[P-PS05] Lunar science and exploration

[PPS05-P07] Development of a system to easily investigate underground structures for space exploration using a miniature seismic array

*Marina Orita¹, Tatsunori Ikeda¹, Takeshi Tsuji¹ (1.Kyushu University)

Keywords:microtremor exploration, Centerless circular array method, Moon Exploration

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 strongly required. In the Moon exploration, it is difficult to bring heavy equipment to the Moon because its weight and size are limited on the spacecraft. Therefore, we developed a small extraterrestrial exploration system based on the microtremor survey without a seismic source. Microtremor survey is an exploration method using ambient noise for S-wave velocity estimation. Ambient noise persistently occurs on the Earth mainly due to wind, ocean waves, and transportations. On the Moon, we can use thermal moonquakes as one of the ambient noise sources. Moreover, if we use the Centerless Circular Array (CCA) method to analyze ambient noise, we can investigate deep underground structures using a small seismometer array. This study evaluates the investigation depth for each arrangement of small seismometer arrays on the Moon. When we use the CCA method, the investigation depth can be evaluated from the given NS ratios (the power spectral ratio of the incoherent noise to the coherent signal), the number of seismometers, the array radius, and the dispersion curve of the field. Since the NS ratio depends on (1) the performance of seismometers and (2) the environment where ambient noises are generated, we estimated the NS ratio using seismometer data of Apollo 14 and 17. We defined two types of signals, (A)thermal moonquakes in the distant area and (B)daytime ambient noise without any detectable events. Noise level is also defined from ambient noise at nighttime without any detectable events. 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) (dots in Fig. 1a). Maximum detectable wavelength (or minimum detectable wavenumber) was estimated from the minimum wavenumber whether the wavenumber k based on the lunar data (Tanimoto et al.,2008;black line in Fig. 1a) exceeds the minimum wave number k_min. We then estimated investigation depth for each array arrangement by calculating one-third of the maximum wavelength (Fig. 1b). Finally, we summarized the points and issues to use the CCA method for exploration on the Moon, considering appropriate array arrangement and noise level to analyze the desired depth. For example, our evaluation demonstrates that we can investigate from surface to 3m depth by using the pentagon-type array with 0.3m radius or the triangle-type array with 0.5m radius.

Acknowledgments: We used Apollo lunar seismic data from Data Archives and Transmission System (DARTS) at JAXA.