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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E06S02, doi:10.1029/2005JE002619, 2006

Absorption and scattering in ground-penetrating radar: Analysis of the Bishop Tuff

Robert E. Grimm

Department of Space Studies, Southwest Research Institute, Boulder, Colorado, USA


Essam Heggy

Lunar and Planetary Institute, Houston, Texas, USA


Stephen Clifford

Lunar and Planetary Institute, Houston, Texas, USA


Cynthia Dinwiddie

Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, San Antonio, Texas, USA


Ronald McGinnis

Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, San Antonio, Texas, USA


David Farrell

Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, Texas, USA


Abstract

Ground-penetrating radar (GPR) signals are attenuated by both absorption and scattering. We performed low-frequency (<100 MHz) GPR surveys at the Volcanic Tableland of the Bishop (California) Tuff to evaluate the factors that control GPR depth of investigation and to develop insight into the capabilities of such radars for Mars. The subsurface reflection character was very different for two different commercial systems used; together, they revealed both internal welding contacts in the tuff and an abundance of discrete scatterers. Attenuation coefficients were computed from profiles that showed distributed scattering: the semilogarithmic signal decay is directly analogous to seismic coda. The absorption (intrinsic loss) was determined to be ∼1 dB/m from low-frequency vertical-electric soundings. The residual attenuation (that is, the attenuation in the absence of absorption) is attributed to scattering. Scattering attenuation of ∼1 dB/m at 25–50 MHz corresponds to mean-free paths as short as 4 m, a fraction of the two-way propagation distances of 20–40 m. Therefore the Bishop Tuff is formally a strong scatterer to GPR. The mean-free path is also comparable to the subsurface radar wavelength in this case, maximizing scattering loss. The scatterers themselves likely originate as welding heterogeneities; contrasts in dielectric constant due to density differences may be supplemented by moisture variations. On Mars, scattering is likely to contribute significant losses to GPR signals in all but the most uniform materials, and unfrozen thin films of water in the lower cryosphere could influence both absorption and scattering.

Received 21 October 2005; accepted 26 April 2006; published 30 June 2006.

Keywords: ground-penetrating radar; attenuation; absorption; scattering; Mars.

Index Terms: 0994 Exploration Geophysics: Instruments and techniques; 5109 Physical Properties of Rocks: Magnetic and electrical properties (0925); 5144 Physical Properties of Rocks: Wave attenuation; 8404 Volcanology: Volcanoclastic deposits; 6225 Planetary Sciences: Solar System Objects: Mars.

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Citation: Grimm, R. E., E. Heggy, S. Clifford, C. Dinwiddie, R. McGinnis, and D. Farrell (2006), Absorption and scattering in ground-penetrating radar: Analysis of the Bishop Tuff, J. Geophys. Res., 111, E06S02, doi:10.1029/2005JE002619.