IMAGING THE SHALLOW SUBSURFACE FOR LUNAR EXPLORATION USING SEISMIC SURFACE WAVES

Abstract

Ground-based geophysical exploration of the Moon has been identified as high priority science in the next decade to be accomplished by landed geophysical networks, payloads on commercial landers, and with instruments deployed by crews at the lunar surface. In-situ seismic experiments will be important for probing the near subsurface and deep internal structures of the Moon. Previous experiments during Apollo 14, 16, and 17 missions used deployed active seismic instrumentation to study the shallow subsurface structure of the Moon. However, no clearly identifiable surface wave signals could be resolved. This is generally attributed to strong signal attenuation and scattering by complex near-surface structures. Alternatively, the coarse receiver geometries used can cause spatial aliasing of surface wave signals to occur making the fundamental mode difficult to identify. Identifying surface waves and developing methods to extract information from them in the highly scattering environment of the Moon will add a valuable tool for lunar exploration efforts.Here we use a geophysical analog study of surface waves in terrestrial complex media to demonstrate how these waves can be potentially used on the Moon. Active 3-component nodal and vertical component geophone surveys were collected over lava flows in the San Francisco Volcanic Field (SFVF) in order to image the velocity structure of a volcanic lunar analog environment. We use multichannel analysis of surface waves (MASW) and horizontal-to-vertical spectral ratio (HVSR) methods to interrogate and constrain the near-surface shear velocity and volcanic structure of the SFVF. We use reflectivity synthetic modeling to address how spatial aliasing caused by coarse receiver spacings in the SFVF and Apollo 16 surveys can affect our ability to resolve dispersive surface wave signals using traditional dispersion curve methods like MASW. Results from this work address the applicability of active source surface wave methods for probing the complex shallow subsurface in a scattering volcanic media analogous to the lunar near-surface structure with implications for future active source seismic survey design on the lunar surface.