Shallow Microtremor Array Survey Using Miniature and Small Arrays: Strategy for Efficient and Feasible Dense Survey.

We investigated the applicability of miniature microtremor arrays with a radius of a few meters or less to shallow surveys (up to a few tens of meters). It is shown that the upper limit wavelength normalized by the seismometer separation distance s that is analyzable by a miniature array does not depend on the observation instrument as long as the instrument has a self‐noise level that is sufficiently lower than the microtremor intensities; however, it generally depends on the average S‐wave velocity of the ground. This means that a miniature array is useful at soft‐soil sites but not hard‐soil sites. A statistical study in central Japan showed that the penetration depth by a miniature array with s = 1 m ranges from 6 to 12 m; specifically, the depth exceeds 12 m in one quarter of the cases but remains below 6 m in one quarter of the cases. This large variation is due to the variation in the average S‐wave velocity. A miniature array should thus be used in combination with a larger array with an s value of several meters to 20 m. In urbanized areas with high industrial activity, where the microtremor wavefield is likely isotropic, these arrays can be replaced by a linear array. In environments with extremely low signal‐to‐noise ratios, a less efficient zero‐crossing method should be applied to the larger array. An appropriate observation strategy should be selected for a given geoenvironment. Field examples show that our strategy enables efficient and feasible dense surveys. Plain Language Summary: A microtremor array survey is a non‐destructive, passive method for subsurface exploration. Because such a survey is also easy to conduct and cost‐effective, it is widely used for seismic hazard assessments. It is expected that the benefits can be extended to lunar and planetary exploration. We demonstrate that a miniature microtremor measurement device with a radius of a few meters or less can be used for shallow surveys. The performance of the miniature measurement device does not depend on the observation instrument. This device is effective at soft‐soil sites but not hard‐soil sites. To increase survey feasibility, miniature measurement devices should be used in combination with a larger measurement device. In ground environments with extremely low signal‐to‐noise ratios, larger measurement devices and somewhat less efficient methods must be applied. It is important to select an observation strategy that is appropriate for a given geological environment considering the balance between the efficiency and feasibility of a shallow survey. Key Points: The applicability of a miniature array with a radius of a few meters or less to shallow surveys is reviewed and summarizedThe applicability of a miniature array depends on the average S‐wave velocity of the ground but not on the observation instrumentIllustrative field examples of our strategy, which enables a dense survey to obtain a shallow two‐dimensional velocity section, are shown [ABSTRACT FROM AUTHOR]