Structural constraints in joint inversion of seismic and EM data: Analysis and visualization

Abstract

Structurally coupled joint inversion of seismic and electromagnetic data is a good candidate for recovering subsurface earth properties if the direction of change of electrical conductivity and acoustic impedance are systematically linked throughout a volume of earth. For example if electromagnetic and seismic material properties change in the same vector direction, then minimization of the cross gradient may be an appropriate basis for joint inversion. However this is moving a little too quickly. First we should be able to visualize and assess gradients within any volume. In particular it is important to consider the potential impact of the scale and resolution that can be achieved with seismic and EM data. We selected a large co-located 3D seismic and MT field data set to illustrate our new methods for assessing structural coupling as a constraint on joint magnetotelluric and seismic inversion. We extract azimuth and dip attributes from the seismic volume and convert these to polar dip.For the magnetotelluric data we complete 3D inversion to yield an electrical conductivity distribution and then compute the direction of maximum change in conductivity throughout that volume. We then compare streamlines, representing the direction of maximum change in electrical conductivity with streamlines representing direction of maximum change in relative reflectivity throughout a common volume. This streamline representation quickly reveals large volumes of earth with significant and systematic mismatch in the direction of change of seismic and electrical parameters. Such mismatches require explanation before a structurally coupled cross gradient methods is applied. Further we are able to quantify the average mismatch angle between vectors throughout any sub-volume. In some cases the mismatch will have physical meaning, such as conductivity gradient connected solute concentration distribution. In other cases, the mismatch can be considered an artifact which potentially may be removed through application of joint inversion, based upon structural coupling constraints such as minimization of the cross gradient.