This patent application is assigned to
The following quote was obtained by the news editors from the background information supplied by the inventors: "Conventional reflection seismology involves generating acoustic waves from a seismic energy source and then detecting reflections of those waves from interfaces in the earth is formation being analysed. When a velocity interface is encountered, the seismic waves are partially reflected back to the surface, where they are detected and recorded. The time taken for the reflected energy to return travelling at the speed of sound through the earth (as attenuated by the different types of rocks and formations encountered) indicates the depth of the reflecting structure. Geophones are used to measure the vertical and/or horizontal components of the reflected seismic waves.
"U.S. Pat. No. 4,904,942 discloses a different scheme for seismic analysis referred to as 'electroseismic' prospecting. In this approach, seismic energy is converted into electromagnetic energy. This conversion is believed to take place as a result of distortion or breakage of bonds between a fluid and the surface of a porous structure containing the fluid. Rapid movement of the fluid caused by an incident acoustical wave front disturbs these dipoles and induces an electromagnetic response. In contrast to a reflected acoustic wave, the electromagnetic wave generated by the electromagnetic response travels at the speed of light through the earth (as attenuated by the materials encountered) with respect to the rock formations.
"EP 1577683 describes a method for characterising a formation including exciting the formation with an acoustic wave and measuring a seismo-electromagnetic response, and then exciting the formation with an electromagnetic field and measuring an electromagnetic-seismic response."
In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventors' summary information for this patent application: "The present invention provides a method of geophysical prospecting for detecting bodies of fluids in underground porous formations, comprising the steps of:
"activating a seismic energy source to transmit seismic energy into the ground;
"detecting an electromagnetic signal generated by interaction between the seismic energy and underground formations containing fluids; and
"recording at least one parameter derived from the detected electromagnetic signal against time, the signal being generated by interaction between a secondary energy impulse from the activation of the energy source and underground formations including fluids, the secondary energy impulse propagating to the formations more slowly than the primary seismic waves from the same activation of the energy source travelling at the speed of sound through the earth (as attenuated by the materials encountered), wherein the detected electromagnetic signal comprises frequencies in the range 0 to 30 Hz.
"The present inventors have identified a secondary electromagnetic signal comprising frequencies in the range 0 to 30 Hz generated in response to activation of a seismic energy source. These signal returns appeared at very late times (extending to many tens of seconds to minutes after the activation times), far outside the times expected for seismic returns or electroseismic phenomena as previously known in this field. It is believed that a secondary slow energy wave travels into the ground, penetrating to significant depths and providing enough energy to move fluids encountered in porous media as it travels down, triggering an electroseismic response. The relatively slow speed of the slow secondary wave (substantially slower than the primary seismic waves) means that the response signal is generated over a longer time period, making it easier to record with greater resolution with respect to times and therefore with respect to the depth below the surface of the detected formations.
"The method preferably includes signal processing involving converting the detected electromagnetic signal via a current-to-voltage converter. It may further include amplifying lower frequencies of the signal output from the current-to-voltage conversion step relative to higher frequencies of the signal output.
"In preferred implementations, the detecting step detects the rate of change of the electromagnetic signal, and the method includes a step of integrating the detected signal with respect to time. For example, where the detector used senses the time derivative of the magnetic field strength, integrating the signal with respect to time will yield the magnetic field strength with respect to time.
"The recording step may comprise recording harmonics of the original electromagnetic signal generated by interaction between the seismic energy and underground formations including fluids are recorded. Thus, the 'detected' electromagnetic signal may be detected or identified with reference to harmonics it generates in the apparatus. Harmonics may come from various sources in the apparatus. For example, they may arise in an antenna, a sound card, in a data acquisition system, and FFT analysis may see very high frequencies as the result of chopping up the signal in the digital conversion process. Techniques associated with harmonics arising in sound cards are described in 'Observations on Sound Card Audio Levels and MFSK16 Spurious Emissions' by
"In further embodiments, a magnetic component of the electromagnetic response waves is detected, preferably in two or three mutually orthogonal directions. Furthermore, an electric component of the response may also be detected. Preferably two horizontal orthogonal directions are detected or three mutually orthogonal components are detected. These measurements may be carried out at two or more locations.
"It is believed that the secondary energy impulse is at least partially propagated as a poroelastic compressional slow wave. In recent years, research has been carried out into wave propagation in anisotropic poroelastic media. For example, the paper 'Wave propagation in transversely isotropic fluid-saturated poroelastic media' by Liu et al, Series A, Vol. 45, No. 3 2002,
"Thus, it is believed that the seismic energy source sets up a diffusive poroelastic pressure loading. This leads to volumetric dilatation and shear strains on hydrocarbon and other fluid bearing pores within the underground formations. An electromotive force is generated by the motion of the charge-bearing fluids in the formation pores. The change in confining pressure also leads to a change in material resistivity, which also influences the magnitude of the resulting electromagnetic fields. The electromotive force results in a secondary magnetic field which diffuses back up to the surface which is detected by the receiving apparatus.
"Recorded signals may then be processed to correlate them with the depth of fluid-containing underground formations with which the secondary energy impulse interacts to generate the electromagnetic waves.
"The inventors have determined that the data is preferably correlated with depth on the basis that the secondary energy impulse propagates from the energy source to the electromagnetic wave generating formations at an average speed in the range 5-30 m/s (or more preferably 10-20 m/s).
"The seismic energy source is preferably located above ground, that is, the source is preferably surface-based.
