Aeromagnetic and total-gamma radiation surveys were flown simultaneously during the 1967-68 dry season over the entire country and consist of approximately 140,000 km of traverse, mostly along north-south lines 0.8 km apart over the land and 4 km apart over the continental shelf. Continuous photography and Doppler navigation provided horizontal control. Flight altitude was 150 m above terrain. Time variations in the magnetic field measured with a fluxgate magnetometer were removed by adjustment at crossings of east-west control lines. Variable contour intervals of 10, 50, 250, and 1000 gammas were used depending on horizontal gradient.
The airborne geophysical data obtained during the Liberian survey are being prepared in a series of maps at 1:250,000 scale showing aero-magnetic and total-count gamma,radiation data for each of ten quadrangles.The index map shows the location of the quadrangles. The radioactivity map of this quadrangle should be used with this report (Behrendt and Wotorson,1971).
The surveys were flown by Lockwood, Kessler, and Bartlett, undercontract to the Liberian Geological Survey.
The purpose of both surveys was to assist in the geologic mapping program undertaken cooperatively by the Liberian Geological Survey and the U. S. Geological Survey under the auspices of the Liberian Government and the Agency for International Development, U. S. Department of State. The entire country is heavily forested, access is difficult, outcrops are sparse, and thick laterite is a characteristic feature. Consequently, it was hoped that the surveys would be useful in interpolating and extrapolating from the known geologic information and in locating potential targets for mineral exploration.
Figure 1 shows the tectonic interpretation for sheets 1 and 2 of the map based on available geologic information (White and Leo, 1969; unpub. data in files of Liberian Geol. Survey). Figure 2 is part of the residual total magnetic intensity map of Liberia obtained by digitizing the data of the map on a one-minute grid, tying to an absolute survey (Lowrieand Escowitz, 1969) by a constant of ±25,980 It 35 gammas and, removing Cain's "Field G" (Cain and others, 1965).
