Guy Marquis

Electrical structure of the Himalaya of central Nepal: High conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp.

Analytic potentials for the forward and inverse modeling of SP anomalies caused by subsurface fluid flow

Classical interpretation techniques developed primarily for gravity and magnetic data have been adapted to electric self-potential (SP) data. From a recently developed wavelet-based technique for potential field interpretation, we propose a novel approach for the forward and inverse modeling of SP anomalies caused by subsurface fluid flow. We use analytic signals and wavelets associated with the electric and fluid flow potentials. Fluid flow singularities have typical signatures in the wavelet-domain SP data that can be used in the inversion scheme. For cases where electrokinetics is unambiguously the origin of SP anomalies, our interpretation method provides key parameters of the subsurface fluid flow (e.g. position, geometry, flow rate). To illustrate the theory, we consider synthetic 2D flows for which analytic solutions exist and actual SP data acquired around the Vulcarolo fissure on Mount Etna that show sources between 3 to 18 m depths.

Investigating alluvial and tectonic features with ground-penetrating radar and analyzing diffractions patterns

Ground-penetrating radar (GPR) experiments were conducted on a Quaternary sedimentary (made up of gravel, sand and loess) site to image the structures and tectonic features. Two sets of antennae, 50 and 100 MHz, have been tested in a water saturated alluvial deposits (mostly sand and gravel). The 100 MHz antennae provided adequate penetration depth and allowed better lateral continuity and resolution of the subsurface targets than the 50 MHz antennae. Results show that most of GPR data are contaminated by strong diffraction hyperbolae caused by above-ground objects near the survey line. Therefore, it is very important to recognize the diffractions through air and not to confuse them with the reflections from underground geologic features. Despite the air diffraction problem, the GPR data allow us to prospect subsurface sedimentary and tectonic structures. Water table, channels and meander bars are observed on GPR data. Most of these observations are correlated with borehole and trench data.

Morphologic dating of slowly evolving scarps using a diffusive analogue

Morphologic dating techniques that have been applied successfully in arid and semi-arid areas are also suitable for slowly evolving scarps that are usually found in temperate climate environments. Two approaches, based on diffusion, to relate the present-day shape of a scarp to its age are used here. The first assumes a model of scarp degradation based on a diffusive process (D method). The second evaluates the state of scarp degradation using the slope distribution along a topographic profile (SD method). Based on an anthropic scarp of known age, we have obtained a mass diffusivity close to 1.5 m²/ka, a reasonable value for areas characterized by a continuous vegetation cover and a temperate climate. We show that the SD method requires an age correction, since it does not take into account the earliest stage for which the mid-height point is not yet affected by erosion. However only the SD method reveals that at several profiles a later scarp reactivation event has occurred. Such information can be important when trying to identity the recent geological or environmental events that have affected a region.

Geophysical investigation of antique iron furnaces: insights from modelling magnetic and VLF data

Magnetic and very low frequency electromagnetics (VLF) surveys at an Antique iron mine near Saales (France) revealed several large magnetic anomalies (of the order of a few hundred nT), strong electrical conductivity contrasts (10‐1000 Ω m) and out-of-quadrant VLF phases (φ >90°). To define both the location and the vertical extent of the underground artefacts, we interpret our magnetic data with Euler deconvolution and a 2D wavelet transform that combines maps of the total field and its gradient. We also explain out-of-quadrant VLF phases using simple 3D EM forward modelling. A joint interpretation of magnetic, VLF and topography data enables us to locate iron-ore exploitation trenches, Antique iron furnaces and their surrounding slags at depths between 1 and 3 m, in good agreement with results obtained during a recent road construction.

Monitoring reservoir fluid flow from surface self potential (SP) measurements: application to geothermal reservoirs

Summary We have carried out monitoring of surface Self Potential (SP) variations around the Soultz-sous-Forets Hot Dry Rock site (France) and Berlin Enhanced Geothermal Field (El Salvador) during injection experiments. We have observed long-period (T > 1 day) SP variations with a maximum amplitude of several mV well correlated to the injection phases: this anomalous potential increases during the injection and decreases after shut-in. We show that this behavior is caused by the electrokinetic effect of the injected water flow in deep fractures. We found that the steel well casing channels the electric current toward the surface; this is why we can record surface SP variations even though the electrokinetic sources are several km below. We also found that the surface SP distribution is mainly controlled by the presence of the casing and hence that it is not possible to map the reservoir flow from surface SP signals. SP and borehole pressure data are usually well correlated but they sometimes show discrepancies because pressure sensors are sensitive to the flow in the vicinity of the well while SP is sensitive to the overall reservoir flow. Monitoring of surface SP therefore adds significantly to the information obtained by standard downhole sensors and could be used in the future as a low-cost decision-making tool to monitor the overall dynamics of reservoirs.