Signal et bruit en magnetotellurique
L. Chaize and M. Lavergne
Journal name: Geophysical Prospecting
Issue: Vol 18, No 1, March 1970 pp. 64 - 87
Info: Article, PDF ( 1.58Mb )
Conventional processes of extracting magnetotelluric signals from noisy records are reviewed: instrument noises and noises that are generated close to the detectors can be eliminated by the usual auto- and crosscorrelation processes. Identification of coherent noises, such as pulses due to field sources that are not uniform over at least 100 km in oil exploration or 1000 km in crustal studies, is much more tedious.
The 5 components Hx, Hy, Hz, Ex, Ey, of the magnetotelluric field have been recorded in many areas in France at different periods of the year, (a) in non-uniform field sources in the vicinity of electric railways and of 50 cycle power lines, and (b) in areas of strong inhomogeneity at depth on the flanks of steep structures and near the sea shore.
Means for detecting non-uniformity are reviewed. Measuring the vertical component of magnetic pulses is a good way of estimating field uniformity: if H vertical/H horizontal <10%, the uniform field assumption is valid, and the classical restitution formulas can be used; if H vertical/H horizontal > 10%, uniformity can not be assumed and there is some difficulty in deciding whether non-uniformity is due to the field source or to anisotropy or inhomogeneities at depth. Several ways to solve this difficulty are described.
The reliability of calculation of actual resistivity at various depths is examined as a function of the precision of apparent resistivity measurements.