Christian Camerlynck

Locating water-filled karst caverns and estimating their volume using magnetic resonance soundings

Magnetic resonance sounding MRS is a geophysical technique developed for groundwater exploration. This technique can be used for investigating karst aquifers. Generally, the study ofakarstrequiresa3Dfieldsetupandcorrespondingmultichannel 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 caverns 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 NahalHever,shiftinga3DtargetinsidetheMRSloopandcalculating the volume-estimation errors for each target position. The calculations covered targets of different sizes.The size and position of a target being unknown factors in a field survey, the numerical 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 m3. For such estimation, only one sounding is

Effective medium theories for modelling the relationships between electromagnetic properties and hydrological variables in geomaterials: a review

The paper reviews the effective medium theories used for modelling the relationships between electromagnetic properties (low-frequency conductivity and high-frequency permittivity) and hydrological variables (water content, salinity, suction, permeability) in soils and rocks. It aims a) to provide a simple presentation of these theoretical approaches, b) to present their theoretical and practical limitations and c) to establish some connections with empirical equations usually used in hydrogeophysics (i.e., Archie relationships, Topp equation and complex refractive index model). This review demonstrates that two groups of effective medium theories can be clearly identified. The first group constituted by the Maxwell-Wagner and Symmetric-Bruggeman rules is characterized by easy-to-use models. When the volumetric water content and the texture are known, they allow to obtain fast estimates of effective electromagnetic properties. In the second group, the differential effective medium schemes that are more complex from a mathematical point of view are preferred when frequency-dependent properties are studied. In particular, differential effective medium schemes are used for providing some insights into the physical basis of spectral induced polarization measurements in hydrogeophysical applications.

Differential effective medium schemes for investigating the relationship between high‐frequency relative dielectric permittivity and water content of soils

Received 15 October 2002; revised 27 January 2003; accepted 4 April 2003; published 3 September 2003. [1] Differential effective medium (DEM) theory is presented and used to calculate the effective HF dielectric permittivity k of unsaturated soils considered as mixtures of solid and two fluid phases. In the case of coarse-grained soils for which dielectric losses are negligible, a good agreement with the empirical equation of Topp et al. [1980] is obtained using only a spherical grain shape. Moreover, results of DEM schemes have shown that ohmic losses induced by salinity modify significantly the relationship between the apparent dielectric permittivity at 120 MHz and the volumetric water content q, for NaCl concentration greater than 0.045 mol/L. In the case of clayey soils, simulations based on the same theoretical approach show that the available data can not be modeled by considering only the geometrical effect associated with the ‘‘platy’’ units and the ohmic losses associated with the surface conductivity. INDEX TERMS: 0699 Electromagnetics: General or miscellaneous; 1866 Hydrology: Soil moisture; 5112 Physical Properties of Rocks: Microstructure;

Numerical modeling for investigating the physical meaning of the relationship between relative dielectric permittivity and water content of soils

The relative permittivity of soils is related to the volumetric liquid water content by a monotonous and general relationship. Among others, Topp et al. [1980] have proposed a simple empirical function to allow a direct determination of the water content. The moment method is applied here to calculate the apparent permittivity of a given volume of soil resulting from the electrical coupling between its different constituents and to attempt to reproduce the empirical function. This is not possible when considering randomly isotropically distributed elementary volumes of water, solid, and air. To reproduce the empirical function, it is necessary to consider that as the water content increases, the elementary volumes of water become more elongated since the liquid phase is continuous.

The Dead Sea sinkhole hazard - new findings based on a multidisciplinary geophysical study

A geophysical study has been carried out in the Dead Sea (DS) coastal area of Israel and Jordan with the goal of better understanding the development of sinkholes in the area. The following surface geophysical methods have been applied: (1) Seismic Refraction method for mapping the buried salt formation; (2) Microgravity and Magnetic Resonance Sounding (MRS) for detecting cavities in the subsurface; (3) Transient Electromagnetic method (TEM) for estimating groundwater salinity. It has been found that: (1) sinkholes have formed within a strip 50-100 m wide along the salt formation edge; (2) a zone with a large density of cavities with a total volume of tens of thousands of cubic meters have been inferred in sinkhole sites; (3) cavities underlying sinkholes are filled with unconsolidated sediments locally reducing hydraulic conductivity. Further development of new cavities has not been detected; (4) groundwater salinity variations along the DS shore are insignificant and changes in the electrical conductivity of the groundwater obtained using TEM were in most cases related to changes of porosity caused by collapse of subsurface sediments. (5) the applied geophysical methods provide valuable data on the development of sinkholes in the DS coastal areas.

Direct estimation of the distribution of relaxation times from induced-polarization spectra using a Fourier transform analysis

The analysis of low-frequency spectral induced polarization data involves the determination of the distribution of relaxation times either from time-domain or frequency domain measurements. The classical approach is to assume a simple transfer function (e.g., a Cole-Cole function) and to determine, by a deterministic or a stochastic fitting procedure, the parameters of this transfer function (for instance the four Cole-Cole parameters). Some other methods (based on optimization) have been developed recently avoiding the choice of a specific transfer function that can bias data interpretation. We have developed a new approach based on the Fourier transform also avoiding the use of a specific analytical transfer function. The use of the Fourier transform is a classical approach to retrieve the kernel of a Fredholm integral equation of the first kind (especially in potential field theory) and this corresponds exactly to the problem we want to solve. We adapt the Fourier transform approach to retrieve the distribution of the relaxation times (for instance to process low-frequency induced polarization data). Problems resulting from the use of this approach with noisy data are prevented by using Wiener filtering. As far as induced polarization is concerned, we found that it is necessary to fit the high-frequency dielectric contribution of the spectra and to remove this contribution from the quadrature conductivity data before inverting the distribution of the relaxation times. Our approach is benchmarked with analytical pair solutions and then tested by using synthetic and experimental data sets.

