(1) Field of the Invention
The present invention relates to a method for electromagnetic geophysical surveying of subsea rock formations. The method comprises towing first and second alternating field emitting sources in first and second depths or elevations above the sea-floor, the transmitter sources controlled for having different phases and amplitudes in order to constitute a phased array emitter antenna with directivity. The directivity is useful for reducing upward-propagating energy which leads to a head wave or “air wave”, and also for ensuring that a major proportion of the transmitted energy propagates downwards into the subsea formations.
The rock formations under the seafloor are water-saturated and the saline pore-fluid contains ions, giving the rock formations a high electrical conductivity, or low resistivity, ρ=0.7-10 Ωm. In this description resistivity will be used, expressed in Ohm-meter, abbreviated Ωm. Hydrocarbons such as oil and gas replace water in the pore spaces of porous petroleum bearing rock formations. Petroleum does not dissolve salts and thus leads to a low electrical conductivity of petroleum-bearing rocks, corresponding to a high resistivity, ρ=20-300 Ωm, and in some instances a resistivity as high as ρ=1000 Ωm. Portions of the otherwise oil-bearing formation filled with brine as pore fluid may not be electromagnetically distinguishable from the overburden. Sea water contains several dissolved salts and usually provides a resistivity of about ρ=0.3 Ωm.
Such buried potentially petroleum-bearing sedimentary rock formations are the target of most geophysical surveys. Geological structures may be subject to seismic surveys, e.g. reflection or refraction seismics, amplitude variation with offset, etc., but petroleum bearing portions of a sedimentary rock formation do not always show seismic characteristics that are distinguishable from water-bearing portions of similar formations that are subject to the seismic analysis.
(2) Description of Related Art
In their patent application US 2003/0052685, “Method and apparatus for determining the nature of subterranean reservoirs”, and an article called “Remote detection of hydrocarbon filled layers using marine controlled source electromagnetic sounding”, EAGE 64th Conference & Exhibition, Florence, Italy, 27-30 May 2002, Eidesmo et al. describe electromagnetic investigations using a horizontally arranged electrical dipole transmitter and electrical dipole receiving antennae arranged over a seafloor with a deeply buried hydrocarbon layer. The receiver dipole antennae are two horizontally arranged orthogonal sets of electrodes. Eidesmo et al. use phase information extracted from a presumably refracted wave response to determine whether there is a high-resistivity potentially petroleum-bearing subterranean reservoir present.
Srnka and Carazzone's US patent application 2003/0050759 relates to a method of simulating emission of a signal of an electromagnetic source using one or more dipole sources. A dipole source is located at an excitation location corresponding to a segment of the electromagnetic source to be simulated. The virtual electromagnetic source comprises a pattern of dipoles distributed in a horizontal plane of the sea. The dipole source is activated, and an electromagnetic signal is recorded at one or more receiver locations. The process is repeated for additional excitation locations corresponding to additional segments in the virtual pattern to be filled in by the electromagnetic source. The data from the sequence of dipole source excitation locations is subsequently processed to determine the simulated signal of the electromagnetic source.
One important purpose of the Srnka patent application is to use a virtual dipole source array in order to simulate a very large circular transmitter in order to simulate focusing of the electromagnetic energy on the target under the overburden. The method of Srnka does not provide a solution to the problem of airborne electromagnetic waves. Much of the electromagnetic energy emitted from the antenna will propagate upward toward the sea surface and propagate through the air. Some of the upward propagating energy emitted from the antenna will also reflect from the sea surface and blur the initially transmitted signal. There is a need to reduce the upward transmitted electromagnetic signal. There is also a prevailing need for producing a stronger electromagnetic signal to propagate downwards from the transmitter antenna and through the seafloor for propagating through the rock formations to be surveyed.