In a method of geophysical prospection a transmitter dipole and a receiver dipole which is sensitive to an electromagnetic field are positioned in cooperation with the subsoil. An impulsive excitation current is applied repeatedly to the transmitter dipole, the excitation current comprising a group of pulses of alternate polarity, each followed by zero current. The transient signal, which is produced thereby at the output of the receiver dipole, is repeatedly detected, and the transient signals are processed to obtain a figurative representation of the transient phenomenon which facilitates geophysical interpretation. Time intervals which are consecutive to excitation pulses and simultaneously with the excitation zero current are defined, these time intervals being of a predetermined duration, chosen to substantially cover all the useful parts of the transient phenomenon. The transient signal is detected as a function of time at least during the time intervals and is sampled, numbered and registered, temporally marked. A processing operation is carried out which commences with a preprocessing in which an unprocessed numerical representation of the transient phenomenon as a function of time is subjected to statistical evaluation of elements of the numerical registration which correspond temporally to the successive time intervals, followed by a smoothing digital filtering which provides a smoothed numerical representation of the transient phenomenon, cleared of a substantial part of the surrounding noise.
A method of geophysical analysis includes transmitting an electromagnetic signal into the earth and measuring the resulting transmission of that signal through the earth to a plurality of spaced recording units. The values of the transmitted signal are locally digitized and stored at each recording unit. The stored data from each recording unit is subsequently transmitted through a telemetric link to a central processing or analyzing unit, the stored data being transmitted from one recording unit at a time and in a preselected sequence. The data may be transmitted from each recording unit in a block and stored in the central processing unit. The digitizing and storing of the measured values at each unit is accomplished through a yield point amplifier. Automatic drift correction processing may also be applied to the measured values obtained from each recording unit. A microprocessor is installed at each recording unit to process, store, and relay the data measured and to test and control the status of the receiving unit.
A transient electromagnetic method for detecting irregularities on container walls by measuring wall thickness. The method utilizes a transmitting antenna and a separate receiving antenna arranged in a loop-loop configuration. The transmitting antenna induces current into the container wall and the receiving antenna and the receiver detect the decay of the induced current, which is then analyzed to detect wall thickness. A receiving antenna array of many receiving antennas is used to increase the spatial resolution. Simultaneous measurement of the induced current by the receiving antennas reduces noise that is coherent across the array. Use of a noise antenna located so as to be unaffected by the transmitting antenna further reduces noise. The received signal from the receiving antenna array is displayed on a two-dimensional display in a spatial arrangement that corresponds to the spatial arrangement of the receiving antennas. The display can be scrolled over the time range of the received signals to produce a moving image, so that irregular areas can be more easily detected than with a static display.
A transient electromagnetic method for detecting irregularities on container walls by measuring wall thickness. The method utilizes a transmitting antenna and a separate receiving antenna arranged in a loop-loop configuration. The transmitting antenna induces current into the container wall and the receiving antenna and the receiver detect the decay of the induced current, which is then analyzed to detect wall thickness. A receiving antenna array of many receiving antennas is used to increase the spatial resolution. Simultaneous measurement of the induced current by the receiving antennas reduces noise that is coherent across the array. Use of a noise antenna located so as to be unaffected by the transmitting antenna further reduces noise. The received signal from the receiving antenna array is displayed on a two-dimensional display in a spatial arrangement that corresponds to the spatial arrangement of the receiving antennas. The display can be scrolled over the time range of the received signals to produce a moving image, so that irregular areas can be more easily detected than with a static display.
The invention relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from the or each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The invention enables subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.
A method of processing electromagnetic signals which are injected into the earth from a capacitor and subsequently detected after reflection from subsurface layers, in order to determine the physical and electrical properties of those layers. The method is based on an iterative process which models the earth as a series of vertically stacked horizontal layers, each characterized by its physical properties of depth beneath the surface and thickness, and its electrical properties of resistivity (or conductivity) and relative dielectric constant. An initial propagation model which specifies the electrical properties of a particular layer or set of layers is constructed and applied to a model input pulse to produce a predicted return pulse which results from the reflection and/or transmission of the input pulse at the boundaries of the layer(s). The predicted return pulse is then compared to actual return pulse data which is selected so as to roughly correspond to a return pulse produced at the location of the modeled layer(s). The result of the comparison forms the basis for adaptively varying the parameters of the propagation model in a manner which is intended to increase the correlation between the predicted return pulse and the actual data. Repetition of the method for different layers or additional locations permits the electrical characteristics of the earth beneath a specified region to be determined. Knowledge of these characteristics can be used to infer the type of geologic formation responsible for transforming the input pulse into the received pulse.
