Noise can be reduced by adding up together results recorded at closely spaced stations and plotting sum at mid-point of station group. This is easiest form of low-pass filter. Asymmetry inherent in dip-angle data may be eradicated by differencing adjacent readings to get average horizontal gradients. Two filters developed to perform both the operations are in common use. Fraser filter utilizes four equi-spaced consecutive readings. First two are added together and halved. Same is done with second two and second average is then subtracted from first. More complicated Karous-Hjelt filter uses six readings, three on either side of central reading which is not itself utilized. ABEM Wadi instrument automatically shows K-H filtered data unless ordered not to do so.
Filtered data are generally simple to contour, particularly if, as is normal practice with Fraser filter, negative values are removed. Steeply dipping VLF dip-angle data (%) steeply conductors generate positive anomalies and are very obvious. Though, it is geophysical axiom which processing degrades data. Filters can demolish slight but possible important features and, more significantly, will distort anomalies because of sources other than simple, conductive sheets. For instance, isolated peak or trough because of a steeply dipping interface between materials of varying conductivity will be transformed by both Fraser and K-H filters into anti-symmetric anomaly. If negative values are then ignored, this characteristic will be interpreted as indicating dipping conductor some distance from region of actual conductivity change.
Displaying VLF data:
As raw dip-angle data are hard to contour and there are valid objections to filtering, VLF dip-angle data are most efficiently presented as stacked profiles. These show all original data, correctly located on map, and sections of profile on which there are gradients pointing out conductor can be stressed by thickened lines. For a map to be interpreted, even qualitatively, direction to transmitter should be shown so that degree of coupling can be estimated. Conductors striking at right angles to this direction will not be well coupled and may not be observed. Map should also show which two possible reading directions has been utilized, as it is otherwise not possible to differentiate normal gradients in which values decrease in facing direction from reversed gradients that may be because of active sources like power and telephone lines.
VLF systems work at relatively high frequencies at which most conductors seem good and generally locate several other anomalies than do CWEM surveys over same ground. Method is best suitable to mapping near vertical contacts and fractures. Conductive mineralization may be detected, but magnitudes of anomalies related with very good conductors may be no greater than those generated by un-mineralized but water-filled fractures, that are likely to occupy larger volumes. VLF measurements can be made quickly and expediently by single operator, and are thus at times utilized to evaluate electromagnetic characteristics of the area before expense of conventional EM survey is incurred.
This is particularly helpful in populated areas where noise from manmade electrical sources is to be expected. VLF surveys are becoming increasingly popular in hydrogeology. Targets are significant in parts of Africa where military signals are weak or poorly coupled to dominant conductors. Portable transmitters are now marketed which permit method to be utilized in the areas.
Natural and controlled-source audio-magnetotellurics:
Use of controlled sources eliminates some problems related with natural fields but introduces others. Very high currents are needed if long-wire sources are to produce adequately strong signals at kilometer distances required by the far-field approximation, and it is seldom easy to find sites where kilometers of wire carrying several amperes of current can be laid out on ground safely (or even at all). Even where this can be done, topographic irregularities may generate important distortions in signal. Closed loop sources can be significantly smaller but need currents even larger than those required for line sources.
Far field for CSAMT measurements is usually considered to start at distance of three skin depths from long-wire source, and is thus frequency dependent. On single sounding plot, onset of intermediate field conditions can generally be identified by implausibly steep gradient in sounding curve. Simple rule of thumb which can be utilized in planning surveys is that, to make sure far-field conditions, distance from source to receiver must be at least six times the required depth of investigation. Layout may, though, have to be modified in light of actual field conditions. In principle, quite different equations should be utilized in intermediate and near-field interpretation, but quality control in field is generally performed using only far-field approximations.
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