Some comments on seabed propagation of ULF/ELF electromagnetic fields

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Abstract
A reference model for the electrical conductivity structure beneath the deep seafloor is proposed and justified using a variety of geophysical evidence. The model consists of relatively conductive sediment and crustal layers of 6.5 km extent overlying a resistive (≈10−5 S/m) subcrustal channel of 30 km thickness and terminated in a deeper conductive layer and half‐space. Its seafloor‐to‐seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a half‐space with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (5 A‐m. The results indicate that significant (≈100 km) ranges can be achieved only around 1 Hz with a bandwidth of ≈1 Hz at a SNR of 10, yielding a very low data rate of <3.5 bits/s. Longer ranges and higher frequencies are precluded by attenuation in the sediment and crustal layers and because the conductivity in the resistive channel is too large.