In a situation where the earth is highly resistive, with discrete bedrock or surficial conductors, Sengpiel or other resistivity sections produced are often inaccurate or misleading. This is not an error! It is a matter of trying to use the incorrect geologic model to describe the true situation. The EM anomaly picking will properly describe the conductivity thickness and depth of the conductor with a model that matches the real situation.

Sengpiel and Differential sections are an approximate model of the resistivity structure of a conductive earth (halfspace) made up of uniform or slowly varying horizontal layers. This is also true of most standard inversion modelling routines used to generate vertical resistivity sections.

Section algorithms calculate the vertical resistivity distribution at each point along the data set as though the ground beneath consisted of horizontal layers of different resistivity. Each layer is flat, has constant resistivity, and is infinite in extent. The variation in layer depths and resistivities apparent in the sections comes from the variation in the modelled depth and resistivity at each new data point. Any lateral inhomogeneities smaller than about 50m in extent will affect the model, and depending on how closely the inhomogeneity approximates a horizontal layer, the model will do a better or worse job of matching the section to the real geology.

In a situation where the earth is highly resistive, with discrete bedrock or surficial conductors, this model can break down and the sections produced can be inaccurate or misleading. A program needs data on at least two frequencies to generate a section other than a perfect homogeneous halfspace. The HEM data drops to near zero as the halfspace resistivity (in ohm-m) gets close to the frequency of the EM channel (in Hz). For example, over 7000 to 8000 ohm-m rock there will be very little signal on the 7200Hz channel. This resisitivity can easily be exceeded by crystalline rocks or sandstones with little overburden.

Under these resistive conditions, the only EM signal that the section program has to work with may be a local surficial or bedrock anomaly. The program will try to calculate what depth and conductivity an infinitely wide horizontal layer must have to give such a response on the coplanar signal. Because infinite layers normally return a stronger signal than a local, discrete conductor, the model will place the layer at great depth to match the signal to the smaller signal from the conductor. The shape will be distorted because there will not be signal on enough channels to define the shape of the feature. If there is enough signal, the program will still model it as a horizontal layer anyway.

 

Greg Hodges, Chief Geophysicist, 2001