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.

The behaviour of near‐surface soils through ultrasonic near‐surface inundation testing

“…Moreover, Taylor et al . (, ) show the potential for shear modulus and stress‐strain behaviour variability as a function of gravimetric water content counter to the assumption of Biot–Gassmann theory, the refraction models assumptions by Bachrach and Nur (,b), and other research wherein a single soil behaviour, e.g. poro‐visco‐elastic behaviour, or fixed value moduli are assumed (Bachrach, Dvorkin and Nur ; Barriere et al .…”

“…) inundate the soil through groundwater table rise, which fails to replicate the dynamic inundation of the ground surface from a brief rain event, as observed by Taylor et al . (). Moreover, Taylor et al .…”

“…Unlike the traditional seismic array deployments discussed above, semi‐permanent ‘operational’ seismic arrays deployed for extended periods of time are often situated within near‐surface soils, not hard rock or other more competent material, and are therefore subject to the meteorologically sensitive behaviour low‐confinement soils can exhibit (Taylor et al . ).…”

“…Moreover, Taylor et al . () observed a significant shift in the shear wave velocities along all principle axes, wherein the soil fabric went from an anisotropic state to an isotropic state signifying a complex realignment of granular stresses. Fratta et al .…”

“…Taylor et al . () observed that near‐surface post‐rain signals are amplified up to 14.7‐fold over the pre‐rain signals in in situ sand testing, with significant shifts in spectral content and shear wave arrival times at low to negligible confining stresses. Moreover, Taylor et al .…”