CSEM/MT - Technology
Controlled source electromagnetic surveys
The controlled source electromagnetic (CSEM) method uses a high powered horizontal electric dipole source to transmit a low frequency signal through the earth to an array of seafloor deployed receivers, which detect and record the electric and magnetic field. By studying the received signal as a function of source-receiver separation, geometry and the frequency of the signal (typically in the range 0.01-10Hz), the resistivity of the seafloor can be determined to depths of up to a few kilometres below the seafloor. The CSEM method is particularly sensitive to resistive structures in the earth, providing resolution at a scale of typically a few tens of metres.
Marine magnetotelluric surveys
The marine magenotelluric (MT) method uses electromagnetic fields naturally generated in the Earth’s atmosphere and ionosphere as a source. The resulting electric and magnetic fields are measured at seafloor receiver stations, and processed to provide responses sensitive to the resistivity structure in the earth to depths of several tens of kilometres. Although the MT method lacks the sensitivity required to resolve the details of resistive structures such as hydrocarbon reservoirs, it is extremely sensitive to bulk background structure, and so is a natural complement to the CSEM method in many situations.
Survey design
Designing a marine EM survey that is tuned to meet the exploration, appraisal or development challenges is key to ensuring a successful campaign. The first stage in this process is to gather all available background information, including seismic and well log information, and use this to construct a detailed and geologically consistent background model of the area. This is used initially to conduct 1D feasibility modelling in which the sensitivity of the CSEM (or MT) method to the resistivity structures of interest is assessed (figure1). Using the iMOSS modelling software, the effect of lithological or fluid property changes on the expected CSEM response can be quantified and appropriate acquisition parameters chosen. Anisotropy can also be accounted for in this modelling process.
Figure 1 -1D feasibility modelling is used to examine the sensitivity of the CSEM method to the structures of interest. In this example the sensitivity to the gas saturation within a North Sea sand reservoir is calculated using the iMOSS® rock physics, seismic and EM modelling package.
A 1D analysis provides a good indication of the sensitivity of a survey to the structure to be surveyed, it cannot take into account higher dimensional effects. It is therefore important following a 1D analysis to construct a more detailed 2D or 3D model which is used to validate the conclusions of the 1D study and account for 2D/3D effects. It is also important to establish whether a given structure can be recovered given a realistic survey dataset and the constraints available in the area. This is accomplished using a forward modelling, hypothesis testing and synthetic inversion approach, an important outcome of which is an interpretation strategy tailored to meet the survey objectives.