Airborne electromagnetic bathymetry

Abstract

Airborne electromagnetic techniques provide an efficient means for determining the bathymetry of shallow seas and coastal regions. Offshore time‐domain data collected with the INPUT® system may be interpreted quickly and inexpensively using the simple table look‐up algorithm described here. This interpretation algorithm is based on two dimensionless parameters which depend on the six channel amplitudes of the recorded INPUT data. These amplitudes are computed and tabulated for a suite of simple two‐layer models in which seawater depth is the only variable quantity. The experimental data can then be interpreted by comparison with the tables of theoretical values to obtain the bathymetry beneath the flight path. The inversion algorithm has proven accurate and stable. In the range of 0–40 m of seawater, the representative difference between the interpreted depths and the charted depths is about 2 m. Bottom gradients lower than 5 percent were resolved accurately beneath 0–25 m of water. Independent interpretations of data sets collected at different altitudes and in different directions show excellent repeatability. The algorithm exhibits low sensitivity to changes in the transmitter‐receiver configuration and to variations in the underlying half‐space conductivities when conductivity values are in the range [Formula: see text]. The fractional error produced in the calculated depths by variations in the conductivity of the seawater is approximately equal to the fractional difference between the actual conductivity of the sea and the value of 4 S/m assumed in the model. ® Trademark of Barringer Research Ltd.