Airborne electromagnetic bathymetry

Abstract

An experimental airborne electromagnetic (AEM) survey was carried out in the Cape Cod Bay area to investigate the potential of extracting bathymetric information over a shallow ocean. A commercially available Dighem III AEM System was used for the survey without any significant modification. The helicopter-borne system operated at 385 Hz and 7200 Hz, both in horizontal coplanar configurations. A concurrent ground truth survey included extensive included acoustic profiles, as well as spot water conductivity measurements.

Because of the lack of knowledge about the absolute system calibration, a postsurvey calibration was made for each flight line, using a small portion of AEM data to derive the zero-level signal, amplitude, and phase calibration factors for each coil pair. The interpreted bathymetric profiles show excellent agreements with corresponding acoustic depth profiles up to one skin depth (possibly more) of the source frequency. It is envisioned that with further improvements in hardware and software, the bathymetric resolution may extend beyond the skin depth. AEM data can also produce (as by-products) conductivity profiles of both seawater and bottom sediments that may find potential applications in various offshore geotechnical engineering works.