"In preferred techniques, the seismic energy source is activated at least twice, with a predetermined time interval between the activations, and the electromagnetic wave responses detected after each activation are then compared. Information may then be determined relating to the properties of a fluid detected underground on the basis of the comparison. It has been found that repeated activations cause changes in signal strength as underground fluids are disturbed, but different fluids with different viscosities and identities may be affected in different ways.
"Statistical analysis may be used to process measurements derived from a plurality of seismic energy source activations carried out at the same location.
"In a further variation, the method includes activating the seismic energy source at at least two different locations, wherein the detecting and recording steps are carried out after each activation.
"The electromagnetic waves may be detected using at least two antennas with the signals generated by the antennas being compared so as to identify signal content attributable to noise which may then be subtracted from the signals.
"Furthermore, the method may include steps of detecting ambient noise present at a detection location and processing signals generated during detection of electromagnetic waves generated in the ground to reduce the amount of signal content attributable to the ambient noise.
"In preferred embodiments, the recorded data is analysed or adjusted having regard to the effects of gravity attributable to the moon and/or the sun. The effect of the moon and/or sun pulling on fluids in the ground may influence the return signal strength. By building up a database of information concerning the relationship between such gravitational effects and signal strength, algorithms may be developed to adjust signal strength to take into account these influences. The recorded data may also be adjusted having regard to solar radiation levels. By determining effects attributable to solar radiation, data can be adjusted having regard to radiation levels measured at the time of signal detection.
"In accordance with embodiments of the present method, information may be determined relating to the properties of a body of fluid detected underground, with reference to the frequency and/or amplitude of detected electromagnetic waves generated by interaction between the secondary energy impulse and underground formations of which the body of fluid forms part. It has been determined that quantative assessment of changes in the frequency content of the detected signal as a function of travel time, can provide information regarding fluid-containing formations. Furthermore, peaks in plots of the amplitude of the signal against frequency may be associated with specific pore fluid compositions.
"The effect described herein appears to stimulate movement of the underground fluids and provides signals when the reservoirs have porosity or permeability. The acoustic wave entering the formation from above is believed to stimulate a movement within the reservoir, the movement being much greater when the reservoir is porous or permeable. Thus, the detected electromagnetic signal may be correlated with the porosity or permeability of the underground formation.
"As well as use in mapping conventional gas and oil reservoirs, the present techniques are also applicable to mapping other fluid-containing formations, including those having relatively low porosity or permeability (such as shales). Recently, extraction technology has been developed which makes it economically viable to extract fluids from these formations. The response achieved using the methods herein appears to be better when the fluids can move, which normally occurs in regions where there are fractures or localised regions of porosity or permeability.
"According to a further aspect, the present invention provides apparatus for geophysical prospecting to detect bodies of fluids in underground formations, comprising:
"a detector assembly for detecting an electromagnetic signal generated by interaction between seismic energy from a user-activated seismic energy source and underground formations; and
"a recorder arranged to record at least one parameter derived from the detected electromagnetic signal against time, the signal being generated by interaction between a secondary energy impulse from the activation of the energy source and underground formations, the secondary energy impulse propagating to the formations more slowly than the primary seismic waves from the same activation of the energy source travelling at around the speed of sound (as attenuated by the materials through which they pass), wherein the detected signal comprises frequencies in the range 0 to 30 Hz.
"One or more coil antennas with a core made of a high magnetic permeability material are preferably used to measure the magnetic portion of the electromagnetic signals. The antenna(s) may stimulate harmonics of the original electromagnetic signal.
"The recorder may be arranged to record harmonics of the electromagnetic signal generated by interaction between the seismic energy and underground formations including fluids. As noted above, harmonics may come from various sources in the apparatus, and may be analysed to provide information relating to the underlying, original electromagnetic signal.
"In some embodiments, the detector assembly comprises an antenna, and magnetic shielding associated with the antenna to reduce the magnitude of signals incident on the antenna which emanate from a source above the ground.
"Preferably, the detector assembly comprises at least two antennas, and the apparatus includes a processor for comparing the signals generated by the antennas so as to identify signal content attributable to noise.
"The detector assembly may include at least one geophone to detect the seismic response to the energy source.
"To process the detected signals, the apparatus preferably includes a current-to-voltage converter electrically coupled to the detector assembly. Furthermore, it may have an amplifier electrically coupled to the current-to-voltage converter and arranged to amplify lower frequencies of the output from the current-to-voltage converter relative to higher frequencies of the output. An integrator may be incorporated for integrating the detected signal.
BRIEF DESCRIPTION OF THE DRAWINGS
"Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:
"FIG. 1 is a flow diagram illustrating a method of geophysical prospecting according to an embodiment of the invention;
"FIG. 2 is a schematic representation of apparatus for geophysical prospecting according to an embodiment of the invention;
"FIG. 3 is a block diagram of an arrangement for processing detected signals;
"FIG. 4 is a representation of a control panel for the amplifier of FIG. 3;
"FIG. 5 is a circuit diagram corresponding to an implementation of the amplifiers of FIG. 3; and
"FIG. 6 is a diagram representing a cross-section through the Earth's surface including a geological fault and an oil reservoir."
URL and more information on this patent application, see: Edwards, Christopher; Nosworthy, Michael. Methods and Apparatus for Geophysical Prospecting to Detect Bodies of Fluids in Underground Formations. Filed
Keywords for this news article include: Electronics, Electromotive, Wave Propagation,
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