2D sections of porosity and water saturation from integrated resistivity and seismic surveys

Abstract

Porosity and degree of saturation – or the water content – are important parameters for hydrogeological, geotechnical and environmental studies. Geophysical methods, especially the resistivity method, are routinely employed to study the spatial variations of these parameters. Resistivity is highly influenced by the presence of water in pore spaces and hence is well suited for studying the presence of fluids on a site and its saturation condition. However, the non‐uniqueness of the solution of resistivity models has led to the joint use of more than one geophysical method in order to reach more accurate geophysical models. In this research, we combined resistivity and seismic refraction profiles. The integration of these methods was particularly aimed to obtain 2D sections for estimating the porosity, water saturation and volumetric water content rather than to obtain a better geophysical solution.Independently inverted resistivity and P‐wave refraction sections are the input data to an iterative analysis process using simulated annealing. Empirical equations taken from the literature are used to relate both the seismic velocity and the electrical resistivity with porosity and water saturation. Several parameters, such as the resistivity of water and clay; the velocity of water, clay, air and matrix; clay percentage and Archie’s parameters remain constant throughout the process. The values considered for each parameter are derive from both the literature and laboratory measurements.Resistivity and seismic refraction profiles were performed on a site located within the LNEC campus. At this site, field measurements of void ratio and volumetric water content were performed at different depths. Soil samples were also collected at three different depths in order to perform laboratory measurements of these parameters and to determine soil composition. The laboratory results were compared with the 2D sections of each parameter. The proposed approach was also applied to two other locations with different and well characterized geology. These tests also allowed us to characterize the dependency of the clay content on the resistivity.This research has potential fields of application in environmental studies, in particular, the determination of probable pathways of pollutants; in hydrological investigations, where it can be useful to transport of nutrients studies; and in geotechnical studies, where, for example, it will be able to give a continuous image of the saturation degree of an embankment.

Subsurface plant‐accessible water in mountain ecosystems with a Mediterranean climate

“…Total integrated porosity, in cm, quantifies the depth-integrated volume-per-unit-area (i.e., the effective thickness) of void space in the subsurface below any given 1 cm 2 area on the land surface. Similar total porosity data for the 2700 m site is not available An alternative approach for estimating total porosity is to use near-surface geophysical methods, specifically P-wave velocities obtained from shallow seismic refraction surveys, in conjunction with a modeled rock-physics relationship (Bachrach & Nur, 1998;Holbrook et al, 2014;Mota & Santos, 2010). P-Wave velocities are controlled by lithology, porosity, fracture density, and the fluid in the pore space (Andersen & Johansen, 2010;Dvorkin, Mavko, & Nur, 1999;Nur, Mavko, Dvorkin, & Galmudi, 1998).…”

Estimating the water holding capacity of the critical zone using near‐surface geophysics

“…The rock physics model defines a relationship between a geophysical property (i.e., seismic velocity, resistivity, or dielectric permittivity) and a hydrologic property of interest (i.e., porosity or hydraulic conductivity). Previous studies have estimated porosity using electrical resistivity (Mota & Monteiro Santos, ; Turesson, ), ground penetrating radar (Rehman, Abouelnaga, & Rehman, ; Turesson, ), and seismic refraction (Hayes, ; Holbrook et al, ; Mota & Monteiro Santos, ). The demand of spatially exhaustive geophysical measurements and the procurement of data required to validate the porosity estimates is one reason why porosity estimates of the CZ over large spatial scales are uncommon.…”

2D characterization of near‐surface : surface‐wave dispersion inversion versus refraction tomography

“…; Othman ). More recently, this approach has also been proposed for hydrological applications to characterize shallow aquifers (Grelle and Guadagno ; Mota and Monteiro Santos ; Konstantaki et al . ; Pasquet et al .…”

“…It is classically carried out for engineering purposes to determine the main mechanical properties of reworked materials in active landslides (Godio et al 2006;Jongmans et al 2009;Socco et al 2010b;Hibert et al 2012), control fill compaction in civil engineering (Heitor et al 2012;Cardarelli et al 2014), study weathering and alteration of bedrock (Olona et al 2010), or assess earthquake site response (Jongmans 1992;Lai and Rix 1998;Raptakis et al 2000;Othman 2005). More recently, this approach has also been proposed for hydrological applications to characterize shallow aquifers (Grelle and Guadagno 2009;Mota and Monteiro Santos 2010;Konstantaki et al 2013;Pasquet et al 2015).…”