Airborne electromagnetics as a tool to image the land-to-sea sedimentary continuum: A complementary geophysical approach to improve coastal characterization
Résumé
Geophysical investigation of the entire coastal zone, including emerged and shallow marine areas, is difficult to conduct especially due to method limitations. Therefore, the land to sea transition zone is often few or not surveyed consequently blanked areas subsist in the backshore, foreshore and upper shoreface domains. This study aims to evaluate the capacity of airborne electromagnetics to image the coastal zone and particularly the land-to-sea sedimentary continuum. It focuses on the Galion and Robert Bays of the Martinique island and is based on the regional survey MarTEM, conducted in 2013 with the SkyTEM 304 system. This survey offers coverage of the whole island and provides information on the subsurface every 30 m along flightlines. Airborne electromagnetics results were compared to topo-bathymetric data, seafloor sediment surface map, ortho-photographs, cores, as well very high-resolution seismic profiles from the CARQUAKES French research program.
The seafloor appears to be relatively well imaged by the electromagnetic technique and corresponds to a resistivity contrast around 0.3 Ω.m. The soft sediment deposits corresponds to the resistivity values between 0.3 and 0.8 Ω.m. The top and the bottom of the soft sediment layer were extracted from the resistivity model and thickness of sediment deposit was then calculated. Grids of the top of the basement and of the sediment thickness were obtained (130 m pixel) for the two bays. Thus, airborne electromagnetics allows imaging the land to sea continuum and giving a continuous view of the emerged part of the coast as well as shallow water area, over the whole bays and up to the penetration limit of the method (around 20 m for the system used in 2013). This is especially advantageous close to rocky coast and over turbid areas, where marine surveys are difficult to carry out. Advantages and disadvantages of the method are discussed. From this case study, it appears that airborne electromagnetics is an efficient method to investigate sediment bodies lying in the land-to-sea transition zone to complement to the other methods traditionally carried out.