Partially saturated soil causing significant variability in near surface seismic signals

ABSTRACT The behaviour of dry, moist, and saturated soils has been studied for over a century without ade quately investigating the behaviour associated with transient saturation in the near surface, i.e. the upper 1 m of overburden, including the effects of rapid meteorological changes, dynamic fluid flow, and variability of saturation on shallow seismic sensors. This paper presents observational data wherein the geophysical instrumentation response was significantly influenced by near‐surface post‐precipitation saturation and additional laboratory experimentation on the effects of satu ration on shear wave velocity. The lack of partially‐saturated data is primarily because transient meteorological events have not been critically important to the types of long‐term deployments performed in the past, where sensors were situated in hard‐rock, collecting data under idealized conditions, as opposed to sedimentary settings. Shorter‐duration deployments and smaller system architectures, e.g. persistent monitoring, now necessitate detailed a priori knowledge of meteoro logical impacts to system design and performance. The purpose of this persistent monitoring geophysical instrumentation is to continually monitor the near surface and relate small perturbations to a specific source type(s) and distance(s) from the receiver. As such, the received signal is compared to known sources within a predetermined geo logical/meteorological condition. Presented herein is the calibration signal generated by a 3.63‐kg (8‐lb) sledgehammer prior to and post 36 hours of steady precipitation. The resulting subsurface seismic velocity time‐histories show a significant increase in signal amplitude, change in fre quency content and no change in duration. Thus, the amplification effects of near‐surface moisture variability combined with dynamic pore fluid could be interpreted as false positives of a specific source signature and/or instrument failure.