INDUCED EM AND THE GEOMORPHIC EVOLUTION OF THE LAGUNA MADRE WIND-TIDAL FLATS: PADRE ISLAND NATIONAL SEASHORE, TX
The Laguna Madre wind-tidal flats and adjacent barrier island (Padre Island National Seashore) are highly-conductive environments (upwards of 3,800 mS/m) that offer a unique opportunity to test the capability of EM to differentiate a variety of coastal depositional environments. For this study, we used a multi-frequency GSSI EMP-400 EM Profiler™ to conduct a series of transects extending across the barrier into the wind-tidal flats. Three shore-normal surveys were taken along the southern, central and northern portions of the sand sheet at a total distance of 10 km, 9 km and 7 km, respectively. In addition, a 20 km long shore-parallel survey intersected the three shore-normal transects along the central portion of the island. For each survey, the EM profiler was oriented to the in-line horizontal dipole mode (HDM) at a 5 m sampling interval. Responses were recorded at three discrete frequencies: 1 kHz, 8 kHz, and 15 kHz, corresponding to skin depths of ~16 m, 10 m and 5 m, respectively. To invert the EM data, we used a Markov Chain Monte Carlo (MCMC) 1D inversion model to distinguish different geoelectric layers. Additionally, wavelet and spectral estimation of the non-stationary time series was performed. Preliminary results suggest that by taking the inverse of the EM data, inflection points become visible, which enables one to distinguish the main stratigraphic features of the barrier and wind-tidal flat system. Further research is currently in progress to incorporate morphometric analyses from a companion study to establish relationships between surface morphometry and subsurface EM results.