Observation of Very Low Frequency Electromagnetic Signals in the Ocean

Abstract

Since magnetic signals recordable on the deep sea floor are very small and since records of long duration must be obtained to extend the frequency range abruptly limited on the high end by ocean shielding, sea floor magnetic variographs depend critically on long term stability. Temperature effects are not limiting since temperature variations at great oceanic depth are minimal, but stability of position in situ at landing depends on little known as well as little controllable conditions.Magnetic variations on the sea floor are presently carried out almost exclusively by means of self contained magnetic variographs (often simply called magnetometers) freely traveling between ocean surface and bottom. The sensors are flux gates, thinfilm single domain transducers and suspended magnets with optical readout. In the first two cases, the achieved stability is dependent upon the stability of the cancelling field and associated electronic circuitry. In suspended magnet types, initial torsion of the suspension fibers can relax the requirements upon electronics considerably.Electric field recording on the sea floor by means of long lines requires a sufficiently large separation to minimize effectively electrode drift. Azimuth and electrode separation must be established with adequate accuracy, a non-trivial requirement. Electrode drift can be eliminated within the frequency range of interest —below a few cycles per minute— by electrode switching. This technique leads to more compact instruments. Its implementation is illustrated in detail.Meaningful electromagnetic observations related to oceanic signals are also made at the periphery of continents or on islands by means of wires specially laid locally or by means of abandoned transoceanic cables.