Contribution of the relative velocity between source and receivers to electromagnetic fields in the sea

Abstract

The analysis of the primary electromagnetic fields caused by steady state or transient electric current flowing along a current loop moving with a constant velocity below the sea surface has several applications. It supports the analysis of submarine physical data and it is useful for protecting ships from the threat of sea mines. The usual approach to the solution for the primary field starts from a hertz vector potential in the frequency domain due to a magnetic dipole. Subsequently it employs Fourier, Laplace, and Hankel transforms to describe the time variation of the primary electromagnetic induced field due to a loop. The result is applicable to both shallow and deep sea water environments. Because of the difference in velocity between source and receiver, a careful application of the convolution integral is necessary in order to adapt the source pulse solution to any type of transmitting current waveform. Furthermore, since the scattered field represents a fraction of the primary field, even minor differences in it caused by the differential velocity renders inadequate interpretation of EMI data.

A moving magnetic dipole in a conductive medium

“…This last difference is only twice larger than the difference between the S and the K systems during the on-time for both σ 2 = 0.1 S/m and σ 2 = 0.01 S/m. Sampaio (2006) estimated a difference larger than 50% in the determination of the apparent conductivity at the early time asymptote and differences at the late time asymptote that may cause interpretation errors of deep layers based on the value of the transverse primary electric field. I will not delve further upon this point because it is not the primary focus of the present analysis and I did not compute any EM secondary field component.…”

“…According to Sampaio (2006), the solution of for the primary Hertz potential of a moving magnetic dipole in medium 1 can be written in the κ x , κ y , z , and ω‐domain as: …”

“…De Hoop (2005), Sampaio (2005) and Sampaio (2006) analysed the behaviour of the primary field for the case of a moving source and fixed observers in a conductive medium. However, the geophysical literature lacks a thorough investigation about the electromagnetic energy scattered by the Earth and its consequence on electromagnetic (EM) data interpretation.…”