Evidence of the magnetotelluric 3-D inversion ability to account for static shift effects: A comparison between MT and heliborne TEM

2pWithin the framework of a global French program towards development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-disciplinary approach, with the aim to identify precisely the potential geothermal resources previously highlighted (Gadalia et al., 2014). Among the investigation methods deployed (geological, geochemical and hydrogeological), we carried out three magnetotelluric (MT) surveys above three of the four most promising areas of Martinique, namely the Anses d'Arlet, the Montagne Pelée and the Pitons du Carbet prospects. A total of about 100 MT stations were collected in the frequency range 1000 Hz to 10-2 Hz together with TEM soundings for potential static shift correction. 3-D MT inversions of the full tensor were run on each sector which took into account the coast effect. Assuming that the static shift observed is the result of near-surface lateral inhomogeneity causing electric field distortion, it can be accounted for by the inversion scheme used by Hautot et al. (2000, 2007) and no prior static correction was applied before inversion. The model results show that most of the soundings displaying static shift effect are accurately reproduced by near-surface resistivity distribution in the model. However it is generally difficult to prove that the shallow structures explaining the static distortion obtained by the MT inversion are real features at the scale of the survey. Here we consider additional high resolution geophysical data to overcome this problem. We demonstrate with the help of the results from the recent heliborne TEM survey covering the whole Martinique (marTEM project) that the sub-surface resistivity distribution obtained from 3-D MT inversion reproduces faithfully the resistivity distribution observed by TEM data (Figure 1). Data of two of the three investigated sectors (Anses d'Arlet and Mount Pelée geothermal provinces) are presented because there is a sufficient spatial density of MT soundings. To compare both datasets, TEM data were first homogenized over the investigated area by krigging the resistivity distribution by layer. The median resistivity of this new grid is then extracted above each cell of the MT model and compared to the MT resistivity. Cells that do not contain TEM information or are located too far away from the fly lines are removed from the statistics. The results show that 82% of the cells considered have a maximum resistivity variation below 400% for the Anses d'Arlet prospect and 79 % for the Montagne Pelée. Thus despite a widely different sampling scale between the two sets of data (53 MT soundings,> 20000 TEM for the Mount Pelée and 32 MT soundings, >6200 TEM), the comparison between the two near surface resistivity models illustrates the ability of our inversion scheme to take into account and reproduce static shift effects

Fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential

Electric self-potential (SP) variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the Roselend and La Gittaz reservoir lakes in the French Alps. The lakes have level variations of at least 50 m over yearly cycles. Seasonal variations of SP associated with lake-level variations are observed on five points of the array. For three points located on the banks of the lakes, a positive correlation to the lake-level variations is observed with a maximal amplitude of about 180 mV, corresponding to an average response of 2.4 mV per meter of water. For two points located on the bottom of each lake, the correlation is negative, with a maximal magnitude of about −50 mV, corresponding to an average response of −1.1 mV per meter of water. Two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed SP variations. In particular, near the Roselend lake, the electrical response to lake-level variations is increasing for decreasing altitude. The measured SP variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes. Numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water. The values of the streaming potential coefficient (SPC), measured in the laboratory with crushed rock samples from the site, range from 14 to 50 mV/0.1 MPa for an electrolyte resistivity of 40 Ω m and are compatible, to first order, with the magnitude of the observed seasonal SP variations. A detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the SPC. This experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity

A new petrological and geophysical investigation of the present-day plumbing system of Mount Vesuvius

Article in PressInternational audienceA model of the electrical resistivity of Mt. Vesuvius has been elaborated to investigate the present structure of the volcanic edifice. The model is based on electrical conductivity measurements in the laboratory, on geophysical information, in particular, magnetotelluric (MT) data, and on petrological and geochemical constraints. Both 1-D and 3-D simulations explored the effect of depth, volume and resistivity of either one or two reservoirs in the structure. For each configuration tested, modeled MT transfer functions were compared to field transfer functions from field magnetotelluric studies. The field electrical data are reproduced with a shallow and very conductive layer (~0.5km depth, 1.2km thick, 5ohm.m resistive) that most likely corresponds to a saline brine present beneath the volcano. Our results are also compatible with the presence of cooling magma batches at shallow depths (~100ohm.m. According to a petro-physical conductivity model, such a resistivity value is in agreement either with a low-temperature, crystal-rich magma chamber or with a small quantity of hotter magma interconnected in the resistive surrounding carbonates. However, the low quality of MT field data at long periods prevent from placing strong constraints on a potential deep magma reservoir. A comparison with seismic velocity values tends to support the second hypothesis. Our findings would be consistent with a deep structure (8-10km depth) made of a tephriphonolitic magma at 1000°C, containing 3.5wt%H2O, 30vol.% crystals, and interconnected in carbonates in proportions ~45% melt - 55% carbonates

3-D magnetotelluric inversion with coast effect modeling to assess the geothermal potential of Anses d'Arlet (Martinique, Lesser Antilles)

