Johan Mattsson

Anisotropic 3D inversion of towed-streamer electromagnetic data: Case study from the Troll West Oil Province

One of the critical problems in the interpretation of marine controlled-source electromagnetic geophysical data is taking into account the anisotropy of the rock formations. We evaluated a 3D anisotropic inversion method based on the integral equation method. We applied this method to the full 3D anisotropic inversion of towed-streamer electromagnetic (EM) data. The towed-streamer EM system makes it possible to collect EM data with a high production rate and over very large survey areas. At the same time, 3D inversion of towed-streamer EM data has become a very challenging problem because of the huge number of transmitter positions of the moving towed-streamer EM system, and, correspondingly, the huge number of forward and inverse problems needed to be solved for every transmitter position over the large areas of the survey. We overcame this problem by exploiting the fact that a towed-streamer EM system’s sensitivity domain is significantly smaller than the area of the towed-streamer EM survey. This approach makes it possible to invert entire towed-streamer EM surveys with no approximations into high-resolution 3D geoelectrical sea-bottom models. We present an actual case study for the 3D anisotropic inversion of towed-streamer EM data from the Troll field in the North Sea.

Rapid Imaging of Towed Streamer EM Data Using the Optimal Synthetic Aperture Method

The mainstream approach to the interpretation of towed streamer electromagnetic (EM) data is based on 2.5-D and/or 3-D inversions of the observed data into the resistivity models of the subsurface formations. However, the rigorous 3-D and even 2.5-D inversions require large amounts of computational power and time. The synthetic aperture (SA) method is one of the key techniques in remote sensing using radio frequency signals. During recent years, this method was also applied to low-frequency EM fields used for geophysical exploration. This letter demonstrates that the concept of the SA EM method can be extended for rapid imaging of the large volumes of towed streamer EM data. We introduce a notion of virtual receivers, which complement the actual receivers in the construction of the SA for the towed streamer data. A numerical study demonstrates that this method increases the EM response from potential subsurface targets and opens a possibility for on-board real-time imaging of EM data during a survey. The method is illustrated by the imaging of towed streamer EM data acquired over the Troll oil and gas fields in the North Sea. Remarkably, the imaging of the entire towed streamer EM survey requires just a few seconds of computation time on a desktop PC. This result is significant, because it opens a possibility for real-time imaging of the towed streamer EM survey data.

Integral Equation Method for Anisotropic Inversion of Towed Streamer EM Data - Theory and Application for the TWOP Survey

We introduce a 3D inversion methodology for towed streamer EM data that takes into account anisotropy and includes a moving sensitivity domain. Our implementation is based on 3D IE method for computing the responses and Frechet derivatives, and uses re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. Interpretation of the towed streamer EM data is a difficult problem because the data are acquired over large areas with huge number of moving towed streamer EM system positions. We overcome this problem by exploiting the concept of moving sensitivity domain, which is implemented using the IE method. In the framework of this concept, for a given transmitter-receiver pair, the responses and Frechet derivatives are computed from a 3D earth model that encapsulates the towed EM system’s sensitivity domain. The Frechet matrix for the entire 3D earth model is then constructed as the superposition of Frechet derivatives from all transmitter-receiver pairs over the entire 3D earth model. This makes large-scale 3D inversion a tractable problem with moderate cluster resources. We present a case study of 3D anisotropic inversion of towed streamer EM data from the Troll West Oil Province.

3D Anisotropic Inversion of Towed Streamer EM Data over the Mariner Field in the North Sea

Towed streamer electromagnetic (EM) data over the Mariner oilfield in the UK sector of the North has been inverted using a fast and efficient 3D anisotropic inversion code. The electric field data were acquired with a single vessel using a horizontal bipole source and sensors housed in a towed streamer in a densely sampled grid over the subsurface volume of interest. The inversion algorithm is based on the 3D contraction integral equation method and utilizing a re-weighted regularized conjugate gradient technique to minimize an objective functional. This inversion method is proven to be fast and efficient for large data sets and is here shown to be suitable for towed streamer EM data from complex geological environments such as the Mariner area. In this case, the final 3D resistivity cube after inversion and with a corresponding misfit of 6.4 %, agrees well with the expected structure from seismic data and well logs. In particular, the 3D cube contains a resistive anomaly of 8-10 Ωm corresponding to the Maureen and Heimdal reservoirs on top of the resistive chalk and basement.

Comparison of PRBS and Square-wave Transient CSEM Data over Peon Gas Discovery, Norway

We examine the application of the transient CSEM method in shallow water (less than 500 m deep) using towed-streamer EM data obtained in a 2009 survey over the Peon gas field, Norway, and consider the in-line electric field component. We compare the results obtained with two different source signatures: a square wave and a pseudo-random binary sequence (PRBS). We show that the PRBS data after deconvolution have more detailed information in the frequency domain than the square wave data and include the DC component, which is absent in the square wave data. In addition, PRBS data have the advantage that further time-domain processing may be applied, including correlated noise removal, which can increase the signal-to noise ratio by as much as 20 dB, and air wave removal. In addition, using the time of the peak of the earth impulse response function and travel-time to resistivity mapping has the potential to provide a starting model for inversion. These techniques must be applied in the time domain and are therefore not available to square wave data.