A method is described for filtering magnetotelluric (MT) data in the wavelet domain that requires a minimum of human intervention and leaves good data sections unchanged. Good data sections are preserved because data in the wavelet domain is analyzed through hierarchies, or scale levels, allowing separation of noise from signals. This is done without any assumption on the data distribution on the MT transfer function. Noisy portions of the data are discarded through thresholding wavelet coefficients. The procedure can recognize and filter out point defects that appear as a fraction of unusual observations of impulsive nature either in time domain or frequency domain. Two examples of real MT data are presented, with noise caused by both meteorological activity and power-line contribution. In the examples given in this paper, noise is better seen in time and frequency domains, respectively. Point defects are filtered out to eliminate their deleterious influence on the MT transfer function estimates. After the filtering stage, data is processed in the frequency domain, using a robust algorithm to yield two sets of reliable MT transfer functions.

The MT interpretation procedure begins with a set of sounding data in the frequency domain. The overall quality of these data can be variable both as a function of frequency and location. Many simple interpretation procedures, such as the assessment of static distortion, act directly on the sounding data. A number of response characteristics, such as the location (in frequency) and number of turning points, are important to the interpretation. Localised scatter (noise) in the response estimates can produce false gradients which degrade the quality of the inferences made from the data.This study considers how the D+ solution can be used to process the raw sounding data to provide a number of interpretational advantages. Although the D+ solution has strict formal roots in I D inverse theory, it is used here simply to enhance those data attributes, particularly that of physical validity, which lead to a more meaningful assessment of data characteristics. The data considered are 84 broadband array soundings from the Parana basin, Brazil. The advantages provided by the D+ processed data set are demonstrated by using the raw and processed data in two main interpretational procedures. The first procedure concerns the ability of the data to provide quantitative assessments of the influence of static distortion. The second procedure concerns the application of transform methods which attempt to recover a resistivity I depth or reflectivity profile directly from the sounding data.

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Magnetotelluric (MT) data collected simultaneously at one or more sites may be processed by a number of different methods. Such methods attempt to remove or suppress the effect of noise on the data channels. The desired results are accurate, unbiased and repeatable estimates of the impedance tensor as a function of frequency and location. In this study we perform an investigation of the analysis of an MT data set sampled at 5s. Both single-site (SS) and remote-reference (RR) techniques are employed to estimate the impedance tensor 2. TWO biased SS estimates of 2 are used to compare the performance of five coherencebased acceptance criteria. It is demonstrated that the RR predicted coherence between local fields can be used for selecting data windows, and provides a necessary assessment of the reliability of a given RR estimate. It is demonstrated that the variance of an RR estimate depends strongly on the local signal-to-noise ratios (as monitored by the local predicted coherence) and depends weakly on the number of data windows, as long as coherences are above a moderate threshold. Although, for our data, an estimate of Z obtained using a remote electric field is grossly inaccurate, its associated predicted coherence is as efficient in selecting low-noise-level data windows as its counterpart obtained using a remote magnetic field. The relation between SS and RR predicted coherences, the latter estimated using both electric and magnetic fields, is investigated. A hybrid selection technique that uses a remote electric field is suggested.

Most of the data acquisition in ground-penetrating radar is done along fixed-offset profiles, in which velocity is known only at isolated points in the survey area, at the locations of variable offset gathers such as a common midpoint. We have constructed sparse, heavily aliased, variable offset gathers from several fixed-offset, collinear, profiles. We interpolated those gathers to produce properly sampled counterparts, thus pushing data beyond aliasing. The interpolation methodology estimated nonstationary, adaptive, filter coefficients at all trace locations, including at the missing traces’ corresponding positions, filled with zeroed traces. This is followed by an inversion problem that uses the previously estimated filter coefficients to insert the new, interpolated, traces between the original ones. We extended this two-step strategy to data interpolation by employing a device in which we used filter coefficients from a denser variable offset gather to interpolate the missing traces on a few independently constructed gathers. We applied the methodology on synthetic and real data sets, the latter acquired in the interior of the Antarctic continent. The variable-offset interpolated data opened the door to prestack processing, making feasible the production of a prestack time migrated section and a 2D velocity model for the entire profile. Notwithstanding, we have used a data set obtained in Antarctica; there is no reason the same methodology could not be used somewhere else.

Magnetotelluric soundings have been made at 25 stations in the Rocky Mountain Trench (RMT) and Main Ranges near 53"N, close to the centre of a major conductivity anomaly which had been mapped in a magnetovariation array study. Most stations covered the frequency range 0.01-500 Hz and three stations 0.0002-500 Hz. The resistivity tensor shows low to moderate anisotropy in the RMT, but is strongly 2-D or 3-D in the Rocky Mountains. Apparent resistivities as a function of frequency are displayed in pseudosections along the Trench and along a transverse profile across the RMT and into the Main Ranges. In preparation for 2-D modelling, 1-D inversions have been used to construct resistivity-depth sections satisfying both magnitudes and phases of the MT responses. These show very low resistivities, in the range 1-10S2m, in the upper crust under the RMT and even lower values under the Main Ranges. The latter values give strong confirmation of the Northern Rockies conductor reported by Bingham, Gough & Ingham and are in agreement with models of the conductors fitted to long-period magnetovariation fields by Ingham, Gough & Parkinson. The MT results here reported add some essential depth and resistivity information. It is suggested-that the conductors beneath the Rocky Mountains Main Ranges and Trench constitute a thickening at the edge of the Canadian Cordilleran Regional (CCR) conductor. Gough has argued that a wide variety of geophysical and geological parameters indicate high temperatures and partial melting in the mantle under the CCR conductor. At the upper crustal depths penetrated in this magnetotelluric study, it is considered more probable that the high conductivity is caused by hot, saline water of mantle origin rather than silicate melt. The CCR in general may have two layers of fluid producing its high conductivity, silicate melt below and saline hot water above. 246K R. S. Hutton et al.

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