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

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

Report of a workshop on technical approaches to construction of a seafloor geomagnetic observatory

This report considers the technical issues on sensors, data recording and transmission, control and timing, power, and packaging associated with constricting a seafloor geomagnetic observatory. Existing technologies either already in use for oceanographic purposes or adapted from terrestral geomagnetic observatories could be applied to measure the vector magnetic field components and absolute intensity with minimal development. The major technical challenge arises in measuring absolute direction on the seafloor because terrestral techniques are not transferrable to the deep ocean. Two solutions to this problem were identified. The first requires the development of an instrument which measures the instantaneous declination and inclination of the magnetic field relative to a north-seeking gyroscope and the local vertical. The second is a straightforward extension of a precision acoustic method for determining absolute position on the seafloor.Funding was provided by the National Science Foundation under grant EAR94-21712 and the National Aeronautics and Space Administration

TIARES Project -Tomographic investigation by seafloor array experiment for the Society hotspot

http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr08-06_leg1/

Electrical structure beneath the northern MELT line on the East Pacific Rise at 15°45′S

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 33 (2006): L22301, doi:10.1029/2006GL027528.The electrical structure of the upper mantle beneath the East Pacific Rise (EPR) at 15°45′S is imaged by inverting seafloor magnetotelluric data obtained during the Mantle ELectromagnetic and Tomography (MELT) experiment. The electrical conductivity model shows no evidence for a conductive region immediately beneath the ridge, in contrast to the model previously obtained beneath the EPR at 17°S. This observation can be explained by differences in current melt production along the ridge, consistent with other observations. The mantle to the east of the ridge at 60 –100 km depth is anisotropic, with higher conductivity in the spreading direction compared to the along-strike direction, similar to the 17°S region. The high conductivity in the spreading direction can be explained by a hydrated mantle with strain-induced lattice preferred orientation of olivine or by partial melt preferentially connected in the spreading direction.This work was supported by NSF grant OCE0118254

Magnetotelluric observations across the Juan de Fuca subduction system in the EMSLAB project

A magnetotelluric (MT)transect has been obtained near latitude 45øN from the active Juan de Fuca Spreading center, across the subduction zone and Cascades volcanic arc, and into the back arc Deschutes Basin region. This paper presents the MT data set and describes its major characteristics as they pertain to the resistivity of the subduction system. In addition, we discuss the measurement and processing procedures employed as well as important concerns in data interpretation. Broadband audiomagnetotelluric (AMT)/MT soundings( approx. 0.01-500 s period) were collected on land with considerable redundancy in site location, and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle. Fifteen long-period MT recordings (about 50-10,000 s) on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers. On the Juan de Fuca plate offshore, 33 out of 39 sea floor instruments at 19 locations gave good results. Of these locations, five magnetotelluric soundings plus two additional geomagnetic variation sites, covering the period range 200-10^(5) s approximately, constitute the ocean bottom segment of our profile. The feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30-50 s period. This phase anomaly, with a corresponding inflection in the apparent resistivity, is continuous eastward from the seacoast and ends abruptly at the High Cascades. It signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle, with the layer conductance decreasing eastward from the coast to a minimum under the Coast Range but increasing suddenly to the east of the central Willamette Basin. The higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100-1000 s over the same distance. The transverse electric mode apparent resistivity and phase on the land display a variety of three-dimensional effects which make their interpretation difficult. Conversely, both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge, indicating that the measurements here are reflecting primarily the large-scale tectonic structures of interest and are little disturbed by small near-surface inhomogeneities. The impedance data near the ridge are strongly suggestive of a low-resistivity asthenosphere beneath resistive Juan de Fuca plate lithosphere. Approaching the coastline to the east, both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater