The rock size-frequency distribution from 10 to 50 cm

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Description

THE NASA GOVERNMENT HAVE LADIR GROUND PENETRATION RADAR THAT SEES 100 MILES INTO THE EARTH. Ground penetrating radar geologicfield studies of the ejectaof Barringer Meteorite Crater, Arizona, as a planetary analog Ground penetrating radar (GPR) has been a useful geophysical tool in investigating avariety of shallow subsurface geological environments on Earth. Here we investigate the capabilities of GPR to provide useful geologic information in one of the most commongeologic settings of planetary surfaces, impact crater ejecta. Three types of ejecta aresurveyed with GPR at two wavelengths (400 MHz, 200 MHz) at Meteor Crater, Arizona,with the goal of capturing the GPR signature of the subsurface rock population. In order to“ground truth”the GPR characterization, subsurface rocks are visually counted andmeasured in preexisting subsurface exposures immediately adjacent to and below the GPRtransect. The rock size-frequency distribution from 10 to 50 cm based on visual counts iswell described by both power law and exponential functions, the former slightly better,reflecting the control of fragmentation processes during the impact-ejection event. GPRcounts are found to overestimate the number of subsurface rocks in the upper meter(by a factor of 2–3x) and underestimate in the second meter of depth (0.6–1.0x), resultsattributable to the highly scattering nature of blocky ejecta. Overturned ejecta that isfractured yet in which fragments are minimally displaced from their complement fragmentsproduces fewer GPR returns than well-mixed ejecta. The use of two wavelengths anddivision of results into multiple depth zones provides multiple aspects by which tocharacterize the ejecta block population. Remote GPR measurement of subsurface ejecta infuture planetary situations with no subsurface exposure can be used to characterize thoserock populations relative to that of Meteor Crater. I. J. Daubar (2013), Groundpenetrating radar geologic field studies of the ejecta of Barringer Meteorite Crater, Arizona, as a planetary analog,J. Geophys. Res. Planets,118, 1915–1933, doi:10.1002/jgre.20145.1. Introduction[2] Ground penetrating radar (GPR) studies can yieldinsight into the physical properties, rock-size distribution,structure, and layering in the shallow subsurface, therebygranting a three-dimensional view of the processes affectingan area over geologic time. The broad motivation and impli-cations of the present work relate to how GPR data fromterrestrial analogs can help constrain models for evolutionof the lunar and martian surface, aid in interpretation oforbital SAR or radar sounding data, and help predict whatmight be encountered in the subsurface during future landedscientific or engineering operations on the Moon or Mars.[3] Broadly speaking, GPR data consist of radar pulsesreturned from the subsurface, where two main factorsinfluence and modify the propagation of waves, contributingtheirfingerprint to thefinal radargram: a material’s dielectricproperties (e.g., electric conductivity, magnetic susceptibi-lity, typically linked to composition) and subsurface struc-ture, or arrangement of different materials (e.g., layering,discrete objects). Our goal is to characterize the structure ofthe subsurface with GPR and to use this GPR characteriza-tion to constrain geological process. While the determinationof subsurface dielectric can inform composition and per-formance of GPR in one environment compared to another(e.g., another planet’s surface) is dependent on relativedielectrics, these aspects of study are not subjects of thisgeologic-process focused work, although they are discussedbelow. Our study’s approach draws on the rationale thatfundamentally different processes (e.g., impact ejection,volcanism,fluival transport) produce characteristic depositswith certain physical properties that are, in some way(s),diagnostic of the process. Investigation and documentation1Center for Earth and Planetary Studies, National Air and SpaceMuseum, Smithsonian Institution, MRC 315, Washington DC, USA.2Department of Earth Sciences and Science Education, SUNY-BuffaloState, Buffalo, New York, USA.3NASA-Goddard Spaceflight Center, Greenbelt, Maryland, USA.4Department of Planetary Sciences, Lunar, and Planetary Lab, Universityof Arizona, Tucson, Arizona, USA.Corresponding author: P. S. Russell, Center for Earth and PlanetaryStudies, National Air and Space Museum, Smithsonian Institution, MRC315, PO Box 37012, Washington DC 20013, USA.