Quantitative Assessment of In-situ Salt Karstification Using Shear Wave Velocity, Dead Sea
Introduction
The Dead Sea (DS) coastal areas of Israel and Jordan have been dramatically affected by sinkhole formation since around 1990 (Fig. 1). This development continues today, with an obvious potential for further collapse beneath roads, dwellings and in the potash works. Most researchers accept that such sinkholes near the shorelines are caused by salt dissolution (karstification) by under-saturated groundwater (Frumkin and Raz, 2001, Abelson et al., 2006, Yechieli et al., 2006, Legchenko et al., 2008a). Our hypothesis proposes that sinkholes form along a dissolution front over the edge of a buried salt layer (western edge in Israel and eastern edge in Jordan). This hypothesis is based on seismic refraction surveys carried out mainly along the western DS shore (Ezersky, 2006, Ezersky et al., 2010) and in Ghor Al-Haditha (El-Isa et al., 1995, Abueladas and Al-Zoubi, 2004).
Following the dissolution front salt edge concept, we have suggested a general model of the salt layer deposited around the DS. This model (Fig. 2) suggests that there is a 10–30 m-thick buried salt belt around much of the DS shore, controlling the sinkhole hazard (Legchenko et al., 2008b, Ezersky and Frumkin, 2013). All known cases of collapses and accidents including the alarming catastrophe at pond 18 in Jordan (Closson, 2005) have occurred within this salt belt. The model presented in Fig. 2 enables us to conclude that: (1) the dissolution front follows an ancient shoreline which existed at the stage of salt unit deposition (10.2–10.8 ka) – modern sinkholes are formed along this dissolution front; and (2) the buried salt layer extends from the modern Dead Sea shoreline landward, towards the dissolution front, permitting its investigation from the surface.
The working hypothesis is that circulating groundwater dissolves salt, causing a gradual increase of salt porosity and permeability. This increase, connected with the evolution of pore space during salt dissolution, was suggested by Bernabé et al. (2003). Shalev et al. (2006) applied this mechanism to DS salt dissolution, demonstrating that porosity and permeability are important factors in both salt dissolution and sinkhole development.
In our previous works, a geophysical methodology was proposed for sinkhole-hazard evaluation based on (1) salt-edge delineation by seismic refraction, and (2) the spatial distribution of bulk resistivities (ρx) and shear-wave velocities in salt (Vs). The mapping of ρx is based on the Transient Electromagnetic (TEM) method (Frumkin et al., 2011), whereas Vs is mapped by the Multichannel Analysis of Surface Wave (MASW) method (Ezersky et al., 2013a). Both studies showed that geophysical parameters are minimal in the 60–100 m zone (dissolution front) next to the salt edge, but that they gradually increase toward the DS reaching a stable value after some 100–200 m from the salt edge (Frumkin et al., 2011, Ezersky and Frumkin, 2013, Ezersky et al., 2013a). This shows that salt karstification is greatest closest to the salt edge and that it gradually decreases toward the DS. Another objective is to correlate the geophysical parameters with a quantitative evaluation of salt porosity and hydraulic conductivity, thus providing a reliable indication of sinkhole hazard. Hereafter, we will consider only the in-situ relationships between shear-wave velocity (Vs) and hydraulic conductivity (K).
It is known that shear-wave velocity is closely related to the porosity and permeability of rocks (Nelson, 1994, Schoen, 2004). Earlier work by Ezersky and Goretsky (2013) (presently manuscript is under review for possible publication in Engineering Geology) found correlative relationships between porosity and permeability, and velocities for DS salt samples saturated with brine. However, there is a problem in extrapolating laboratory-based relationships to field conditions. Comparison of ultrasonic velocities (Vp, Vs) measured on borehole samples in the laboratory with data from ultrasonic logging in the same boreholes, has shown that in-situ salt is characterized by significantly lower velocities than salt samples in the laboratory selected from the same depth. Both porosity and permeability values are thus higher in the borehole. It is suggested that hydraulic conductivity values in a salt layer will also be greater, because Vs velocities are significantly lower than those in laboratory salt specimens. To extrapolate the Vs and K parameters measured in samples to the field scale (100 m2 lateral size), we used the field parameters measured on the Nahal Hever South (NHS) site (Site II in Fig. 1) using MASW and Magnetic resonance Sounding (MRS), also known as Surface Nuclear Magnetic Resonance (Legchenko et al., 2011, Legchenko, 2013, Song, 2013), which is a geophysical application of the Nuclear Magnetic Resonance (NMR) phenomenon.
