Source shape impact on controlled source EM surveys

Abstract

Impact of the shape of grounded-line sources on the frequency domain response observed in the controlled source electromagnetic (CSEM) method has not been addressed thoroughly. In this paper, we use the electromagnetic (EM) fields from a half space as examples to discuss the impact of shape. The EM fields from current dipoles along a curved source are summarized. To illustrate the impact, the ratios of the fields from a curved line source to that of a straight line source with the same current are computed. Numerical results show that the impact of source shape on EM fields occurs mainly from a few hertz to higher frequencies. In our examples, shape deformation causes about 7% variation in the real part, and up to a relative error of ~22.5% in the imaginary part at middle frequencies. However, for the derived impedance, the impact can be ignored. Spatially, in the coaxial zone, the impact of source curvature can also be ignored; but in the equatorial zone near the source, the impact needs to be considered. In our examples, the maximal distance between a curved source and the corresponding straight line source is about 1/4 of the spacing between two current poles, more curved than usually seen in field work. Therefore, our results indicate that we do not need to care about source shape when working in a far zone or with source curvature smaller than that of the sources here. There are two possible ways to lessen the shape impact. One is to lay out grounded line source in a Z shape, but a more complete way is by using impedance derived from orthogonal electrical and magnetic fields to present observed data.