Inversion of Multi-Transient Electromagnetic (MTEM) Data

The multi-transient electromagnetic (MTEM) technique, for detecting and monitoring hydrocarbons, works by injecting a transient current into the ground (a source) and recording the potential difference between two receiver electrodes (the receiver) some distance, or offset, from the source. Field layout is similar to that of seismics, with many receivers per source and many sources per profile. After processing, the data for each source-receiver pair consist of the earth’s impulse response function and step response function. Two methods of inversion are described in this paper. The first uses a single value from each step response (the late time value) and transforms this to an apparent resistivity. These apparent resistivities are inverted in a 2D scheme to give a cross-section of resistivity versus depth. In the second method the full waveforms of either the step or the impulse response functions, are inverted simultaneously for several offsets at each common mid-point position. The resulting 1D resistivity models are collated to form a 2D resistivity image of the subsurface. The methods are illustrated with data from a demonstration survey over a gas storage reservoir.<br><br><br>