A geophysical survey has been performed in the area of Litoměřice (Czech Republic) where a geothermal exploration drilling has been performed in the Bohemian massif in 2000. Measurements have been performed from 6 to 11th September by BRGM and Geomedia staff, with the logistic help of Litoměřice city. The survey aims at measuring electrical resistivity at depth to image the main geological features underground until few kilometers in urban areas with the challenging problem of cultural noise.
Because the first method (MT) can be severely affected by electromagnetic (EM) noise induced by anthropogenic activities such as cities and industries and DC trains such as in Czech Republic (and many regions in Europe), CSEM (active controlled-source electromagnetics) and MT (passive method, natural sources) data were both collected to be able to provide a resistivity imaging of the area. 27 CSEM stations and 7 collocated MT stations have been recorded.
The CSEM survey has been conducted using surface and borehole high power emissions and two pseudo-orthogonal polarizations. A set of frequencies ranging between 0.125 Hz to 128 Hz has been selected to investigate the medium. Results provide apparent resistivity maps at various frequencies and for both polarizations reflecting the geological variations from north to south.
MT robust signal processing has been performed using synchronous recordings performed locally (30 km) and at far (> 500 km) remote stations. Data were processed using advanced robust techniques in each possible configurations (3 remote reference stations and single site processing). As the ambient noise and MT sources are not stationary, a time-lapse approach based on an error threshold was used to select the best MT estimates. MT soundings were obtained in the [7mHz-1kHz] frequency band (such as station 06 shown on Figure 1). Their quality will be discussed in the presentation.
SUMMARY Time-lapse resistivity tomography bring valuable information on the physical changes occurring inside a geological reservoir. In this study, resistivity monitoring from controlled source electromagnetics (CSEM) data is investigated through synthetic and real data. We present three different schemes currently used to perform time-lapse inversions and compare these three methods: parallel, sequential and double difference. We demonstrate on synthetic tests that double difference scheme is the best way to perform time-lapse inversion when the survey parameters are fixed between the different time-lapse acquisitions. We show that double difference inversion allows to remove the imprint of correlated noise distortions, static shifts and most of the non-linearity of the inversion process including numerical noise and acquisition footprint. It also appears that this approach is robust against the baseline resistivity model quality, and even a rough starting resistivity model built from borehole logs or basic geological knowledge can be sufficient to map the time-lapse changes at their right positions. We perform these comparisons with real land time-lapse CSEM data acquired one year apart over the Reykjanes geothermal field.
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.
Exploring for underground resources using land-based electromagnetic methods can be very challenging due to the presence of strong human-generated and geological noise. In such context, some passive EM techniques like the Magneto-Telluric method may not be applicable at all and a dedicated toolbox of EM techniques capable of dealing with these issues is required. We focus here on the challenges encountered while exploring for geothermal resources in volcanic islands, namely a highly heterogeneous near-surface creating strong static-shifts, the presence of a sea/land transition zone and a high degree of urbanization over the area of interest. We will show that a combined airborne and land-based controlled-source EM approach provide a robust approach and illustrate these aspects with an EM dataset acquired over the Martinique Island, in the French Lesser Antilles. We believe this work sets the foundations for more robust geophysical exploration workflows in such challenging EM environments. Introduction Land-based electromagnetic (EM) methods, attempting to detect contrasts in electrical resistivity between target resources and their surroundings, have been developed and utilized for exploring buried resources such as minerals, hydrocarbons, geothermal energy, or groundwater for more than a century (Streich, 2016 and reference therein). Noise has complicated EM recordings ever since the first EM measurements were made. Various types of noise have been recognized (Szarka, 1988): human-generated noise emitted, as examples, by the power grid, power plants, railways, pipelines, industrial and agricultural facilities. Large metallic bodies, such as well casings, can also strongly alter EM field behavior locally. Magnetotelluric signal is regarded as noise in the context of active EM surveying. Instrument noise limits measurable EM field levels. Finally, subsurface heterogeneity like bodies at depth, small-scale structure near the sources or receivers not resolvable by the measurement technique used, can mask target responses and hence be considered as geological noise (Streich, 2016). In order to explore for underground resources in a large variety types of environments (e.g. urbanized areas, mountainous terrains, volcanic areas, sedimentary basins), we have developed a comprehensive toolbox of EM techniques capable of dealing with such challenging EM environments. In this paper, we will focus on the challenges encountered while exploring for geothermal resources in volcanic islands, namely a highly heterogeneous near-surface, the presence of seawater and a high urbanization over the areas of interest. We will show that combined airborne and land-based controlled-source EM methods provide a robust approach and illustrate these aspects with an EM dataset acquired over the Martinique Island, in the French Lesser Antilles.
ABSTRACT We propose here a new, robust, methodology to estimate the errors on a magnetotelluric (MT) impedance tensor. This method is developed with the bounded influence remote‐reference processing (BIRRP) code in a single site configuration. The error is estimated by reinjecting an electric field residual obtained after the calculation of an impedance tensor into a tensor function calculation procedure. We show using synthetic examples that the error tensor calculated with our method yields a more reliable error estimate than the one calculated from Jackknife statistics. The modulus of realistic error estimates can be used as a quality control and an accurate inversion constraint of MT surveys. Moreover, reliable error estimates are necessary for new applications of MT to dynamic subsurface processes such as reservoir monitoring.
