Mapping thin resistors and hydrocarbons with marine EM methods: Insights from 1D modeling

Abstract

The use of marine controlled-source electromagnetic EM (CSEM) sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling. We examine the sensitivity of the CSEM method to such layers with forward and inverse modeling in one and three dimensions. The 3D modeling demonstrates that if both source and receivers are over a tabular 3D target, 1D modeling predicts the observed response to very high accuracy. Experimental design can thus be based on 1D analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure. Modeling in three dimensions shows that the vertical electric-field response is largest over the edges of a 3D target. The 3D modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by CSEM sounding. A simple air-wave model (energy propagating from source to receiver via the atmosphere) allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range, frequency, and seafloor resistivity. Unlike DC resistivity sounding, the marine CSEM method is not completely T-equivalent and, in principle, can resolve resistivity and thickness separately. Smooth inversion provides an estimate of the method's resolving power and highlights the fact that although the radial CSEM fields contain most of the sensitivity to the thin resistive target, inverted alone they produce only increasing resistivity with depth. Inclusion of the radial mode CSEM data forces the recovery of the thin resistor, but magnetotelluric data can be used more effectively to achieve the same result.

Sensitivity of the near-surface vertical electric field in land controlled-source electromagnetic monitoring

“…Synthetic feasibility studies suggest that CSEM monitoring should be feasible in marine environments using presentday standard survey configurations (Orange et al, 2009;Constable and Weiss, 2006). On land, though, reservoir volumes affected by resistivity change need to be large before they become detectable using standard surface-based horizontal dipole-dipole configurations (Wirianto et al, 2010).…”

Monitoring of hydrocarbon reservoirs using marine CSEM method

“…This is slightly above, or approximately at, the noise level of 10 −15 V/m set by Um and Alumbaugh (2007). This noise estimate is slightly higher than that given by Constable and Weiss (2006). The magnitude of the source-momentnormalized anomalous reservoir magnetic field x and y components is in the range 10 −8 − 10 −11 nT/m.…”

Effects of the airwave in time-domain marine controlled-source electromagnetics

“…Additionally, by using numerical models, Constable and Weiss (2006) show that, in contrast to CSEM with horizontal receivers, vertical receivers are more sensitive to the edges of a subsurface resistor. Consequently, measurements of the vertical electric field are more suitable for mapping the lateral extent of a resistor than recordings of the horizontal electric field.…”