Abstract
The estimation of the earth transfer functions in MT prospecting method poses the greatest difficulty. As in the seismic prospecting method this task requires the development of advanced processing techniques. In order to assess the performance of each technique, controlled synthetic data and different noise types, which simulate the observed signals, are required. This paper presents a procedure to generate a wide-band noise-free electromagnetic field to be used both for magnetotelluric and audio-magnetotelluric studies. Furthermore, an effort was made to extend the simulation procedures to generally stratified and simple inhomogeneous earth structures. The discrete-time magnetic field values are generated through the inverse Fourier transform of a continuous amplitude spectrum and a sampling procedure. The electric field time series are obtained by the convolution of the magnetic field time series, calculated in the interested frequency band, with a non-causal impedance impulse response. Polarized fields, which are important when inhomogeneous media are considered, are also generated.
Highlights
Computer simulation of electromagnetic field components has frequently been used for testing estimation techniques of the magnetotelluric impedance tensor (Goubau et al, 1978; McMechan and Barrodale, 1985; Yee et al, 1988; Larsen et al, 1996) and for assessing preprocessing methods of extraction of the stationary and coherent part of signals corrupted by noise
The present paper describes an approach to generate wide-band electromagnetic time series
The electric field time series is calculated by convolving in time the magnetic field time series with the earth impulse response (eq (2.13))
Summary
Simple pseudo-random numbers were used to represent the noise-free random components of the time series (Lamarque, 1999) or the real and imaginary parts of the incident magnetic field components (Goubau et al, 1978) The latter authors calculated the electric field spectra using a bi-dimensional impedance tensor made by simple complex relationships. San Filipo and Hohmann (1983) and Yee et al (1988) assume a function which tends to zero exponentially with increasing values of frequency to represent the main spectral characteristics of the natural magnetic field, simulating this behaviour through recursive digital filters These same authors generated the magnetic time series by convolving a pseudo-random number sequence with the filter coefficients. The results were applied to 1D and simple non 1D structures
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