The MASW method is known as an efficient tool for measuring Vs (Miller et al., 1999, Park et al., 1999, Xia et al., 1999). It is especially effective when the object is located below the groundwater table, because Vs is slightly sensitive to groundwater filling the pores in rock. It is simple to undertake a MASW survey in the field and it is relatively cheap. The MRS method allows in-situ measurements of hydraulic conductivity (K) values for comparison with those measured by pumping tests (Legchenko et al., 2008a, Nielsen et al., 2011, Knight et al., 2012, Vouillamoz et al., 2012a). However, the method is very sensitive to electromagnetic noise and is most suitable for measuring relatively high hydraulic conductivity values (Costable and Yaramanci, 2011).
The main goal of the present study is to extrapolate relationships between the hydraulic conductivity (K) and shear wave velocities (Vs) measured on salt specimens in the laboratory, to the same parameters measured in salt under field conditions. A classification of the salt conditions is suggested, based on Vs values. Finally, the obtained relationships are applied for generating a hydraulic conductivity (K) map of the Mineral Beach sinkhole development sites, in order to evaluate sinkhole hazard.
Section snippets
Salt of the Dead Sea coastal area
The salt from the coastal Dead Sea (DS) area is about 10.2–10.8 ka old (Yechieli et al., 1995, Stein et al., 2010). The Early Holocene halite layer was deposited in the DS area during an extremely arid period in the shrinking phase between Lisan Lake and the present Dead Sea (Stein et al., 2010). The salt unit lies below the groundwater table at a depth of 20–50 m below surface and is 6–30 m thick. Salt layers are very conductive hydraulically (Yechieli et al., 1995), and are saturated with highly
Surface-wave methods
Surface-wave dispersion inversion (SWDI) is a standard approach for inferring a 1D Vs structure for global earth seismology, near-surface geophysics, and geotechnical and civil engineering applications (Nazarian and Stokoe, 1984, Rix and Leipski, 1991, Park et al., 1999, Socco and Strobia, 2004). Surface waves, commonly known as ground roll, are always generated in all seismic surveys, have the strongest energy, and their propagation velocities are mainly determined by the medium's shear wave
Nahal Hever South site and layouts of geophysical studies
To generate correlative Vs–K relationships, we have used results from the MRS, MASW and TEM methods jointly carried out during 2005–2007 on the Nahal Hever South (NHS) site (Site II on Fig. 1). The MRS measurements were carried out at NHS by Legchenko et al., 2007, Legchenko et al., 2008a, Legchenko et al., 2008b. Fifteen MRS soundings were made on the site during 2005, and 23 100 × 100 m2 MRS soundings were made in 2007. The MRS station location is shown in Fig. 3 (Legchenko et al., 2007,
Discussion
The inter-relationship Vs–K presented in Fig. 11 consists of two different components: K values at large Vs values are measured by direct laboratory testing, wherein velocity was measured by the ultrasonic pulse method. K values at low shear-wave velocities are based on K measured by the MRS method (e.g. KMRS) and converted to the pump test-based parameter Kpump by means of an optimal calibration coefficient Cp = 7 ∗ 10− 9, wherein Vs was derived from MASW measurements. Thus, the graph in Fig. 11 is
Conclusions
A quantitative estimation of in-situ salt karstification can provide an important indicator of sinkhole hazard. One indication of salt karstification is its increased hydraulic conductivity, caused by the development of dissolution cavities forming conducting channels within the salt. We aimed to investigate the Dead Sea (DS) salt hydraulic conductivity (K) versus shear-wave velocity (Vs) in the field, for estimating the real salt properties in sinkhole development sites. These parameters were
Acknowledgements
This work was made possible through support provided by the U.S. Agency for International Development, under the terms of Award No M27-050. We thank the Ministry of Energy and Water Resources of Israel for supporting this investigation. Y. Hazor, A. Salamon, and E. Livne are thanked for helping with the geologic interpretation of borehole data. We are grateful to W. Wittke for fruitful communications during our work on the Project. We thank M. Asten and a second anonymous reviewer for the
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