Marine downhole to seafloor dipole-dipole electromagnetic methods and the resolution of resistive targets

Abstract

Most active marine electromagnetic surveys use an array of transmitting and receiving electric dipoles located on the sea-floor. There are many reports of successful detections of buried resistors with these arrays. Nevertheless, the concept has a significant drawback: Large offsets between the transmitter and receiver with respect to the target depth are needed. A logical alternative is the use of a vertical electric-dipole transmitter in a borehole deeper than the target, combined with receivers located at the seafloor. Because current must pass through the target, even data recorded at short offsets with respect to target depth should be sensitive to the target. We derived the equations for the electric and magnetic field components produced by the vertical electric dipole in a 1D layered medium. The equations are used to model the fields for resistive layers. An eigenparameter analysisis applied to the different field components, and the resolution is compared with the standard seafloor inline array. The analyses show that data for the downhole transmitter are more sensitive to the resistance between the source and the receiver than the sea-floor array. However, model geometry such as depth to the target is resolved less easily. The resolution of the model geometry is improved significantly when a long bipole transmitter is used, extended from below the target layer to a level above it. We carried out 3D modeling for the downhole transmitter through the use of a finite-difference algorithm. Results indicate that these data are very sensitive to lateral changes within the target layer. Differences between electric fields simulated at the seafloor for 3D models and a reference model show a correlation to lateral resistivity differences. This finding suggests that the array is suitable for mapping and monitoring resistivity anomalies outward from the borehole.

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

“…Evans (2007) presents the case for using shallow CSEM techniques on the continental margins for a variety of purposes, including sediment characterization, facies mapping and the evaluation of groundwater discharge. Scholl and Edwards (2007) propose a method using a vertical borehole time-domain EM source with seafloor receivers in order to map shallow resistive structure and present several model studies documenting the sensitivity and resolution of the method.…”

The use of rotational invariants for the interpretation of marine CSEM data with a case study from the North Alex mud volcano, West Nile Delta

“…To get further insight into how the interpretation of invariants compares to the standard interpretation of the radial (inline) and/or azimuthal (broadside) component(s), we will look at the sensitivity of transient responses to the parameters in terms of the eigenparameter analysis described in Edwards (1997). Additional application examples of the analysis can be found in Scholl & Edwards (2007) and Swidinsky et al (2012). Contrary to the aforementioned authors we will not use the term eigenparameter (and derived terms) but rather singular value (SV) (and derived terms), because ultimately the analysis is based on the singular value decomposition (SVD) of a scaled Jacobian matrix, which is usually non-square.…”

“…For calculations we use a Fortran code originally developed at the University of Toronto, which implements the theory described in Edwards (1997) and Scholl & Edwards (2007). We have modified the code to allow for calculations with layered water columns and adapted the code to be directly accessible from within Matlab.…”

An electromagnetic-based streamer for near-shore investigations

“…Some systems have been developed in universities (e.g. [1,2]) or transposed from land application to marine environment by some companies with limited success, but none of these trials led to the industrialisation of the methods and instruments for near-shore targets. The most encouraging application of marine EM method has been in the recent years in the oil and gas industry where giant transmitters towed by ships and static receivers laid out on the seabed are used to image deep electrical structures ( > 1km) (e.g.…”