CSEM data are inverted to derive anisotropic resistivity models. The inversion is performed in stages. Firstly, an unconstrained inversion is run in order to examine the resistivity structure obtained in the absence of any a priori information. However unconstrained inversions in general have lower resolution than seismic data for example. Resolution can be improved by including structural information from the seismic data. Integration of resistivity and seismic data is key in improved exploration, two complementary yet distinct and separate datasets provide increased understanding of the sub-surface compared to one dataset (seismic alone).

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  Step 1: Unconstrained to Seismically Guided Inversion
  Figure shows:
  • Top image – unconstrained anisotropic inversion (vertical resistivity) of a 2.5D resistivity section showing a 44 km long line over Albatross and Snohvit in the Barents Sea, Norway. Unconstrained inversion starts with a uniform half space with a homogeneous resistivity value assigned to it, no a priori information is included, thus it’s a purely data driven process. The known hydrocarbon discoveries and seen as areas of higher resistivity when compared to the background resistivity, here higher resistivity is shown in red.
  • Middle image – Simultaneously acquired with the resistivity data (see slides 1 and 2) 2D GeoStreamer broadband seismic data, same section length and depth, single horizon interpreted.
  • Lower image – Integration of resistivity and seismic data – seismically guided inversion, anisotropic inversion of the vertical resistivity is guided using, in this example, a single seismically derived horizon, improves resolution and structural conformity of resistivity information.
  Integration of resistivity and seismic data is key in improved exploration, two complementary yet distinct and separate datasets provide increased understanding of the sub-surface compared to one dataset (seismic alone).
  Dense sampling reduces bias in the recovered resistivity and increases sensitivity. It is Particularly important in low contrast resistivity settings or when data are used to derived quantitative estimates of e.g. hydrocarbon saturation/volumes.


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  Step 2: 1000 m shot spacing
  Result of Snohvit and Albatross seismically guided inversion if three of every four EM shots is removed, I.E. effective EM shot spacing is 1000m compared to PGS’ standard EM shot spacing of 250m, which provides exceptionally dense EM data.


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  Step 3: 250 m shot spacing
  For comparison to previous slide, here we include an EM shot every 250m as per standard acquisition, note the improvement on the data when a higher density dataset is used.
  The Snohvit example shows that EM inversion which has been guided using a seismic horizon improves resolution and structural conformity. The line length is 44 km.