CR1Dinv: A Matlab program to invert 1D spectral induced polarization data for the Cole-Cole model including electromagnetic effects

An inversion code has been constructed using Matlab, to recover 1D parameters of the Cole-Cole model from spectral induced polarization data. In a spectral induced polarization survey, impedances are recorded at various frequencies. Both induced polarization and electromagnetic coupling effects occur simultaneously over the experimental frequency bandwidth, and these become progressively more dominant when the frequency increases. We used the CR1Dmod code published by Ingeman-Nielsen and Baumgartner [2006]. This code solves for electromagnetic responses, in the presence of complex resistivity effects in a 1D Earth. In this paper, a homotopy method has been designed by the authors to overcome the local convergence problem of normal iterative methods. In addition, in order to further condition the inverse problem, we incorporated standard Gauss-Newton (or quasi-Newton) methods. Graphical user interfaces enable straightforward entering of the data and the a priori model, as well as the cable configuration. Two synthetic examples are presented, showing that the spectral parameters can be recovered from multifrequency, complex resistivity data.

Pre-existing caverns in salt formations could be the major cause of sinkhole hazards along the coast of the Dead Sea

Since 1990 hundreds of sinkholes have appeared along the coast of the Dead Sea. In the literature the rapid development of sinkholes is explained as a result of a drop in the level of the Dead Sea. This model assumes very fast dissolution of large volumes of salt and the creation of new caverns that cause sinkholes in 10 to 20 years. However, the results of our geophysical study do not confirm the fast dissolution assumption. To explain the available field observations, we propose the following model: (1) slow dissolution of salt (much longer than 20 years) with the creation of caverns without development of sinkholes; (2) sinkhole development is triggered by the lowering of the groundwater level because the rocks overlying the salt formation become unsaturated; (3) the time of sinkhole appearance is controlled by the mechanical properties of the rocks that overlie pre-existing caverns.

The use of ground penetrating radar for remote sensing the organic layer – mineral soil interface in paludified boreal forests

Black spruce forests that are located in the Clay Belt, within the boreal region of eastern North America, are prone to paludification. Paludification is a natural process where organic layer accumulates on the forest floor, leading to substantial decreases in forest productivity. This study assessed the ability of using ground penetrating radar (GPR) to remotely sense the organic layer – mineral soil (OL–MS) interface (representing organic layer thickness (OLT)), which has a major influence on the occurrence of paludification in this region. The two chosen sites for this study represented different types of soil and organic layer thicknesses that are linked to different degrees of paludification: low to moderately paludified (site A) and highly paludified (site B). At each site, GPR measurements were collected along three 40 m parallel transects at 20 cm intervals with 200 MHz antenna. GPR interpretations were compared with field manual probing measurements. Detection of this continuous interface was successful at site A (r = 0.93, P < 0.001), but mesic and humic horizon clay content limited radar depth penetration, rendering the OL–MS undetectable at site B. However, we found that GPR data, coupled with ground truth information, were effective in mapping the thickness of the organic fibric horizon (r = 0.79, P < 0.001) at site B, which could be considered as an indicator of the OLT in highly paludified areas. Overall, GPR appeared effective for mapping the OL–MS interface in the low to moderately paludified site, which is attractive for implementing forest management strategies that will help to stop the advance of paludification.

Evidencing a large body of ice in a rock glacier, Vanoise Massif, Northern French Alps

Abstract The achette rock glacier is an active rock glacier located between 2660 and 2480 m a.s.l. in the anoise assif, orthern rench lps (45° 29′ , 6° 52′ E). In order to characterize its status as permafrost feature, shallow ground temperatures were monitored and the surface velocity measured by photogrammetry. The rock glacier exhibits near‐surface thermal regimes suggesting permafrost occurrence and also displays significant surface horizontal displacements (0.6–1.3 ± 0.6 m yr–1). In order to investigate its internal structure, a ground‐penetrating radar survey was performed. Four constant‐offset profiles were performed and analyzed to reconstruct the stratigraphy and model the radar wave velocity in two dimensions. Integration of the morphology, the velocity models and the stratigraphy revealed, in the upper half of the rock glacier, the good correspondence between widespread high radar wave velocities (>0.15–0.16 m ns–1) and strongly concave reflector structures. High radar wave velocity (0.165–0.170 m ns–1) is confirmed with the analysis of two punctual common mid‐point measurements in areas of exposed shallow pure ice. These evidences point towards the existence of a large buried body of ice in the upper part of the rock glacier. The rock glacier was interpreted to result from the former advance and decay of a glacier onto pre‐existing deposits, and from subsequent creep of the whole assemblage. Our study of the achette rock glacier thus highlights the rock glacier as a transitional landform involving the incorporation and preservation of glacier ice in permafrost environments with subsequent evolution arising from periglacial processes.