Geophysical investigation of underground cavity in Bimah Sinkhole, Northern Oman

Abstract

The purpose of the study is to investigate the ability of geophysical methods to detect an exposed karstic cavity in Northern Oman. The geophysical methods examined involve Seismic Refraction Tomography (SRT), Ground Penetrating Radar (GPR), and Electrical Resistivity Tomography (ERT). The three surveys were conducted over a line crossing the cavity. Each method profile was processed and analyzed apart. The resulting model from SRT has shown horizontal stratification similar to the trend of the layers exposed on surface. However, the data revealed a noticeable drop in velocity immediately above the cavity. This was associated to the presence of the air-filled cavity. In the processed GPR image, we have noticed strong hyperbolic reflections that have disturbed the continuity of the horizontal features. These strong reflections have been interpreted to be a response to the large contrast in electric and dielectric properties between the cavity and its surrounding. Below the strong reflections, the amplitude has undergone severe attenuation which might be associated to the water present in the karstic cavity. Moreover, the resistivity tomography section has depicted anomalously high resistivity responses that are very common characteristic of air-filled voids in subsurface. The refraction tomography, GPR, and resistivity images have responded in different ways to the presence of the cavity. Each method has revealed a characteristic property of it. The integration of the three methods has led to a confident detection and characterization of the cavity. To conclude, the three geophysical methods have demonstrated their ability to detect the targeted cavity. These promising results have opened the door for further exploration for cavities that might be hidden in the area.