Abderrezak Bouchedda

Sferics noise reduction in time-domain electromagnetic systems: application to MegaTEMII signal enhancement

Two noise reduction techniques are proposed for the removal of sferics noise from airborne transient electromagnetic data. Both techniques use multi-resolution analysis via a stationary wavelet transform of the data. The analysed signal is divided into several successive lower resolution components. The transient character of the sferics can be seen as high amplitudes of the wavelet detail coefficients close to the time of the sferics event. The first noise reduction strategy, named the wavelet extraction technique, identifies sferics in the first detail coefficients using an energy detector. The corresponding detail coefficients are set to zero, and the electromagnetic signal is reconstructed by inverse transform. This technique is very robust and successful both for on-time and off-time data and even in the case where several sferics are present. However, when sferics occur near the switch on or the switch off times of the airborne electromagnetic transmitter signal, or if the low frequency components of the spheric are very high, this technique becomes less effective. To overcome this problem, the second strategy, named the wavelet stacking technique, uses the shift invariance and linearity of the stationary wavelet transform to perform data stacking in the wavelet domain. Tests on synthetic data results show that the wavelet stacking technique performs better than the mean and median stacking techniques. The wavelet extraction and median stacking present equivalent performance. On very noisy real data, the wavelet stacking technique makes the detection of weak anomalies more straightforward. After additional smoothing by filtering, wavelet extraction and median stacking can produce similar results to wavelet stacking. However, the amplitude and temporal decay of anomalies can be affected by high residual sferics noise. The wavelet extraction technique has the advantage that it can be used to extract sferics for an audio frequency magnetic-like method to map subsurface conductivity changes. When a large number of sferics are observed, the current practice is to stop data acquisition; these techniques allow data collection to continue.

Assisted traveltime picking of crosshole GPR data

We introduce two new traveltime picking schemes developed specifically for crosshole ground-penetrating radar (GPR) applications. The main objective is to automate, at least partially, the traveltime picking procedure and to provide first-arrival times that are closer in quality to those of manual picking approaches. The first scheme is an adaptation of a method based on cross-correlation of radar traces collated in gathers according to their associated transmitter-receiver angle. A detector is added to isolate the first cycle of the radar wave and to suppress secon-dary arrivals that might be mistaken for first arrivals. To improve the accuracy of the arrival times obtained from the crosscorrelation lags, a time-rescaling scheme is implemented to resize the radar wavelets to a common time-window length. The second method is based on the Akaike information criterion(AIC) and continuous wavelet transform (CWT). It is not tied to the restrictive criterion of waveform similarity that underlies crosscorrelati...

Iron Grades Estimation Using Bayesian Sequential Simulation - Lalor Case Study.

Summary This work presents the first step of the use of a Bayesian sequential simulation algorithm to estimate the iron grades of Lalor VMS deposit. The approach is based on an in situ petrophysical relationship between iron grades and both conductivity and density. This statistical relationship is used to interpolate and extrapolate the iron grades over a 3D conductivity and density volume. Furthermore, this stochastic algorithm allows estimating the spatial uncertainty among a set of equiprobable realizations.

Numerical Feasibility Study for Borehole Magnetometric Resistivity Monitoring of CO2 Storage in Deep Saline Reservoirs

In this paper, we study the efficiency of borehole magnetometric resistivity method for the detection and qualitative monitoring of CO2 plume evolution in deep saline aquifers. We have considered two configurations of source electrodes. The numerical simulations show that surface configuration allows following the lateral plume extension with Bz and By components, with y orthogonal to the injection dipole. In the surface-borehole configuration, all components are sensitive to the evolution of the plume but the amplitude is not necessarily correlated to its extension.

Time Domain EM Soundings for the Delineation of the Fresh Water - Salt Water Interface in Eastern Canada Islands

A hydrogeophysical study involving electromagnetic and electric methods to map depths to the saltwater-freshwater interface into coastal aquifers was carried out. Time domain electromagnetic (TDEM) soundings were acquired along electric resistivity tomography (ERT) profiles perpendicular to the coastline in the Magdalen Islands (Eastern Canada). Departures from hydrogeological models allowed evaluating the influence of water pumping on the studied aquifers. As a result, except in zones closer to the pumping wells, a good agreement between the hydrogeological model and the position of the interface estimated by the coupling of TDEM and ERT is observed.