SPECIAL TOPIC: MARINE SEISMIC & EM
Figure 6 Depth section with 3D CSEM inversion anomalies from the south of Gjøa dataset. Vertical resisitivity superimposed on seismic and resistivity log 35/12-1 (3D seismic courtesy of CGG).
computations. The code can model arbitrary transmitter-receiver configurations. 3D inversions are done with a preconditioned gradient-based method (Mackie et al., 1994). Data from the area south of Gjøa was inverted in 3D through unconstrained anisotropic inversion. The transmitter-receiver offset range for the transient input data was 500 to 3000 m, and the inversion start model was an anisotropic half-space with hori-
zontal resistivity 1.8 Ωm and vertical resistivity 3.6 Ωm. From the inversion a moderate resistivity anisotropy around 2 is observed, close to the start model value. It should be noted that while the vertical CSEM method has high sensitivity to vertical resistivity, the sensitivity to the horizontal resistivity is low. The inversion volume covers three dry wells: 35/12-1, 35/12-3 and 35/12-5. No elevated resistivity is observed from the inversion at the well locations, and this is confirmed by the resistivity logs. Offset from the dry wells the inversion resolves several resistive events in the lower cretaceous to triassic interval, suggesting the presence of additional exploration possibilities in the area. Figure 6 displays a depth section with two resistive events. Impact on 3D inversion from infrastructure Investigation of the 3D inverted resistivity volume has been performed to evaluate the pipeline impact on the imaging result. Pipeline effects are expected to appear as shallow anomalies in the vicinity of the pipelines, with associated deeper compensation artefacts. No strong artefacts have been identified around the
Figure 7 3D inversion result. Vertical resistivity depth slice at 500 m (top) and 1200 m (bottom). Dots indicate receiver positions while the solid lines are the two pipelines within the survey area.
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Figure 8 Vertical resistivity depth slice at 2350 m showing two resistive events in the lower cretaceous to triassic interval.