We present a simple and feasible approach to analyze and identify two dimensional (2D) effects in central loop transient electromagnetic (TEM) sounding data and the correspondingly derived quasi 2D conductivity models. The proposed strategy is particularly useful to minimize interpretation errors. It is based on the calculation of a semi-synthetic TEM-tipper at each sounding and for each observational transient time point. The semi-synthetic TEM-tipper is derived from the measured vertical component of the induced voltage UI;z and the synthetically calculated horizontal component UI;x. The approach is computationally in-expensive and involves one two-dimensional forward calculation of an obtained quasi 2D conductivity section. Based on a synthetic example we demonstrate that the TEM tipper approach is applicable to identify which transient data points and which corresponding zones in a derived quasi 2D subsurface model are affected by 2D inhomogeneities. The 1D inversion of such data leads to false models. An application of the semi-synthetic TEM tipper to field data from the Azraq basin in Jordan reveals that in total eight of 80 investigated soundings are affected by 2D structures although the field data can be fitted optimally using 1D inversion techniques. The largest semi synthetic tipper response occurs in a 300 m wide region around a strong lateral resistivity contrast. The approach is useful to analyze structural features in derived quasi 2D sections and to qualitatively investigate how these features affect the transient response. To avoid misinterpretation, these identified zones corresponding to large tipper values are excluded from the interpretation of a quasi 2D conductivity model. Based on the semi-synthetic study, we also demonstrate that a quantitative interpretation of the horizontal voltage response (e.g. by inversion) is usually not feasible as it requires the exact sensor position to be known. Although a tipper which is derived purely from field data is useful as a qualitative tool to identify 2D distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi-synthetic TEM tipper approach is particularly feasible as an a-posteriori approach if no horizontal components are recorded or if the sensor setup in the field is not sufficiently accurate.
Yogeshwar, P., Tezkan, B. (2017): Analyzing two-dimensional effects in central loop transient electromagnetic sounding data using a semi-synthetic tipper approach. Wiley – In: Geophysical Prospecting, DOI: 10.1111/1365-2478.12520
|Authors||Yogeshwar, Pritam and Tezkan, Bülent|
|Title||Analyzing two-dimensional effects in central loop transient electromagnetic sounding data using a semi-synthetic tipper approach|