Scenario Planning for Electromagnetic Surveying

Abstract

The Multi-Transient ElectroMagnetic (MTEM) technique is used to delineate resistive targets that may correspond to hydrocarbon reservoirs. Prior to actual surveys, feasibility studies are conducted to determine the probable detectability of the target of interest, set in the background resistivity structure where known or postulated. From the 2D model cross-section, resistivity profiles are extracted at positions along a profile. Modelling comprises determining the earth’s impulse responses for appropriate source-receiver offsets and these may be varied in the feasibility process. Two methods are then used to assess detectability. One exploits the amplitude of the impulse responses by plotting common offset sections. Shallow offsets illuminate targets near-surface, if they are addressable, whilst longer offsets see deeper. In an alternative method, the arrival time of the peak of the earth’s impulse response is used to determine an apparent resistivity. Values may be determined for all offsets at all selected profile positions and the result plotted as a 2D resistivity pseudo-section. Detectable targets are those that can be seen to correspond with the target reservoir of interest. Resolvability of individual targets within a sequence of overlapping stacked reservoirs on land and in marine studies will be addressed.