Effective skin depth of EM fields due to large circular loop and electric dipole sources

Abstract

Definition and concept of plane wave skin depth is applied to study the effective skin depth due to large circular loop and horizontal electric dipole sources, and attempt is made to illustrate its application to the real field survey design and data interpretation problems. The effective skin depth, the depth at which the amplitude of a frequency-domain electromagnetic field due to a large circular loop source or an electric dipole source falls to 1/e of its value at surface of a homogeneous half space, is compared with the usual plane wave skin depth for variation of different survey and model parameters, viz. source frequency, source moment or loop dimension, source-receiver offset and half space conductivity. The results show their characteristic variations, and depict that the effective skin depth due to large loop source and/or electric dipole source depends on source-frequency, source moment, source-receiver offset and half-space conductivity. The effective skin depth may vary from a fraction of plane wave skin depth in near vicinity of the source to more than twice the plane wave skin depth at points away from the source. However, for large source-receiver offsets and/or high source frequencies and half-space conductivities, it tends to the plane wave skin depth. For receiver positions in near vicinity of the source, the effective skin depth curves exhibit variations specific to the source, and show different characteristic variations for different sources, whereas for receiver positions away and far away from the source, the effective skin depth curves show similar characteristic variations for all sources, irrespective of nature and geometry of the source. The results depict distinctive effect of geometry and nature of the source on effective skin depth. The examples presented for illustrating the application of effective skin depth depict that effective skin depth has direct relation with the EM response of the local source over the layered earth and thus, can be used as an aid for estimating the optimum parameters in EM survey design and data interpretations.