University of Southampton

Statoil Asa

Lucy M MacGregor

Martin C Sinha

Jennifer L Rust

An electromagnetic survey method for surveying an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir, comprising obtaining a first survey data set with a vertical electric dipole (VED) antenna for generating vertical current loops and a second survey data set with a vertical magnetic dipole (VMD) antenna for generating horizontal current loops. In an alternative embodiment, the VMD antenna is dispensed with and the horizontal electromagnetic field is derived from the naturally occurring magnetotelluric (MT) electromagnetic field. In another alternative embodiment, the VED data is compared with a background geological model instead of VMD or MT data. Also described are a survey apparatus comprising VED and VMD antennae, planning a survey, and analysis of survey data. The first and second survey data sets allow the galvanic contribution to the detector signals collected at a detector to be independently contrasted with the inductive effects.

Electromagnetic surveying for hydrocarbon reservoirs

An electromagnetic survey method for surveying an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir, comprising obtaining a first survey data set with a vertical electric dipole (VED) antenna for generating vertical current loops and a second survey data set with a vertical magnetic dipole (VMD) antenna for generating horizontal current loops. In an alternative embodiment, the VMD antenna is dispensed with and the horizontal electromagnetic field is derived from the naturally occurring magnetotelluric (MT) electromagnetic field. In another alternative embodiment, the VED data is compared with a background geological model instead of VMD or MT data. The invention also relates to a survey apparatus comprising VED and VMD antennae, to planning a survey using this method, and to analysis of survey data taken using this survey method. The first and second survey data sets allow the galvanic contribution to the detector signals collected at a detector to he independently contrasted with the inductive effects. This is important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes or structure.

An apparatus for generating electronic signals for application in subsea electromagnetic exploration. A surface generated high-voltage low-current source signal stabilized at a first frequency is supplied to a deep-tow vehicle () via an umbilical cable (). The high-voltage low-current signal is transformed at the deep-tow vehicle to a high-current low-voltage a.c. signal by a transformer () within a cycloconverter (). A semiconductor relay bridge () provides switchable rectification of the high-current low-voltage a.c. signal to provide a quasi-square wave at a second frequency, lower than the first frequency, for supply to a transmitting antenna () towed by the deep-tow vehicle. The times of the rectification switching are dependent on zero crossings of the high-current low-voltage a.c. signal. Allowable rectification switching times may be gated to occur only within pre-determined time windows to avoid noise-induced zero-crossing switching. The apparatus allows multiple transmission frequencies to be derived from a single stabilized source and improves spectral integrity by avoiding rectification switching at zero-crossings not occurring at the first frequency.

Signal generation apparatus and method for seafloor electromagnetic exploration

An electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir, comprising obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination allow the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the effects of signal attenuation, which are highly dependent on local properties of the rock formation, overlying water and air at the survey area. This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of structure.

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Southampton, United Kingdom

Lucy M MacGregor