Summary The local resistivity within a reservoir is linked to the saturation of CO2, thereby allowing imaging and quantitative estimation of the distribution of injected CO2 over time through measurements of the subsurface resistivity. We present results of EM sensitivity modelling for monitoring applications in different settings, and investigate the potential of technology improvements to CSEM hardware and methodology.

Summary Vertical-vertical CSEM data collected in the Norwegian Sea with a signal optimized for time domain processing was processed in frequency domain (FD) and in time domain (TD). In FD the data was inverted using the regularized Gauss-Newton style 2D inversion code MARE2DEM. For the TD inversion a modified version of MARE2DEM was used that includes a function for the frequency to time-domain transform. A line from a recently acquired data set from the Norwegian Sea was used as test example. Results of the different processing and inversion schemes were similar. Both recovered a resistivity drop in the shallower section and an increase in resistivity in the Jurassic. An uplift of the Jurassic in the eastern part of the line was also recovered by both methods. Well log data confirms the results. Gamma Ray and resistivity logs indicate an increase of porosity in Cretaceous sediments. The TD inversion shows a more pronounced resistivity in the deeper part of the section. This may be contributed to its higher sensitivity for resistive targets at depth. Computationally the FD inversion is considerably more efficient.

Several different CSEM technologies are currently commercially available for offshore exploration, with different characteristics and advantages. Few direct and realistic comparisons between the different methodologies have been made. In this paper we use a 2.5D finite element code capable of handling both frequency domain and time domain to investigate potential for depth penetration for two methods, horizontal source frequency domain and vertical source time domain. Synthetic data with realistic noise levels has been generated for specific simple models and then inverted. Our results show that vertical time domain solution has potential to resolve deeper targets for the assumed conditions.

Summary Electromagnetic (EM) methods are an accepted tool to reduce offshore hydrocarbon exploration risk. One of the companies offering such a service is the Norwegian geophysical company PetroMarker, which acquires offshore time domain CSEM data with a vertical stationary transmitter and vertical E-field sensors placed on the sea bed. In many cases a 1D inversion approach will give a good enough understanding of such data. Still, to fully exploit the information inherent in the data set and enhance the resolution of subsurface features it is necessary to account for geometrical effects such as bathymetry and the lateral extension of the resistive features through 2D or 3D inversion. Recently a new 2D time domain inversion code for CSEM data with induced polarization (IP) capability was developed based on the well-established frequency domain MARE2DEM finite element code. Accounting for induced polarization allows for a wider range of transmitter receiver offsets in a single 2D inversion run. In this paper we show some first examples of this code on synthetic and field data. It is found that a resistivity structure can fast and reliably be recovered, also in the presence of induced polarization.

The acronym CSEM has widely become synonymous for frequency domain Controlled Source ElectroMagnetics with seabed nodes and horizontal towed dipole transmitters. While this incarnation of marine CSEM is certainly the best known CSEM variant, it is not necessarily always the best option for acquisition over a certain prospect. Based on numerical modelling the differences between horizontal FD CSEM and vertical TD CSEM for given models were explored. The results show that time domain CSEM with vertical transmitter and vertical receiver dipoles is a viable alternative that provides high sensitivity, high depth of penetration as well as low disturbance by air wave and 3D structures. It's much smaller transmitter receiver distance creates an advantage in lateral resolution as well as in the detection of narrower or smaller structures. Case studies with comparisons between inverted vertical-vertical CSEM data and well log results show the real world usefulness of the method as well as the necessity for close integration of the results with other geophysical data. We argue that the marine CSEM landscape will become more divers and that acquisition layout and methodology for a given target needs to be evaluated on a case by case basis to achieve optimum results.

Vertical-vertical CSEM is an electromagnetic offshore technology that uses specialized sea floor receivers for accurate measurement of the vertical electrical field generated by a vertical transmitter dipole in the sea column. We present four case studies from the North Sea, Norwegian Sea and Barents Sea. The results of the vertical-vertical CSEM method are corendered with seismic and compared to well results. In three of the four cases the wells have been drilled after the CSEM survey had been completed. The results of the drilling confirmed the interpreted resistivity distribution. In one case the vertical-vertical CSEM data was acquired over a known reservoir. The results show that vertical-vertical CSEM can contribute highly valuable information to the drill/drop decision process. This is especially true in cases where no resistive anomaly is found.

Stefan L. Helwig, Abdul Wahab El Kaffas, Terje Holten, Oyvind Frafjord y Kjetil Eide presentan el metodo “vertical-vertical” para las prospecciones electromagneticas marinas de fuente controlada y discuten sus potenciales ventajas en comparacion con los metodos convencionales de dipolo horizontal.

During the last decade electromagnetic (EM) methods have become an accepted tool to reduce offshore hydrocarbon exploration risk (Constable, 2010). As an alternative to CSEM with moving horizontal sources PetroMarker developed an offshore, time domain EM method based on vertical, stationary transmitters and receivers (Barsukov et al., 2007). In this paper we present a case study for this method over a Norwegian oil prospect. Kakelborg PL 370 lies about 25 kilometers northwest of the Statfjord Nord field in the North Sea and is operated by Wintershall Norge. A vertical-vertical time domain EM survey was conducted over the Kakelborg prospect by PetroMarker in summer of 2009. After processing and inversion of the EM data it was predicted that the prospect is dry. In July 2012 wildcat well 33/6-4 was drilled without discovering hydrocarbons.

Stefan L. Helwig, Abdul Wahab El Kaffas, Terje Holten, Øyvind Frafjord and Kjetil Eide present the 'vertical–vertical' approach to marine controlled source electromagnetic surveys and discuss its potential benefits compared with conventional horizontal dipole methods.