Integrated study of the sinkhole development site on the Western shores of the Dead Sea using geophysical methods

This paper presents a combination of high-resolution seismic diffraction, reflection, refraction, continuous vertical electric sounding (CVES), ground-penetrating radar (GPR) and microgravity methods to study the structure and properties of the shallow subsurface at the sinkhole development site in the Nahal Hever southern area (on the shore of the Dead Sea) in Israel. These methods have different resolutions and penetration depths. The methods are complementary to each other and reveal useful information about the subsurface. Each one investigates certain characteristics, but when combined they provide us with new information about the subsurface. The seismic refraction method is utilized to map salt layers and to detect dissolved zones. The analysis of all of the seismic refraction data obtained at the Dead Sea area, verified by the information gained from a borehole, has shown that the P-wave velocity of the salt layer varies from 2900 to 4250 m/s. The velocity of 2900 m/s was used as the minimum velocity criterion for salt-layer identification at this site. A low-velocity zone ( V P V P = 2900–4250 m/s) is interpreted as a dissolved salt or non-saline unit. The site is characterized by two significant geological features. The first feature is a water-filled cave, detected recently by a borehole in the depth interval 23–28.5 m, and the second feature is a depression at the surface. Geophysical results show that the depression site is presently characterized as a heavily perturbed zone in both lateral and vertical directions. Soil decompaction in the subsurface is clearly identified by the CVES method as a high-resistivity anomaly. This decompaction is assumed to be associated with the voids and fractures occurring down to a depth of 100–120 m (by diffraction data) and in the uppermost subsurface (by GPR data). The zone containing the cave is characterized by a lateral (funnel) and vertical (faults) distortion of the subsurface structure (by seismic reflection data) and has a faint diffraction anomaly in the shallow subsurface. A slightly increased resistivity anomaly appears in the depth interval 6–12 m (from CVES data). Drilling data reveal this interval as a decompaction zone. No GPR anomalies were detected at the buried cave site. The microgravity survey at the buried void site in 1999 detected an anomaly that is very similar to the funnel-shaped one.