SPECIAL TOPIC: MARINE SEISMIC & EM
Vertical dipole CSEM: 3D acquisition and data impact from infrastructure Kjetil Eide1* presents improvements to the method and a 3D field data example. Introduction Marine CSEM methods for subsurface investigations were developed nearly 30 years ago (Cox, 1981), and have found extensive applications within the offshore hydrocarbon exploration industry over the past 15 years (e.g. Constable, 2010). These methods detect contrasts in electrical conductivity, exploiting the fact that the electrical conductivity of hydrocarbon-saturated reservoirs is significantly smaller than in the surrounding sediments (Ellingsrud et al., 2002). While marine CSEM has proven itself as a valuable tool in exploration and mapping of frontier areas, there is growing increased interest in applying CSEM for near-exploration and reservoir monitoring applications. A vertical-based time domain EM exploration method (Barsukov et al., 2007) has been developed by the Norwegian geophysical company PetroMarker, founded in 2005. In the past the method has primarily been used for exploration, but since the vertical method relies on a stationary acquisition mode with very high accuracy in transmitter positioning it allows both repeatability and freedom of operation close to existing infrastructure and installations, and in vulnerable environments. Over the past few years there have also been improvements in receiver technology, allowing efficient 3D acquisition. We will present improvements to the method and a 3D field data example. We will also present investigations of the impact of the method on field data from infrastructure.
Figure 1 Vessel used for 2018 acquisition with transmitter and receiver systems. The transmitter electrodes are lowered into sea from the A-frames on the side of the vessel. Dynamic positioning ensures verticality of source dipole during transmission.
1
PetroMarker
*
Corresponding author, E-mail: kjetil.eide@petromarker.com
The vertical CSEM method operates in the time domain and uses a stationary vertical electric dipole (VED) source which transmits a transient electromagnetic signal. The potential of using vertical dipoles as transmitters was suggested at an early stage by, for example, Kaufman and Keller (1983) and Edwards et al. (1985). Goldman (1990) demonstrates the high sensitivity of vertical transient measurements towards resistivities in between resistive air and a resistive basement. When the vertical source is turned on a DC field diffuses outwards both into the sea water and into the subsurface. When the source is turned off, it induces a secondary electromagnetic field which diffuses from the subsurface and back to the receivers, which record the
Figure 2 CSEM receiver with active verticality correction. The vertical field is measured along the vertical centre pole within a tripod structure. A sensor in the top of the receiver measures the verticality of the dipole, and an active system adjusts the position to achieve verticality within 0.1°.
DOI: 10.3997/1365-2397.2019033
FIRST
BREAK
I
VOLUME
37
I
NOVEMBER
2019
79