Smoke rings

Abstract

A loop with a d.c. current flowing is laid on the ground (the transmitter). Field lines due to this loop are shown in figure 1. The d.c. current is abruptly shut off. By E.M. theory, when one abruptly interrupts the current in the transmitter, a system of induced currents is generated in the halfspace, such as to preserve the magnetic field that existed before switch off. Nabighian (1979) has shown that at any given time after switch off, this system of induced currents can be represented by a simple current filament, i.e. a line of current flowing in the ground. This line of current has the same shape as the transmitter loop, it moves downward and outward with an increasing velocity and diminishing amplitude. Figure 2 shows a schematic of the line of current in the ground at three different times after switch off. The equivalent line of current moves downward and outward at an angle of 47 degrees with a velocity given by [see full text for equation]. Note that the maximum current density moves at an angle of 30 degrees. This is the result given by Lewis and Lee (1978). The equivalent line of current which is obtained by integrating over the current density moves at 47 degrees.