Using in situ vertical displacements to characterize changes in moisture load

Abstract

Changes in soil moisture content alter the load on underlying material, and we have developed a technique for characterizing this effect by using an extensometer to measure the displacement caused by the load change. The extensometer is pushed into soil at depths of 5 m or more, and displacement between two anchors separated by 1.5 m is measured with a resolution of better than 0.01 lm (10 28 m). The instrument is sensitive to load changes at the ground surface within a radial distance that is roughly twice its depth, potentially providing a method for averaging changes in water content over hundreds of m 2 or more. During a field trial at a site in South Carolina, compressive displacements in unsaturated saprolite were strongly correlated to rainfall with a calibration factor of 0.16 lm displacement per mm of rainfall 60.002 lm/mm (R 2 5 0.95). Estimates of the net change in water volume per unit area made using the calibration factor from rainfall were similar to independent estimates of evapotranspiration. The technique was affected by barometric pressure variations, but the sensitivity was less than expected and does not hinder meaningful application. A companion instrument demonstrated the displacement signal was repeatable.

Deformation‐Based Monitoring of Water Migration in Rocks Using Distributed Fiber Optic Strain Sensing: A Laboratory Study

“…It is well known that fluid injection can lead to an increase in pore pressure in a reservoir, a reduction in the effective stress, and a dilation deformation of reservoir rocks. Owing to this fluid-to-solid coupling (Wang, 2017), measuring rock deformation could provide information on the pore pressure and fluid migration (Barbour & Wyatt, 2014;Murdoch et al, 2015). The magnitude of a hydromechanical deformation is dependent on the fluid pressure change and rock stiffness.…”

In situ hydromechanical responses during well drilling recorded by fiber-optic distributed strain sensing

“…Although the main purpose of DSS is the monitoring of geomechanical deformations or earth subsidence (for safety considerations) (Kogure and Okuda, 2018;Krietsch et al, 2018;Murdoch et al, 2020;Zhang et al, 2018), DSS could also be used to understand reservoir formation and reservoir Published by Copernicus Publications on behalf of the European Geosciences Union. flow owing to hydromechanical coupling.…”

“…Deformation-based reservoir monitoring methods have been recently applied to obtain the lateral permeability distribution (at coarse scales) of underground reservoirs with surface deformation monitored by InSAR technique (Bohloli et al, 2018;Vasco et al, 2008Vasco et al, , 2010 and estimate the vertical compressibility with vertical deformation measured by well-based techniques (e.g., radioactive maker technique and extensometer stations) (Ferronato et al, 2003;Hisz et al, 2013;Murdoch et al, 2015). However, such vertical deformation monitoring tools are usually only available at limited points and over limited time intervals.…”

Understanding the Hydromechanical Behavior of a Fault Zone From Transient Surface Tilt and Fluid Pressure Observations at Hourly Time Scales

“…Relative deformation instruments like tiltmeters or extensometers have emerged as interesting tools for groundwater hydrology (Barbour & Wyatt, 2014;Chen et al, 2010;Fabian & K€ umpel, 2003;Longuevergne et al, 2009;Murdoch et al, 2015;Vasco et al, 2001). This is because they are able to record accurately and precisely small deformations at high sampling rates compared to spaceborne methods, up to 1 Hz.…”