Cross-Plot Analysis By Using Rock Physics-Based Thresholds For An Evaluation Of Unsaturated Soil

Cross-plot of resistivity and S-wave velocity (Vs) is one of the effective methods to evaluate earth structure, e.g. river levee, because both properties are relatively easy to be obtained by geophysical methods such as capacitive resistivity survey and surface wave method. From the cross-plot of the two geophysical properties, we can interpret the soil types and looseness based on the observations that resistivity and Vs are sensitive to the grain size and looseness of the soils, respectively. For example, dividing the cross-plot by two individual thresholds for resistivity and Vs is the easiest way to interpret the data; however, setting appropriate thresholds is the most challenging issue in this methodology. In order to overcome this problem, in this study, I propose the way to determine the objective thresholds by using rock physics models. Particularly, the effect of water saturation is significant to interpret shallow subsurface; thus I modify the parallel circuit model for the resistivity prediction to account for the saturation effect. For the Vs prediction, I apply the soft sand model with considering clay content. These rock physics models enable us to connect the two properties by two common parameters: porosity and clay content. Both parameters are included in the models. Therefore, if we assume that porosity and clay content are proxies for looseness and grain size respectively, we are able to select certain values of porosity and clay content as the thresholds to discriminate the data indicating vulnerable area in the cross-plot. Preliminary analysis shows that the segmentation done by the rock physics-based thresholds gives a reasonable result that agrees with a previous study and the geophysical dataset obtained at various river levees.