Electromagnetic footprint measurements from a towed platform for characterizing sub-bottom conductivities and structures in the Stockholm archipelago

In the ocean there are mainly two ways of characterizing the properties of the sediment structure. One technique is to observe the properties by physically collecting core samples by in-situ instrumentation. The other is to use remote techniques to estimate the acoustic properties of the sediment and from these observations invert the sediment thickness and sound velocity by interpreting the reflected sound. One of the drawbacks of the acoustic technique is that its range sometimes may be limited by gas trapped in the sediments, which severely impedes the sound propagation. This situation is often encountered in the Stockholm archipelago. For these areas it is possible to use low-frequency electromagnetic fields to achieve basically the same type of sediment property description. A model-based technique has been developed for characterizing the sediment conductivity and thickness by inversion of electromagnetic data. A field trial was undertaken in May 2007 where electromagnetic data was collected by a towed sensor which also included a one-axis electromagnetic multi-frequency source in addition to the electric receiver. Results from the analysis are presented and discussed in terms of its appropriateness for use in footprint techniques for determining sediment layer thickness and sediment conductivity along the tow track.