An evaluation of the marine magnetic gradiometer

Abstract

This study examines the ability of a marine gradiometer to produce diurnal‐free total magnetic intensity data. Using Gulf’s research vessel HOLLIS HEDBERG, an experimental program was carried out in the Santa Barbara channel offshore California. A shore‐based station was operated continuously during the survey period to monitor the temporal magnetic activity. The gradiometer data are adversely affected by the magnetic field of the towing vessel. An analysis of gradiometer bias as a function of ship’s heading and deployment distance shows remarkable agreement with a model of the ship’s magnetic field proposed by Bullard and Mason in 1961. Based upon this model, analytic azimuth correction factors are developed and applied to the gradiometer data. Comparisons are made among single‐sensor, ship‐minus‐shore, and integrated total magnetic intensities. Residual mistie statistics, contour maps, and profile plots form the basis of the comparison. At this location, the ship‐minus‐shore and integrated total intensities produce maps with standard residual misties of 0.55 and 0.62 gammas, respectively. Because of the gradiometer’s operational simplicity and the uncertainty of spatial coherency of diurnal activity, the gradiometer is recommended for routine use in the marine environment. The gradiometer is very effective in eliminating diurnal activity, but it requires moderately more sophisticated data analysis procedures.