[1] 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 lowfrequency 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.

No abstract

No abstract

Multi-temporal image analysis of very-high-resolution historical aerial and recent satellite imagery of the Ahnewetut Wetlands in Kobuk Valley National Park, Alaska, revealed the nature of thaw lake and polygonal terrain evolution over a 54-year period of record comprising two 27-year intervals . Using active-contouring-based change detection, high-precision orthorectification and co-registration and the normalized difference index, surface area expansion and contraction of 22 shallow water bodies, ranging in size from 0.09 to 179 ha, and the transition of ice-wedge polygons from a low-to a high-centered morphology were quantified. Total surface area decreased by only 0.4% during the first time interval, but decreased by 5.5% during the second time interval. Twelve water bodies (ten lakes and two ponds) were relatively stable with net surface area decreases of ≤10%, including four lakes that gained area during both time intervals, whereas ten water bodies (five lakes and five ponds) had surface area losses in excess of 10%, including two ponds that drained completely. Polygonal terrain remained relatively stable during the first time interval, but transformation of polygons from low-to high-centered was significant during the second time interval.

[1] We conducted low-frequency (16 to 100 MHz) ground-penetrating radar surveys on the eroded lava flows at Craters of the Moon (Idaho, USA) volcanic field to evaluate the potential of future radar-sounding investigations on Mars to map shallow subsurface features. Radar-sounding profiles were obtained from three locations: above a lava tube, across a volcanic rift, and over a scoria cone. Results were combined with laboratory permittivity and magnetic permeability measurements of field-collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth. Our results demonstrate a constrained performance for low-frequency sounding radars to characterize mafic, arid volcanic terrains that contain a significant amount of ferro-oxides ($14%), mainly in the form of olivine and magnetite. Penetration depths of 35 m were achieved at a frequency of 100 MHz, and depths of 80 m were achieved at 16 MHz, with an effective dynamic range of 60 dB. Results indicate that for frequencies below 100 MHz, the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses. Over our frequency range, the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses, ground porosities, and degree of heterogeneity rather than the sounding frequency. In light of these results, we suggest average attenuation and scattering losses measured in terms of dB/m and discuss the expected penetration depth for the Mars orbital radar-sounding instruments SHARAD and MARSIS in mafic volcanic terrains.