On the theory of sea‐floor conductivity mapping using transient electromagnetic systems

The electrical conductivity of the sea floor is usually much less than that of the seawater above it. A theoretical study of the transient step‐on responses of some common controlled‐source, electromagnetic systems to adjoining conductive half‐spaces shows that two systems, the horizontal, in‐line, electric dipole‐dipole and horizontal, coaxial, magnetic dipole‐dipole, are capable of accurately measuring the relatively low conductivity of the sea floor in the presence of seawater. For these systems, the position in time of the initial transient is indicative of the conductivity of the sea floor, while at distinctly later times, a second characteristic of the transient is a measure of the seawater conductivity. The diagnostic separation in time between the two parts of the transient response does not occur for many other systems, including several systems commonly used for exploration on land. A change in the conductivity of the sea floor produces a minor perturbation in what is essentially a seawater response. Some transient responses which could be observed with a practical, deep‐towed coaxial magnetic dipole‐dipole system located near the sea floor are those for half‐space, the layer over a conductive or resistive basement, and the half‐space with an intermediate resistive zone. The system response to two adjoining half‐spaces, representing seawater and sea floor, respectively, is derived analytically. The solution is valid for all time, provided the conductivity ratio is greater than about ten, or less than about one‐tenth. The analytic theory confirms the validity of numerical evaluations of closed‐form solutions to these layered‐earth models. A lateral conductor such as a vertical, infinite, conductive dike outcropping at the sea floor delays the arrival of the initial crustal transient response. The delay varies linearly with the conductance of the dike. This suggests that time delay could be inverted directly to give a measure of the anomalous integrated conductance of the sea floor both between and in the vicinity of the transmitter and the receiver dipoles.