The Dead Sea hazard: geophysical assessment of salt sinkholes hazard in Israel

A geophysical approach is presented for analyzing processes of subsurface salt dissolution and associated sinkhole hazard in the Dead Sea. The implemented methods include Seismic Refraction (SRFR), Seismic Reflection (SRFL) and Diffraction (SDF) methods, Transient Electromagnetic method (TEM), Electric Resistivity Tomography (ERT), and Ground Penetration Radar (GPR). The combination of these methods allows the detection of the faults and other
scattering objects in the subsurface, delineation of the salt layer boundaries, estimating its porosity distribution, finding caves within the salt layer, and identifying deformations in the overlying sediments. The geophysical methodology has been recently modified in framework of the International Project sponsored by MERC program (USAID).
The zero-offset Common Shot Point (CSP) stacking and diffraction imaging method were rewritten using a weighted multipath summation technique. The diffraction method serves as a tool for detection of faults and voids, while the CSP stacking contains information about the structure of the subsurface.
Knowing the salt layer interface location we can separate the study of salt and no-salt areas. It allowed avoiding of 3D effects near the salt – aquifer border. In such interpretation TEM method allows the fast spacious mapping of the bulk resistivity through great areas. The resistivity is interpreted in terms of groundwater salinity based on resistivity-salinity calibrations generated for the DS region. New methodology allows also 2D and 3D presentation of acquired results although the methodology itself of data acquisition is 1D in its origin.
GPR method permits the detailed detection of subsurface deformations preceding collapse of surface, a long time before it will happen.
This approach is shown to be useful for anticipating the occurrence of specific sinkholes, as demonstrated on both shores of the Dead Sea. These sinkholes at present are observed mainly along the edge of a salt layer deposited during the latest Pleistocene, when Lake Lisan receded to later become the Dead Sea. This salt layer is dissolved by aggressive water flowing from adjacent and underlying aquifers which drain to the Dead Sea. This process is accelerating today
due to the rapid fall of Dead Sea levels during the last thirty years, caused by anthropogenic use of its water.
The geophysical methodology was applied to study the Shalem-2 area. Results will be discussed during the presentation.