Miniature seismometer array system for Lunar underground structures investigation: Evaluation of its exploration depth based on Apollo seismometer data

Abstract

　Currently, water on the Moon surface is considered a potential resource. Furthermore, we need to know geological information to build base camps on the Moon. Therefore, estimating the S-wave velocity of the underground of the Moon has been crucial. In the Moon exploration, it is difficult to bring heavy and large-sized equipment because of the spacecraft’s limited capacity. Therefore, we investigated the applicability of a small extraterrestrial exploration system based on the microtremor survey without a seismic source. This study evaluates the investigation depth for each arrangement of small seismometer arrays on the Moon when analyzing microtremor data by the Centerless Circular Array (CCA) method. The result of the CCA method depends on a given NS ratio (the power spectral ratio of the incoherent noise to the coherent signal), the number of seismometers, and an array radius. Therefore, we estimated the NS ratio on the Moon using seismometer data of Apollo 14, 15, 16, and 17. Using the estimated NS ratio, the number of seismometers and the array radius, we calculated the minimum detectable wavenumber k_min at each frequency by the approach of Cho et al. (2006). The minimum wavenumber k' – its lunar data-based wavenumber k_obs exceeds the minimum detectable wavenumber k_min – estimated the maximum detectable wavelength. We then calculated one-third of the maximum wavelength to estimate the investigation depth for each array arrangement. First, we analyzed the earth data using the CCA method and confirmed that the investigation depth could be evaluated from the three parameters and the observed dispersion curve. Second, we analyzed Apollo data and evaluated the investigation depth on the Moon. Finally, we summarized applicability of the CCA method for the Moon explorations, considering appropriate array arrangement and noise level to analyze the desired depth. For example, our evaluation demonstrates that we can investigate from a surface to 3 m depth by the pentagon-type array with 0.3 m radius or the triangle-type array with 0.5 m radius. Thus, our evaluation approach helps design seismic arrays in lunar exploration projects in the future.