Analysing two-dimensional effects in central loop transient electromagnetic sounding data using a semi-synthetic tipper approach
P. Yogeshwar and B. Tezkan
Journal name: Geophysical Prospecting
Issue: Vol 66, No 2, February 2018 pp. 444 - 456
Info: Article, PDF ( 1.49Mb )
We present a simple and feasible approach to analyse and identify two-dimensional effects in central loop transient electromagnetic sounding data and the correspondingly derived quasi two-dimensional conductivity models. The proposed strategy is particularly useful in minimising interpretation errors. It is based on the calculation of a semi-synthetic transient electromagnetic tipper at each sounding and for each observational transient time point. The semi-synthetic transient electromagnetic tipper is derived from the measured vertical component of the induced voltage and the synthetically calculated horizontal component. The approach is computationally inexpensive and involves one two-dimensional forward calculation of an obtained quasi two-dimensional conductivity section. Based on a synthetic example, we demonstrate that the transient electromagnetic tipper approach is applicable in identifying which transient data points and which corresponding zones in a derived quasi twodimensional subsurface model are affected by two-dimensional inhomogeneities. The one-dimensional inversion of such data leads to false models. An application of the semi-synthetic transient electromagnetic tipper to field data from the Azraq basin in Jordan reveals that, in total, eight of 80 investigated soundings are affected by two-dimensional structures although the field data can be fitted optimally using onedimensional 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 for analysing structural features in derived quasi two-dimensional sections and for qualitatively investigating 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 two-dimensional 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 derived purely from field data is useful as a qualitative tool for identifying two-dimensional distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi-synthetic transient electromagnetic 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.