Lars Ole Løseth

Decomposition of electromagnetic fields into upgoing and downgoing components

This paper gives a unified treatment of electromagnetic EM field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia,wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity,magneticpermeability,andelectricalconductivityareknown constants with respect to space and time. Above and below the plane of measurement, the medium can be arbitrarily inhomogeneousandanisotropic. In particular, the proposed decomposition theory applies to marine EM, low-frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir, that are of interest for the interpretation. The direct-source field, the refracted airwave induced by the source, the reflected field from the sea surface, and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements. The viability and validity of the decomposition method is demonstrated using modeled and real marine EM data, also termed seabed logging SBL data. The synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealSBLwerecollected.Theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320-mwaterdepthaboveaknowngas province offshore Norway. The effect of the airwave is seen clearly in measured data. After field decomposition just below the seabed, the upgoing component of the recorded electric field has almost linear phase, indicating that most of the effect of the airwavecomponenthasbeenremoved.

Low-frequency electromagnetic fields in applied geophysics: Waves or diffusion?

Low-frequency electromagnetic (EM) signal propagation in geophysical applications is sometimes referred to as diffusion and sometimes as waves. In the following we discuss the mathematical and physical approaches behind the use of the different terms. The basic theory of EM wave propagation is reviewed. From a frequency-domain description we show that all of the well-known mathematical tools of wave theory, including an asymptotic ray-series description, can be applied for both nondispersive waves in nonconductive materials and low-frequency waves in conductive materials. We consider the EM field from an electric dipole source and show that a common frequency-domain description yields both the undistorted pulses in nonconductive materials and the strongly distorted pulses in conductive materials. We also show that the diffusion-equation approximation of low-frequency EM fields in conductive materials gives the correct mathematical description, and this equation has wave solutions. Having considered both a wave-picture approach and a diffusion approach to the problem, we discuss the possible confusion that the use of these terms might lead to.

Investigating the exploration potential for 3D CSEM using a calibration survey over the Troll Field

We demonstrate the ability of the controlled source electromagnetic (CSEM) technique to reveal hydrocarbon reservoirs by reinvestigating the North Sea Troll field with a 3D acquisition grid. Unlike previous applications of 2D CSEM surveys over this field, which have shown a large electromagnetic response over the gas province, the focus in this study is on the thinner and smaller oil reservoir. A weak but consistent anomaly enables detection and delineation of the oil zone and reduces the ambiguity associated with traditional 2D lines. This holds true even for a sparse receiver grid where the target is located between the receiver and source lines. The 3D data show the extent of the oil zone and that it is matched to the top reservoir structural high as well as north-south aligned faults. Variation in magnitude of the anomaly is qualitatively in good correlation with resistivity logs at the well locations. This suggests that 3D CSEM data pave the way for exploration use because of its superiority over 2D data in terms of reduced sensitivity to survey layouts.

A solution to the airwave-removal problem in shallow-water marine EM

A new method is presented for removing the airwave in shallow-water electromagnetic (EM) data. By using pairs of receivers or pairs of source points along a towline and forming weighted differences of an EM field component, the airwave is significantly attenuated. The weights are related to the geometric spreading of the airwave. Specifically, the horizontal electric and magnetic field components are weighted with the horizontal offset to the power of three before their differences are calculated. One can also first calculate difference data without weights in the common-source domain to remove correlated noise and then use the resulting data to perform a second-order difference with weights. In this case, the weights should equal the horizontal offset to the power of four. Moreover, the weighted difference data can be integrated to obtain an estimate of the physical field void of the airwave. Additionally, the airwave contribution to the physical field can be estimated. The theory behind the method is presented and demonstrated on a real controlled-source EM (CSEM) data set.

TIV Contrast Source Inversion of mCSEM data

We present a 3D contrast source inversion scheme for electromagnetic data in conductive media. We consider only contrasts in electric conductivity but allow the medium to be transversely isotropic in the vertical direction. This has applications in, for instance, inversion of marine controlled-source electromagnetic data. The contrast source inversion (CSI) method is based on the integral equation formulation of electromagnetic field propagation and solves the inverse problem of determining the conductivity structure of the subsurface. The method minimizes a cost functional that enforces both data fidelity and that the solution satisfy the Lippmann-Schwinger equation. Further regularization is introduced linearly into the cost functional to incorporate prior model information. Although the problem is nonlinear, the chosen strategy splits the minimization problem into two linear problems, which are solved alternatingly. To this end, contrast sources are introduced, which may be interpreted as sources emitting the scattered field from a scattering object. Two synthetic and two real field examples are inverted, which demonstrates the method and how the transversely isotropic in the vertical direction (TIV) inversion performs compared with isotropic inversion. The CSI method is found to be applicable to real field examples, and the results show that a TIV inversion is preferred over isotropic to identify weak anomalies in these examples. The reason for this is that both the horizontal and vertical conductivity affects the signal propagation in the overburden.

Detecting Skrugard by CSEM — Prewell prediction and postwell evaluation

AbstractThe discovery of Skrugard in 2011 was a significant milestone for hydrocarbon exploration in the Barents Sea. The result was a positive confirmation of the play model, prospect evaluation, and the seismic hydrocarbon indicators in the area. In addition, the well result was encouraging for the CSEM interpretation and analysis that had been performed. Prior to drilling the 7220/8-1 well, EM resistivity images of the subsurface across the prospect had been obtained along with estimates of hydrocarbon saturation at the well position. The resistivity distribution was derived from extensive analysis of the multiclient CSEM data from 2008. The analysis was based on joint interpretation of seismic structures and optimal resistivity models from the CSEM data. The seismic structure was furthermore used to constrain the resistivity anomaly to the Skrugard reservoir. Scenario testing was then done to assess potential alternative models that could explain the CSEM data in addition to extract the most likely re...

On the Effects of Anisotropy in Marine CSEM

SUMMARY In marine CSEM (controlled source electromagnetic) exploration and SBL (SeaBed Logging), the seabed subsurface is investigated by emitting low-frequency signals from a dipole source close to the seabed. The resulting electromagnetic (EM) field is recorded by receivers that usually sit on the seafloor. The main goal is to describe possible thin resistive layers within the conductive surroundings beneath the seabed. In the following, various anisotropy effects in a stratified earth are studied. The synthetic data examples that are presented have been generated by a modelling tool for stratified media that handles arbitrary anisotropy models. The results show that a model without a thin resistive layer, but with anisotropy effects in the overburden, might resemble the response from a thin resistive layer in an isotropic model.