Marine Controlled-source Electromagnetic Surveys Research Articles

Electric field responses of different gas hydrate models excited by a horizontal electric dipole source with changing arrangements

Abstract The exploration of marine controlled source electromagnetic survey for gas hydrate generally consists of a dynamic horizontal electric dipole source towed in the sea water and a static receiver array allocated on the seafloor. In this paper, two orthogonal horizontal components of electric field excited by a horizontal electric dipole source upon one-dimensional horizontal-layered seafloor are calculated to study the effect of different arrangements and gas hydrate models on the response of electric field. There are several dominant factors determining the variation of induced field, including transmitting frequency of the source, distance between the source and the seafloor, water depth, thickness of the overburden layer, as well as physical and chemical structure of the gas hydrate model. Modeling results suggest that the amplitude of received electric fields can reach up to 10-16–10-10 V /(A·m 2 ) by adjusting transmitting frequency; keeping the source being towed several tens of meters above the seafloor can improve the survey efficiency but not decrease the quality of collected data; deep water environment and shallow thin overburden layer are favorable for the survey because of their attribution to signal-to-noise ratio; the porosity and saturation of gas hydrate have significant influence on medium resistivity-the higher resistivity the medium has, the bigger the amplitude of electric field response will be. Forward modeling can provide valuable information for the development of instruments and the design of survey to detect gas hydrate.

Unlocking the value of CSEM

Lucy MacGregor1 and Richard Cooper suggest that for marine controlled source electromagnetic survey methods to achieve more general adoption in the E&P industry, the applications need to be refocused with more emphasis on prospect appraisal and reservoir monitoring rather than exploration.

3D finite-difference frequency-domain modeling of controlled-source electromagnetic data: Direct solution and optimization for high accuracy

Three-dimensional modeling of marine controlled-source electromagnetic (CSEM) data is vital to improve the understanding of electromagnetic (EM) responses collected in increasingly complex geologic settings. A modeling tool for simulating 3D marine CSEM surveys, based on a finite-difference discretization of the Helmholtz equation for the electric fields, has been developed. Optimizations for CSEM simulations include the use of a frequency-domain technique, a staggering scheme that reduces inaccuracies especially for horizontal electric-dipole sources located near the seafloor, and a new interpolation technique that provides highly accurate EM field values for receivers located in the immediate vicinity of the seafloor. Source singularities are eliminated through a secondary-field approach, in which the primary fields are computed analytically for a homogeneous or a 1D layered background; the secondary fields are computed using the finite-difference technique. Exploiting recent advances in computer technology and algorithmic developments, the system of finite-difference equations is solved using the MUMPS direct-matrix solver. In combination with the other optimizations, this allows accurate EM field computations for moderately sized models on small computer clusters. The explicit availability of matrix factorizations is advantageous for multisource modeling and makes the algorithm well suited for future use within an inversion scheme. Comparisons of simulated data for (1) 1D models to data generated using a 1D reflectivity technique and (2) 3D models to published 3D data demonstrate the accuracy and benefits of the approach.

A Magnetic Gradiometric Method as an Adjunct for Marine CSEM

The marine Controlled-Source Electromagnetic (CSEM) method has proven itself an invaluable adjunct to the seismic method in petroleum exploration in the last few years. The problem is that it is not easily adapted to the Australian scene because most of Australia's petroleum deposits are in relatively shallow waters where CSEM does not work very well. Depending on the target, depths up to 500 m can be considered shallow water for CSEM. CSEM surveys in shallow water can have problems with the airwave dominating the received CSEM signal and also with electromagnetic noise induced by ocean waves. Modelling studies suggest that it is feasible to measure CSEM magnetic field gradients as a supplement to E and B field measurements and use this extra information to remove environmental noise (wave effects and magnetotelluric noise), airwave and direct wave components, and other unwanted contributions to the measured E and B fields. This can improve the signal/noise ratio of the CSEM measurements, thereby producing more reliable interpretations, and extend the useful range of the method, which can reduce survey costs. We have been developing a magnetic gradiometer that can be used for marine CSEM surveys. This poster will present the results from preliminary field trials conducted off Sydney's coast, in water depths up to 62 m, which show that the OceanMAG gradiometer was able to measure the magnetic field induced by ocean waves. Approaches for using magnetic gradient measurements as an adjunct to marine CSEM data will also be described.

Rigorous 3D inversion of marine CSEM data based on the integral equation method

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of offshore petroleum exploration. However, due to enormous computational difficulties with full 3D inversion, practical interpretation of MCSEM data is still a very challenging problem. We present a new approach to 3D inversion of MCSEM data based on rigorous integral-equation (IE) forward modeling and a new IE representation of the sensitivity (Fréchet derivative matrix) of observed data to variations in sea-bottom conductivity. We develop a new form of the quasi-analytical approximation for models with variable background conductivity (QAVB) and apply this form for more efficient Fréchet derivative calculations. This approach requires just one forward modeling on every iteration of the regularized gradient-type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method, which provides a sharp boundary image of the petroleum reservoir. The methodology is tested on a 3D inversion of the synthetic EM data representing a typical MCSEM survey conducted for offshore petroleum exploration.