RIMFAX GROUND PENETRATING RADAR FIELD TESTS in the WESTERN USA. Patrick S. Russell1, Daniel C. Nunes2, Svein-Erik Hamran3, David A

51st Lunar and Planetary Science Conference (2020) 3012.pdf

RIMFAX GROUND PENETRATING RADAR FIELD TESTS IN THE WESTERN USA. Patrick S. Russell1, Daniel C. Nunes2, Svein-Erik Hamran3, David A. Paige1, Tor Berger3, Hans E. F. Amundsen4, Sverre Brovoll3, Lynn M. Carter5, Henning Dypvik6, Rebecca Ghent7, Jack Kohler8, Michael Mellon9, Dirk Plettemeier10; 1Univ. California Los Angeles, CA, USA, 2Jet Propulsion Lab, CA, USA, 3FFI, Norway, 4Vestfonna Geophysical, Norway, 5Univ. Ar- izona, AZ, USA, 6Univ. Oslo, Norway, 7Planetary Science Inst., AZ, USA, 8Norwegian Polar Inst., Norway, 9Cornell Univ, NY, USA, 10Tech. Univ. Dresden, Germany.

Introduction: The Radar Imager for Mars' subsur- The Flight Model (FM) of the Rimfax antenna and face eXperiment (RIMFAX) ground penetrating radar electronics box is already integrated onto the M2020 (GPR) for the Mars 2020 Rover will image the shallow Rover (Fig). In addition, a qualified Engineering Model subsurface beneath the rover (Fig. 1). A GPR instrument (EQM) is used for tests, and a separate EM is used for can provide subsurface imaging capabilities at sufficient terrestrial field analog studies and performance tests. depth, resolution, and timing to be of operational value Utah Field Sites: In Utah, we chose two of the sites to the rover mission, while also providing valuable geo- because they had previously been studied with GPR [4- logic context. A GPR on the Chinese Lunar rover 6]. Chang’E-3 successfully penetrated several meters into Lone Mesa: The first is around the rim of Lone Mesa the lunar subsurface [1]. The WISDOM GPR is planned [Campground], located just off Utah Rt. 313, between on the ExoMars mission to be launched in 2020 [2, 3]. Arches and Canyonlands Natl. Pks., NW of Moab, UT. Here we report on results from field tests of a RIMFAX Results (Figs. 2&3) show that Rimfax did an excellent prototype. job of picking up the unconformity between the sand- The RIMFAX radar uses a Gated Frequency Modu- stone forming the bulk of the mesa and the overlying lated Continuous Wave (FMCW) waveform operating limestone capping it. from 150 MHz to 1200 MHz. The radar electronics are Modification Based on Field Test: Tests on the housed in a box mounted in the rover body in a temper- smooth, flat, hard surface of the mesa-capping lime- ature controlled zone. The antenna is a slot bow tie an- stone revealed that the 2nd reflection of the surface (ie, tenna mounted outside on the back of the rover. signal that has followed a path of reflections: emitter- RIMFAX will collect soundings every 10 cm along the surface-detector-antenna-detector) occurs in radar- rover traverse. On each location three soundings will be grams at a key depth of interest: ~30-60 ns or ~2-3 m, collected; Surface Sounding where the radar see the re- depending on subsurface dielectric properties. A solu- flection from the antenna and the surface, Shallow tion was proposed, modeled, tested, and implemented Sounding where the surface and the shallow subsurface involving the addition of an attenuator to the electron- is imaged and Deep Mode where the surface is gated out ics-end of the cable to the antenna. This reduces the am- and only deeper reflections are imaged. The full radar plitude of the passing signal by a factor of two times the bandwidth, 150 – 1200 MHz, is used in the Surface and attenuator value of -4 dB, effectively reducing the am- Shallow Modes. A bandwidth from 150 – 600 MHz is plitude of the 2nd surface reflection relative to the true used in the Deep Mode to reduce data volume. Addi- subsurface reflection. tionally, time-lapse soundings will be done while the Coral Pink Sand Dunes: The second site is within rover is stationary to measure the effects of thermal the dune field of Coral Pink Sand Dunes St. Pk., W of changes in the subsurface over the diurnal cycle. Kanab, UT. As previous workers here [4&5] and else- where have noted, dry, well-sorted, quartz sand is an excellent medium for collecting deep GPR data (both we and [4&5] reached ~9-10 m). The medium also reveals fine, distinct structures, namely packets of dune faces preserved as dune movement progresses laterally and vertically over earlier dune deposits (Fig. 4). We were also able to identify and trace the underlying bedrock interface by crossing over an outcrop in an inter-dune swale. [4&5] used the bedrock interface to describe a fault-controlled dune catchment and migration. Tests of

RIMFAX Antenna a classic lithified dune also revealed good penetration RIMFAX Electronics Box and internal structure. Mojave Field Sites: Red Rock Canyon: In the Mo- Fig. 1. Mars 2020 Rover with location of Rimfax. jave Desert of Southern CA, we chose the site of Red 51st Lunar and Planetary Science Conference (2020) 3012.pdf

Rock Canyon St. Pk., along CA Rt. 14, NNW of Mojave References: [1] Xiao et. al (2015) Science 347 (6227), and California City, CA. The varied lithologies and se- p1226-1229. [2] Ciarletti V. et al. (2011) Proc. IEEE, verely tilted beds in the park, as well as access by ATV 99 (5). [3] Ciarletti V. et al. (2017) Astrobiology 17 (6- trails, provided lots of geologic diversity in a small area. 7), p565-584. [4] R.L. Ford, et. al (2010) Utah Geol. Attempts to discern layering within basalts or the base Assn. Pub. 28, D.A. Sprinkel et. Al, eds., p371-398. [5] of thin (2-4 m thick) basalt flows were minimally suc- E.J. Rozar (2015) Boise St. Univ. MSc. Thesis 987, 96p. cessful. The reason is likely related to alteration to ob- [6] O.C. Akinpelu (2010) Univ. Toronto PhD Thesis, served clay minerals. Silica layers up to 1-2 m thick pro- 295p. vided good penetration but revealed no internal structure.

Locat 1 Locat 2

Fig. 2. Rimfax EM radargram along rim of Lone Mesa. Notable underlying layer is contact of overlying inter-dune carbonates [6] and underlying eolian sandstone. Transect is ~6 meters long and ~3 meters deep. Data taken with attenuator in place.

Locat 2 Fig. 3. Outcrop view of Lone Mesa scarp. Overlying limestone cap is more resistant than underlying sandstone. Location 2 is marked both here and in radargram, above, obtained along the mesa edge. Photo-based limestone thickness estimates are ~1.2-1.7 m, consistent with depth of prominent layer in radargram, above.

Fig. 4. Rimfax EM radargram over a large dune at Coral Pink Sand Dunes St. Pk. Transect is ~110 m long; vertical axis is ~ 11 m . Upper portions show fine-scale patterns within dunes; discontinuous, sloping single reflector is likely interface with bedrock.