Magnetotelluric measurements at sea

Abstract

In a paper by one of us (A.T.P.) entitled ‘The theory of magnetotelluric methods when the source field is considered’ [1962], various cases were examined where the dimensions of the source field might have to be considered when interpreting magnetotelluric measurements. Most of the paper was about measurements in land areas, but a brief reference was made on page 1916 to measurements at sea. It was rightly concluded that for measurements—made over seas of depth greater than about 200 meters—of relatively rapid oscillations of the geomagnetic field (period ∼ 100 sec), the finite dimensions of the source field could be ignored and the relatively simple Cagniard formulas could be applied. For slower geomagnetic fluctuations of the order of a day, Price stated that the magnetotelluric results even over deep oceans ‘will be almost entirely determined by the source field and practically independent of the induced field.’ That statement is incorrect, however; it resulted from a numerical slip in the calculation of a scaling factor. The factor 0.19 in the last paragraph on page 1916 should, in fact, be 1.90. When this correction is made, it appears that the Cagniard formulas would be applicable over an ocean of depth 5 km for an oscillation with a period of 1 day, if the wavelength of the field source were greater than about 3×104 km. The maximum possible wavelength is 4×104 km, i.e. the circumference of the earth. For oscillations of shorter period, say n−1 days, the minimum permissible wavelength of the field source would be reduced by about the same factor. Hence it appears that the Cagniard formulas could be used over the oceans for geomagnetic oscillations of periods up to 1 day, provided that the source fields are of sufficiently large dimensions.