Patent ID: 7737699
Filing Date: 2010-06-15
Classification: G01V

Abstract:
1. A marine geological EM survey method comprising the steps of: excitation of electromagnetic field in a surveyed medium by passing rectangular current pulses therethrough, with pauses therebetween, and measured along series of circularly shaped probe profiles, the pulses are passed from a dipole electric source located on one of the circular profiles, wherein two identical rectangular pulses are passed through the medium, as follows: a first pulse is passed by moving the probe device clockwise, and the second pulse is passed by moving the probe device in the counterclockwise direction; measuring a first and a second differences of electric potentials during the pauses, while the transient signal still exists, with a constant time interval, at every probe point located behind the dipole source being towed at the angular distance of 90 degree along the profile line, wherein the following condition is provided: the resultant axial potential difference along the profile is equal to zero, to eliminate the effect of the axial component of the current density under the probe point; obtaining a focusing coefficient κ(t three sets of the dipole-source-independent normalized EM parameters are calculated from the measured differences of electric potentials: where: Δt is a time interval between two nearest measurements of instant electric potential differences during the transient process signals existence; ΔU ΔU performing group probe measurements at predetermined neighboring profile points along the entire profile line, and normalized EM measured parameters for each group are summarized, and the following average values are obtained: where: n is a number of probe points in each of groups; using said values and the differential equation of mathematical physics for electric field of a dipole source in an electrochemically polarizing conductive medium where: ∇ Ė(iω) is an electric field excited by a dipole source, expressed for an equation for harmonic alterations of the electric field in time; σ(iω,σ σ η is a coefficient of induced polarization, τ is a time decay constant for potential difference due to induced polarization; solving an inverse mathematical problem to determine three electro-physical parameters proprietary for each element of the medium: the electric conductivity σ plotting three cross-sections graphs of these parameters in the time domain.