Acoustic, magnetic and seismic investigation of the sea floor off the uraniferous district of Piriac, France

Abstract

A geophysical survey was carried out off the Piriac coast in France. A sidescan sonar, a high-accuracy magnetometer and a high-resolution seismic system in conjunction with a very precise positioning system were used together. These geophysical methods were employed in order to follow the offshore extension of a metamorphic synform including pitchblende deposits and to identify the nature of the surrounding bedrock. Following the results of the seismic and the magnetic survey, complemented by onshore core-sample magnetic studies, it was possible to outline the termination of the offshore synform extension and to locate a new marine metamorphic unit to the west of the synform. This unit includes some strong magnetic anomalies which may be associated with mineralized districts. Moreover, the most eastern anomaly could represent the prolongation of the synform, displaced by a fault towards the northwest. A mosaic of the extremity of the western synform was compiled with the sidescan sonar images. The different rock types and sediments, extensive fracturing of the granitic basement and metre-scale ripples in the sediments are clearly visible, allowing a geological interpretation of the end of the synform to be made at a scale of 1 2500 . The synthesis of all data including sidescan sonar mosaic, seismic and magnetic maps and the magnetic study of onshore core samples contributes considerably to the knowledge of the sea floor off Piriac and should be valuable for uraniferous mineral prospecting of the western part of the synform.