2pWithin the framework of a global French program towards development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-disciplinary approach, with the aim to identify precisely the potential geothermal resources previously highlighted (Gadalia et al., 2014). Among the investigation methods deployed (geological, geochemical and hydrogeological), we carried out three magnetotelluric (MT) surve ys at three of the four most promising areas of Martinique, namely the Anses d'Arlet, the Montagne Pelée and the Pitons du Carbet prospects. A total of 32 MT tensors were collected in the Anses d'Arlet area in the frequency range 1000 Hz to 10-2-10-3Hz togetherwith TEM soundings for potential static shift correction. A 3-D MT inversion of the full tensor was carried out including the coast effect. The 3-D resistivity model reveals a major resistive body elongated in the NNO -SSE direction, corresponding to the main structural volcanic axis of the area, and interpreted as a deep intrusion almost reaching the surface. Based on geological observations -an alteration zone located between Anses d'Arlet and Petite Anse-the shallow conductive layer identified eastward is interpreted as the remaining of an old cap-rock partly eroded that becomes thicker southwards. The latter could be related with the altered core of the Morne Larcher. Other studies allowed the reconstructing of the geothermal system evolution to its present and moderate apparent activi ty mainly located south of the resistive anomaly. This intrusion could act as heat source for the geothermal system. This sector is identified as the most interesting for further exploration wells. In order to better understand structures highlighted by the real data distribution and test the strategy to integrate correction coefficients for the coast effect (modeled separately) during the inversion, we designed a forward model using the s ame bathymetry, topography and MT sounding distribution of the survey. Impedance tensors were calculated for the 32 sites. The geometry of the 3-D structure has been designed quasi-independently of the data distribution to assess the impact of un-appropriate acquisition grid. The 3-D inversion was run with these synthetic data (Hautot et al, 2000, 2007). The results indicate that the general structure is recovered for the first 1000 m. Deeper, and southward, the absence of MT soundings surrounding the resistive body prevented a good lateral constraint, causing its disappearance. To the north, the conductive body "pseudo caprock" is well defined, especially on layer 5 (Figure 2), were the apex of a supposed geothermal reservoir could be identified (sites 3 and 6). Deeper the eastern border remains very well constrained by the eastern MT synthetic soundings

3-D Magnetotelluric Investigations for geothermal exploration in Martinique (Lesser Antilles). Characteristic Deep Resistivity Structures, and Shallow Resistivity Distribution Matching Heliborne TEM Results

Within the framework of a global French program oriented towards the development of renewable energies, Martinique Island (Lesser Antilles, France) has been extensively investigated (from 2012 to 2013) through an integrated multi-methods approach, with the aim to define precisely the potential geothermal ressources, previously highlighted (Sanjuan et al., 2003). Amongst the common investigation methods deployed, we carried out three magnetotelluric (MT) surveys located above three of the most promising geothermal fields of Martinique, namely the Anses d'Arlet, the Montagne Pel{\'e}e and the Pitons du Carbet prospects. A total of about 100 MT stations were acquired showing single or multi-dimensional behaviors and static shift effects. After processing data with remote reference, 3-D MT inversions of the four complex elements of MT impedance tensor without pre-static-shift correction, have been performed for each sector, providing three 3-D resistivity models down to about 12 to 30 km depth. The sea coast effect has been taken into account in the 3-D inversion through generation of a 3-D resistivity model including the bathymetry around Martinique from the coast up to a distance of 200 km. The forward response of the model is used to calculate coast effect coefficients that are applied to the calculated MT response during the 3-D inversion process for comparison with the observed data. 3-D resistivity models of each sector, which are inherited from different geological history, show 3-D resistivity distribution and specificities related to its volcanological history. In particular, the geothermal field related to the Montagne Pel{\'e}e strato-volcano, is characterized by a quasi ubiquitous conductive layer and quite monotonic typical resistivity distribution making interpretation difficult in terms of geothermal targets. At the opposite, the resistivity distribution of Anse d'Arlet area is radically different and geothermal target is thought to be connected to a not so deep resistive intrusion elongated along a main structural axis. Beside these interesting deep structures, we demonstrate, after analyzing the results of the recent heliborne TEM survey covering the whole Martinique, that surface resistivity distribution obtained from 3-D inversion reproduce faithfully the resistivity distribution observed by TEM. In spite of a very different sampling scale, this comparison illustrates the ability of 3-D MT inversion to take into account and reproduce static shift effects in the sub-surface resistivity distribution.Comment: Wordl Geothermal Congress 2015, Apr 2015, Melbourne, Australi

Modeling temporal variations of electrical resistivity associated with pore pressure change in a kilometer-scale natural system

International audienceFrom 1995 to 1998 the natural electric field was monitored with an array of 20 dipoles on a ridge separating two reservoir lakes in the French Alps. The experiment was run to study the correlation between the electric potential variations and transient deformations of the ridge in association with the annual cycle of lake level variations. Large distortion of the induced electric field is observed and is found almost purely static and well correlated to the geology. A simple DC 3-D model is constructed, and resistivity structures that create the distortion are identified. The electrically resistive crystalline bedrock strongly amplifies the static distortion caused by the heterogeneous geology on the ridge. The temporal variations of the electric distortion observed over two years are associated with the lake level cycle. The model suggests that a resistivity variation of the order of 20% in the bedrock can account for the observed seasonal time-varying distortion. The resistivity change could be explained in terms of pore and crack geometry change controlled by stress. This study suggests that in particular geological contexts, electrical resistivity changes in structures can be detected through an amplification of the static distortion of the induced electric field. The results provide a framework to interpret some observations of electric field variations possibly associated with tectonic activity. The galvanic coupling model proposed here is an alternative to the streaming potential effect model, and it defines new criteria for the surface detection of groundwater in the crust

Effective electrical conductivity of 3-D heterogeneous porous media

International audienceThe relationship between the physical properties and the effective electrical conductivity of porous structures is studied using three-dimensional (3-D) models of random porosity. Synthetic electric (DC) and electromagnetic (EM) field data are calculated for a 3-D electrical conductivity model with a random porosity p (p = 2 - 70%) embedded in a homogeneous half-space. The effective conductivity of the random porosity model is obtained from inversion of the synthetic data and agrees with a modified Archie's law. We applied percolation theory to our random porosity model to explain the variation of effective conductivity with p. We found that EM and DC data do not provide the same effective conductivity at a particular porosity but they do provide the same volume fraction of interconnected conductive elements. This volume fraction depends on the percolation threshold pc. It follows a law of the form ~p above pc and ~p2.2 below pc