Mathieu Darnet

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.

Detecting hydrocarbons in carbonates: joint interpretation of CSEM and seismic

Quantitative interpretation (QI) of seismic data has been successfully used to predict reservoir properties such as porosity and fluid fill. In specific cases, however, adding resistivity estimates of the subsurface to the QI can reduce ambiguities in the properties prediction. Controlled-source electromagnetic (CSEM) sounding methods provide these estimates. By making use of their complementary nature, integration of seismic and electromagnetic datasets has been used to estimate the height of hydrocarbon-filled porous carbonate. In this paper, we present a methodology for joint interpretation, which can help us to discriminate tight carbonates from porous carbonates, and to differentiate between brine and hydrocarbon fill. When applied to a field in a carbonate setting, the estimated hydrocarbon column height corresponds qualitatively with the saturation height estimate based on production data.