In the differentially normalized method of electromagnetic sounding (DNME), the transmitter and receiver are grounded electrical circuits. The conduction and polarization properties of a section are studied by measuring the electrical potential difference (ΔU(t)) and the second potential difference (Δ2U(t)); the latter characterizes the spatial inhomogeneity of the electromagnetic field. Measurements of Δ2U(t) are strongly influenced by three-dimensional inhomogeneities within the receiver spread. To reduce this effect, measurements are made in two positions (left and right) of the transmitter circuit relative to receiver with subsequent averaging of the measured data. Often in field studies, the transmitter and receiver circuits are at an angle to each other, and the use of two transmitters in measurements leads to the need to determine a generalized transmitter for one-dimensional forward numerical modeling of field data. The effect of the off-axis (diagonal) position of the transmitter and receiver circuits on the data of electromagnetic pulse sounding and their inversion for a one-dimensional polarizable conducting medium have been studied in real and numerical experiments. In modeling, the effect of induced polarization (IP) is taken into account by introducing the resistivity frequency dispersion (Cole-Cole equation). Validity of the calculation of the generalized transmitter is estimated for the solution of the one-dimensional forward problem with the inversion of field diagonal measurements. The effect of three-dimensional objects on the results of measurements using the above observation system is estimated by solving the 3D forward problem for a polarizable conducting medium.

No abstract

No abstract

No abstract

Considerable costs of onshore and offshore wells drilling require effective geophysical methods of HC deposits exploration and study. One of such methods is DNME dealing with conductive and polarization medium properties. In some cases electrical resistivity can indicate HC presence in reservoir that have increased resistivity or water having reduced resistivity. Polarizable medium properties alter in presence of HC. Such alterations are observed to the first regional confining layer, which typically occurs at depth to 1000m. Hence studying medium polarization properties HC occurrence can be detected even at relatively small depth. DNME exploration is conducted both off and onshore. Horizontal electric lines are used as the source and the receiver of the electromagnetic signal. At marine exploration electric field potential differences of the first and second order are registered at several spans. Offshore measurements are carried out on the move of a vessel. At that inductive sounding based on skin-effect is combined with induced polarization (IP) based on the geometric principle. Combined inversion of measurements on several spans allows studying distribution along with conductivity depth and polarization properties of geological environment. In order to obtain quality electromagnetic data the series of measuring techniques was developed. Digital filtration also significantly increases S/N ratio. Onshore and offshore DNME explorations are carried out all over the world: shelves of Sea of Okhotsk and Barents Sea, Caspian Sea, Sea of Azov, Baltic and Caribbean Seas, Gulf of Ob and Tar Bay, Volga-Ural Timan-Pechora, West Siberian, East Siberian, Peri-Caspian, Baltic, North-Caucasian oil-and-gas provinces, China, Cuba, Peru. One of the examples of successful DNME usage offshore is the exploration in Barents Sea (Severo-Gulyaevskoye field). Technique of induction (EM) and IP fields' separation in the whole measuring range allowed to map deposit’s contour occurring in IP field. Geoelectric modeling showed polarizability coefficient increase in deposit's contour at depth complying to regional confining layer. According to well testing data this depth interval (500-600 m) contains high concentration of epigenetic pyrite. The same is observed directly above the oil-saturated reservoir at depth around 2500.