Identification of sinkhole origin using surface geophysical methods, Dead Sea, Israel

Highlights

• Piping and salt dissolution mechanisms of sinkhole formation are considered.

• Potential of geophysical methods is analyzed to distinguish sinkhole origin.

• Geophysical data are applied to Israeli Dead Sea site Newe Zohar (Dead Sea).

• Model of sinkhole formation in the Ghor Al-Haditha is compared with Israel site.

• The underlying voids form primarily by salt dissolution with some cases of piping.

Abstract

The western and eastern Dead Sea (DS) shores are hit by intensive sinkhole collapse during the last 30 years. The first researchers have considered a piping model of sinkhole formation, based on washing out fines by underground flows. Then, it was proved by numerous boreholes and seismic refraction surveys that sinkholes along western shore are caused by dissolution of buried salt layers and collapse of the surface into dissolution caverns. However, signs of piping and subsurface flows were observed in some sinkhole sites. In this paper, we show that robust identification of sinkhole origin can be achieved when proper geophysical methodologies and their application are used. We consider Newe Zohar site located in the southern part of the Dead Sea in order to analyse different signs of sinkhole formation models using various geophysical methods. The Seismic refraction method (SRFR), enables us to discover the salt layer based on longitudinal wave velocity Vp; the Multichannel Analysis of Surface Waves (MASW) method allows to determine the salt layer properties (rigidity) based on shear wave velocity Vs; finally, the Time Electromagnetic (TEM) method allows us to evaluate the degree of aggressiveness of groundwater with respect to salt, based on bulk resistivity values. Here we analyse competitive models of sinkhole formation and suggest geophysical methods to determine the subsurface geomorphology. We show that various geophysical methods should be applied in concert to explore the subsurface for the occurrence of salt, as well as understanding sinkhole formation processes. The underlying voids along the Dead Sea are shown to form primarily by salt dissolution, with some cases of additional piping. Applying the right geophysical parameters for groundwater and salt sediments classification proves to be crucial for understanding the subsurface geomorphology.

Introduction

Intensive sinkhole development along the Dead Sea shores both in Israel and Jordan (Fig. 1) has attracted great attention of the scientific community (Arkin and Gilat, 2000; Yechieli et al., 2006; Legchenko et al., 2008b, Legchenko et al., 2008c; Closson and Abou Karaki, 2009; Frumkin et al., 2011; Abelson et al., 2017; Al-Halbouni et al., 2017; Polom et al., 2018; Arav et al., 2019).

There are two competitive geological models that may explain the Dead Sea sinkhole formation: (a) piping and (b) salt dissolution. The first model in unconsolidated sediments is associated with high gradients of flow, such as the frontal areas of young alluvial fans or high benches of the Lisan Formation (Arkin and Gilat, 2000). These sinkholes are typically funnel-shaped with a surface diameter ranging from 1 to 30 m. The sinkhole depth is commonly not more than 15 m, and downstream from its bottom it may extend tens to hundreds of meters sub-horizontally. Existing flow lines may form the focus for developing sinkholes. Fine particles are washed out along the flow path, followed by the formation of a hollow pipe, or subsurface channel. The process may continue in an upward direction by consecutive breakdown. As the collapsing void approaches the surface, sudden collapse may occur, forming a funnel-shaped sinkhole.

Another type of sinkhole formation model (salt dissolution) requires the presence of (a) a salt layer (lithological factor), (b) under-saturated groundwater flowing in contact with the salt layer (hydrological factor), and (28a) fractures or faults allowing the unsaturated water to flow in contact with the salt layer (tectonic factor) (Yechieli et al., 2006). Frumkin and Raz (2001) discussed two types of salt dissolution underlying the sinkholes. The first is associated with vadose dissolution, as occurs in Mount Sedom salt diapir (Frumkin, 2013). The second is associated with salt dissolution under the watertable along the retreating Dead Sea shore. The underwater dissolution model is accepted presently as the main mechanism of sinkhole formation along the Dead Sea. None of the models, however, fully explains the sinkhole formation mechanism. Pollution of the DS by suspended silt arriving with underground flows has rarely been confirmed by observation in Israel, but has been noted by Taqieddin et al. (2000) in the springs of Wadi ibn Hammad at the Jordanian coast.

Many geophysical investigations in the DS karst areas have been carried out in Israel and Jordan (Shtivelman et al., 1994; El-Isa et al., 1995; Sawarieh et al., 2000; Eppelbaum et al., 2008; Legchenko et al., 2008a; Ezersky et al., 2010, Ezersky et al., 2013a; Bodet et al., 2010; Frumkin et al., 2011; Keydar et al., 2013; Al-Zoubi et al., 2007, Al-Zoubi et al., 2013; Polom et al., 2018). We have shown that the geophysical methods are very sensitive to the anomalies that correspond to changes produced by dissolution and subsidence processes (Ezersky et al., 2006). For example, using high resolution seismic reflection method we detected salt top subsidence. A cave was detected using seismic diffraction imaging (Keydar et al., 2010). Prominent anomalies were detected by microgravity method in the sinkhole sites (Rybakov et al., 2001; Eppelbaum et al., 2008; Al-Zoubi et al., 2013). Comprehensive review of the Dead Sea geophysical investigations is given in Ezersky et al. (2017).

This article is aimed to show how geophysical methods can be used to identify the type and formation process of subsurface voids, causing sinkhole appearance. The Newe Zohar sinkholes site in the southern DS shore of Israel is used as a case study.