ELECTROMAGNETIC INVESTIGATION OF THE SEA FLOOR

Abstract

Numerical evaluation of integral expressions for the fields about a vertical magnetic dipole in the sea allows analysis of the electromagnetic response over wide ranges of sea induction number and sea floor conductivity. Our analysis indicates that a marine electromagnetic system for measurement of bottom conductivity variations could readily be designed, with such applications as oceanographic and geologic studies, and mineral exploration. For a source‐receiver system on a homogeneous sea bottom, it is found that: (i) when the ratio k=(sea‐bed conductivity)/(seawater conductivity) is greater than about 0.03, both horizontal and vertical magnetic fields are useful for measurement of bottom conductivity at sea induction numbers less than 30 [induction number =√2 (horizontal transmitter‐receiver separation/skin depth)]. A separation of 30 m and frequencies in the range 300–3500 hz appear suitable for investigation of the upper few meters of unconsolidated bottom sediments. (ii) When the ratio k is less than 0.03, sea induction numbers from 10 to a few hundred are required for detection of seabed conductivity variations. In this case, the horizontal magnetic field, resulting from energy transmission mainly through the seafloor, is the suitable field to use. Electromagnetic sounding of indurated rocks may thus call for frequencies of 100 to 20,000 hz at a separation of 200 m. Field strengths vary strongly with relative sea depth D/R (D=sea depth, R=horizontal source‐receiver separation) when D/R is small; but sensitivity to bottom conductivity is little affected by D/R. Elevation of source and receiver above a seafloor less conductive than seawater reduces field strengths and sensitivity to seabed properties.