MIM and nonlinear least‐squares inversions of AEM data in Barataria basin, Louisiana

Abstract

An airborne electromagnetic survey was performed over the marsh and estuarine waters of the Barataria basin of Louisiana. Two inversion methods were applied to the measured data to calculate layer thicknesses and conductivities: the modified image method (MIM) and a nonlinear least-squares method of inversion using two two-layer forward models and one three-layer forward model, with results generally in good agreement. Uniform horizontal water layers in the near-shore Gulf of Mexico with the fresher (less saline, less conductive) water above the saltier (more saline, more conductive) water can be seen clearly. More complex near-surface layering showing decreasing salinity/conductivity with depth can be seen in the marshes and inland areas. The first-layer water depth is calculated to be 1-2 m, with the second-layer water depth around 4 m. The first-layer marsh and beach depths are computed to be 0-3 m, and the second-layer marsh and beach depths vary from 2 to 9 m. The first-layer water conductivity is calculated to be 2-3 S/m, with the second-layer water conductivity around 3 to 4 S/m and the third-layer water conductivity 4-5 S/m. The first-layer marsh conductivity is computed to be mainly 1-2 S/m, and the second-and third-layer marsh conductivities vary from 0.5 to 1.5 S/m, with the conductivities decreasing as depth increases except on the beach, where layer three has a much higher conductivity, ranging up to 3 S/m.

Airborne Electromagnetic Bathymetry

“…Bergeron and co-workers (1989) examined the same Cape Cod Bay dataset using a different method to invert the AEM data, to obtain good agreement with measured altimetry, seawater conductivity and known water depths. Bryan et al (2003) used field data obtained over marsh and estuarine waters in Barataria Basin (Louisiana, USA) with a frequency domain AEM system using a primary waveform digitally constructed from six harmonic frequencies (Mozley et al, 1991a,b). The results of their inversions showed good agreement with measured water conductivities and depths, identifying horizontal water layers with the less saline, less conductive, fresher water layer overlying a more saline, more conductive water layer.…”