Doubling the effective skin depth with a local source

The depth at which the amplitude of the frequency-domain electromagnetic fields due to dipole and square loop sources over a homogeneous half-space fall to lie of their value at the surface is compared to the conventional plane-wave skin depth. The skin depth due to a local source depends on the transmitter frequency, halfspace conductivity, transmitter altitude, and transmitter-receiver offset, and may range from a fraction of to more than twice the plane-wave skin depth. Unlike the plane-wave skin depth, the "local-source skin depth" is different for electric and magnetic fields, and may be nonunique for some transmitter geometries and field components. For all transmitter geometries, however, the local-source skin depth approaches the plane-wave skin depth as the transmitter altitude and/or receiver offset increase. The concept of the local-source skin depth has direct application to survey design and data interpretation. A theoretical example demonstrates that it is possible to predict, for a given survey geometry and frequency range, whether or not an electromagnetic sounding can detect a conductive basement below a thick overburden layer.