Velocity–resistivity versus porosity–permeability inter-relations in Dead Sea salt samples

Abstract

This study is intended to investigate Dead Sea (DS) salt porosity–permeability (hydraulic conductivity) inter-relations, as well as their relationship to geophysical parameters such as compression and shear wave velocities (Vp, Vs, respectively), and specific resistivity (ρx) in laboratory and field conditions, to estimate representative properties affected by porosity changes during sinkhole development. Some 41 salt samples were subjected to porosity measurements, and 25 samples were tested to reveal hydraulic conductivity. The salt of the coastal Dead Sea area, composed of halite, is revealed in 10,200–10,800year-old layers. The halite commonly appears as rock salt. The 7–30m thick salt unit is located below the groundwater table at depths of 20–50m from the surface. Salt layers are very conductive hydraulically and water-saturated with the DS brine is of very high salinity with total dissolved solids [TDS] of 340–380g/l. Two types of salt have been studied: Crystalline transparent salt (CTS) distributed along the entire DS's western shore and white massive salt (WMS) found locally along the southern DS and along the dike. Salt porosity measured in samples varies in the range of 5–21%, whereas hydraulic conductivity varies between 10−5 to 10–9m/s. Density is changed in the range of 1681–2116kg/m3 in wet samples and between 1638 and 2106kg/m3 in dry samples. During the laboratory testing of DS salt samples, we have measured the following geophysical parameters: Vp=3700–4460m/s, Vs=1900–2620m/s and ρx=2–100Ωm. On the other hand, our previous field studies based on S-wave seismic refraction, surface wave prospecting (SWP) and Transient Electromagnetic (TEM) methods have shown that in in-situ salt velocities Vp range between 2900 and 4240m/s, Vs varies between limits of 750m/s to more than 1650m/s, and ρx is of 0.8 to 2.5Ωm. These values are considerably less than the analogous parameters measured in the salt samples. The same conclusion has been made from the comparison of salt sample properties and borehole logging; both studied by ultrasonic method. Geophysical data testify that in-situ porosity calculated using relationships obtained during this study exceeds those in the laboratory samples, especially in zones of heightened voidness (where it is >25%). Hydraulic conductivities in the same zones exceed a value of 10−6m/s and even 10−5m/s. We expect that in larger salt volumes these parameters can yet increase.