gpr12771

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Geophysical Prospecting, 2019, 67, 1582–1594

doi: 10.1111/1365-2478.12771

Vertical–vertical controlled-source electromagnetic instrumentation and acquisition Stefan L. Helwig1,2∗ , William Wood3 and Bernard Gloux3 1 Consens,

Houston, TX 77019, USA, 2 Formerly PetroMarker, Houston, TX 77055, USA, and 3 PetroMarker, Stavanger, 4029, Norway

Received March 2018, revision accepted January 2019

ABSTRACT Vertical–vertical controlled-source electromagnetic is an alternative to other techniques for providing three-dimensional resistivity images of the subsurface. It utilizes a large and powerful vertical dipole transmitter and arrays of E-field receivers with vertical and horizontal dipole sensors. The necessary instrumentation and acquisition procedures which differ strongly from other controlled-source electromagnetic methods are described in the paper. Key words: Controlled-source electromagnetic, Electromagnetics, Acquisition, Instrumentation.

1 INTRODUCTION When marine controlled-source electromagnetic (CSEM) was commercially introduced to the hydrocarbon industry in the early 2000s (Eidesmo et al. 2002; Ellingsrud et al. 2002), the methodology was based on previous developments in academia for studying ocean basins and active spreading centres. The roots go back to the 1980s and before when experiments with active source units were carried out by SCRIPPS in the Pacific (Cox, Deaton and Pistek 1981; Young and Cox 1981). Potential usefulness for hydrocarbon exploration was also realized very early on (Srnka 1986) but was commercially not available until the early 2000s when several companies started to provide services that in essence all relied on the same data-acquisition principles. A grid of autonomous offshore magnetotelluric (MT) receivers that continuously acquire horizontal E and H-field data are deployed on the seafloor. To allow for the detection of thin resistive layers, electrical current flow in the vertical plane is necessary, which is not provided by the natural MT-fields, thus an active source is needed and provided in the form of a deep-towed horizontal dipole, transmitting a low-frequency source signal. While many important improvements in acquisition, processing and interpretation have been made to the

∗ E-mail:

1582

slh@consens.technology

technology over the last one and a half decades, the fundamental acquisition scheme of horizontal source CSEM has remained the same. An alternative marine CSEM acquisition approach was introduced by Barsukov, Fainberg and Singer (2007) and Holten et al. (2009). It is based on a vertical dipole transmitter and the measurement of the vertical electrical field component on the seafloor. Contrary to the horizontal dipole which emits a transverse electric mode with current flow in the horizontal plane and a transverse magnetic (TM) mode with current flow in the vertical plane, a vertical dipole transmitter only creates a TM mode. This leads to differences in the interaction of the EM fields with the environment and to differences in the sensitivity towards thin hydrocarbon layers. The detrimental effect of the air wave in shallower water acquisition, for example, can be mitigated by acquisition and interpretation strategies that generate and extract only the TM-mode component of the field (Andréis and MacGregor 2008). The differences in the sensitivity distribution are important for the detection of smaller targets. In horizontal–horizontal acquisition, the maximum sensitivity is at large offsets and low frequencies. Vertical–vertical CSEM is typically measured in time domain, and the maximum sensitivity occurs at small offsets and late times. This smaller source–receiver offset also makes the vertical–vertical approach less prone to disturbing influences from three-dimensional (3D) bodies outside of the

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2019 European Association of Geoscientists & Engineers


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