Locating water-filled karst caverns and estimating their volume using magnetic resonance soundings A. Legchenko 1 , M. Ezersky 2 , C. Camerlynck 3 , A. Al-Zoubi 4 , K. Chalikakis 3 , and J-F. Girard 5 ABSTRACT Magnetic resonance sounding MRS is a geophysical tech- nique developed for groundwater exploration. This technique can be used for investigating karst aquifers. Generally, the study of a karst requires a 3D field setup and corresponding multichan- nel data-acquisition instruments. Now only single-channel MRS equipment is available; i.e., the time needed for a 3D MRS field survey is multiplied by a factor of four or five. Where karst cav- erns are natural hazards, as in the Dead Sea coastal area at Nahal Hever, Israel, even an approximate localization of potentially dangerous zones and a corresponding estimation of the hazard dimensions are useful. We studied numerically the accuracy of MRS estimations of the volume of different 3D targets around Nahal Hever, shifting a 3D target inside the MRS loop and calcu- lating the volume-estimation errors for each target position. The calculations covered targets of different sizes. The size and posi- tion of a target being unknown factors in a field survey, the nu- merical data were considered as random values to be analyzed statistically. Using a 1D approximation of the MRS solution and assuming a 100-  100-m 2 MRS loop, the volume of a 3D target under Nahal Hever conditions is estimated within a 75% error when the target is smaller than the MRS loop, and within a 50% error when the target size is about the same as the MRS loop. The lower threshold of karst-cavity detection with MRS is about 6500 m 3 . For such estimation, only one sounding is required. INTRODUCTION To investigate and address problems related to sinkholes in a karst environment, the location of karst caverns must be known. Drilling commonly is used for detecting karst, but the challenge of drilling alone lies in the probability of detection. When covering an area of significant size, the drilling costs rapidly become unacceptable. Thus, a balance must be found between the cost of obtaining infor- mation and the probability of detecting features of importance. Geo- physical methods can identify anomalous zones within which a drill- ing program can be targeted. Similarly, geophysics can be used to in- terpolate between boreholes and ensure that the drilling has not missed features of potential concern. By using geophysical methods, the probability of detecting features of interest is increased while re- ducing, or stabilizing, the drilling costs. When investigating karst, surface geophysical methods are se- lected in the context of existing consensual standards. The American Society of Testing and Materials ASTM publishes a guide for se- lecting surface geophysical methods ASTM International, 2006. For assessing sinkholes and voids, ASTM recommends seismic re- fraction, electrical DC, electromagnetic frequency and time do- main, ground-penetrating radar, and gravity methods. Survey ob- jectives, site constraints from interference, and rock properties dic- tate the final investigation method. In addition to commonly used geophysical techniques, we now propose the magnetic resonance sounding MRS method. MRS, a recently developed geophysical technique for groundwater investi- gations, is specifically sensitive to groundwater and can reliably identify karst aquifers Vouillamoz et al., 2003. The linear depen- dence of the MRS signal upon water quantity in the subsurface al- lows estimation of the water volume in karst caverns. MRS is a Manuscript received by the Editor 11 January 2007; revised manuscript received 31 January 2008; published online 26 September 2008. 1 Institut de Recherche pour le Développement, Laboratoire d’étude des Transferts en Hydrologie et Environnement, Grenoble, France. E-mail: anatoli.legtchenko@hmg.inpg.fr. 2 Geophysical Institute of Israel, Lod, Israel. E-mail: mikhail@gii.co.il. 3 Université Pierre et Marie Curie, Paris, France. E-mail: camerl@ccr.jussieu.fr; chalikak@ccr.jussieu.fr. 4 Al-Balqa’Applied University, Engineering Faculty, Salt, Jordan. E-mail: aalzoubi@go.com.jo. 5 Bureau de Recherches Géologiques et Minières, Orléans, France. E-mail: jf.girard@brgm.fr. © 2008 Society of Exploration Geophysicists. All rights reserved. GEOPHYSICS, VOL. 73, NO. 5 SEPTEMBER-OCTOBER 2008; P. G51–G61, 15 FIGS., 1 TABLE. 10.1190/1.2958007 G51