoes and, thus, significantly decrease the penetration depth. Such an effect could also influence radar sounding performance on Mars, where a thin layer of conductive material, such as hematite, maghemite, clay or silt, may significantly attenuate the radar signal. Finally, operating at lower frequencies does not necessarily yield better penetration in an electrically conductive subsurface environment. Project results and subsequent work led to presenting ten papers, publishing four journal articles, convening a workshop and an American Geophysical Union technical session, and securing two funded NASA grants.