The differential parameter method for multifrequency airborne resistivity mapping

Abstract

Helicopter EM resistivity mapping began to be accepted as a means of geologic mapping in the late 1970s. The data were first displayed as plan maps and images. Some 10 years later, sectional resistivity displays became available using the same ''pseudolayer'' half-space resistivity algorithm developed by Fraser and the new centroid depth algorithm developed by Sengpiel. Known as Sengpiel resistivity sections, these resistivity/depth images proved to be popular for the display of helicopter electromagnetic (EM) data in conductive environments. A limitation of the above resistivity and depth algorithms is that the resulting Sengpiel section may imply that the depth of exploration of the EM system is substantially less than is actually the case. For example, a target at depth may be expressed in the raw data, but its appearance on the Sengpiel section may be too shallow (which is a problem with the depth algorithm), or it may not even appear at all (which is a problem with the resistivity algorithm). An algorithm has been adapted from a ground EM analytic method that yields a parameter called the differential resistivity, which is plotted at the differential depth. The technique yields the true resistivity when the half-space is homogeneous. It also tracks a dipping target with greater sensitivity and to greater depth than does the Sengpiel display method. The input parameters are the apparent resistivity and apparent depth from the pseudolayer half-space algorithm and the skin depth for the various frequencies. The output parameters are differential resistivity and differential depth, which are computed from pairs of adjacent frequencies.

Extending Geobandwidth using the Multipulse Configuration

“…In this configuration, the waveform was a 4 ms, 800,000 Am 2 half-sine with a 1 ms, 40,000 Am 2 trapezoid pulse.. The receiver is concentric with the transmitter and flown with a nominal height of 30 m. Figure 3 shows profile data for the half-sine and trapezoid pulse (top) as well as Differential Resistivity TM (Huang and Fraser, 1996) for each pulse (bottom). The Differential Resistivity section from the trapezoid pulse not only shows more detail in the near-surface, but, since it is able to measure earlier in time, is also able to map conductivity at shallower depths.…”

Helicopter electromagnetic and magnetic geophysical survey data, Swedeburg and Sprague study areas, eastern Nebraska, May 2009

“…The differential resistivity and depth transformation (Huang and Fraser, 1996) is one simple depth imaging method that has proven effective for HEM survey data (Smith and others, 2003). Both the An important part of the data processing is leveling the EM signals for system drift and calibrations.…”

Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data

“…Industry standard practice for interpretation of frequency-domain AEM data is to use half-space inversions, i.e. apparent resistivity transformations, that invert for the resistivity of a halfspace and its depth below the receiver (Huang and Fraser, 1996). These transformations are fast and stable and they can produce a good representation of horizontal resistivity variations.…”