Acoustic velocity measurements in materials using a regenerative method

© Holder (s):

(74) Agent ^): Brevatome.

(54) Apparatus for measuring the rate of propagation of acoustic energy in materials using a reaction method.

(57) The apparatus for characterizing material s according to the invention comprises an acoustic energy generator 18 arranged to supply acoustic energy to the material to be characterized 12 at a first location 14. this acoustic energy propagating in the material, an acoustic energy receiver 24 arranged to receive the acoustic energy propagated in the material at a second location 16 and to convert this received energy into electrical signals, an amplifier device 26, 28 connected between the receiver and the generator in a closed loop 30 to amplify the electrical signals and to supply these amplified signals to the generator, the amplifier device I having a reaction gain sufficient to produce oscillations which indicate the characteristics of the material, and an indication means 40, 42, 44, 46, 48 to indicate the character of the oscillations and thus of the material.

Application to geological exploration.

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FR 2 534 026 □

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The invention relates generally to the characterization of materials by measuring the propagation of acoustic energy in these materials. More specifically, the invention relates to geological exploration in which acoustic energy is propagated in a part of the earth to characterize the materials found in the earth through which the acoustic energy is propagated.

The Government of the United States of America acquired rights to the invention under contract No. W-74O5-EMG-U8 between the US Department of Energy and the University of California.

It is common practice to characterize materials by sending an energy wave, constituting a certain form of energy, such as acoustic energy, into the material to be characterized. This technique is used ®i geology, to characterize a volume of earth in the search for materials such as oil and water deposits. The technique is also used in technology relating to the non-destructive evaluation of building materials to detect imperfections, such as voids, cracks, or inclusions "in materials such as steel, 1" aluminum and concrete.

This technique uses an energy source to generate an energy pulse signal which crosses the material to be characterized. A receiver is used to receive and interpret the pulse or signal. The distance between the source and the receiver is known, as well as the energy of the transmitted pulse. By measuring the time taken by the signal to move from the source to the receiver, one can evaluate the speed of propagation and compare it to tables of known characteristics of materials. In this way, it is possible to characterize the type of material. By taking a series of measurements along different paths in the material, and by detecting and characterizing the echoes, it is possible to detect and characterize irregularities in the material. ''

In geological exploration, the energy source and the receiver can be placed at different locations on the earth's surface, or in first and second boreholes which are spaced apart from each other by a known distance. . The source or generator can produce signals as a series of discrete pulses that travel the known distance between the generator and the receiver. Typically, the generator can be constituted by devices such as dynamite or other explosive charges, or a source of electrical sparks. These generators can produce pulses comprising compression or pressure waves, oxidation of rarification, and rotation waves having two horizontal and vertical components as desired. Rotation waves are often used as energetic pulses because rotation waves travel at a speed of approximately one-half to one-third the speed of the compression waves, thereby allowing the receiver more time to receive and analyze the impulses. Existing techniques are problematic and have drawbacks, including operating difficulties in developing and using existing techniques, difficulties in making precise measurements of the pulse propagation times, and difficulties in interpreting the test data, so interpretation by highly qualified personnel is required. Comparing test data with known material characteristics is also difficult.

An object of the present invention is a new and improved method for characterizing materials by the propagation of acoustic energy in these materials.

Another object of the invention is to have this new improved process which is more precise, simpler and easier to implement than those of the prior art.

3.

Another object of the invention is a new and improved apparatus for characterizing materials by the propagation of acoustic energy in these materials.

To achieve these and other objects, the present invention preferably provides a method for characterizing materials comprising providing acoustic energy at a first location to the material to be characterized, the acoustic energy being propagated in the material, to be received the acoustic energy in a second location identify in the material and convert the received acoustic energy into an electrical signal, amplifying the electrical signals to produce amplified electrical energy, converting the amplified electrical energy into this acoustic energy, the amplification operation being performed with sufficient feedback amplification to produce oscillations of the electrical energy and the energy, and to indicate the frequency of the oscillations as an indication of the character of the material.

The indication operation preferably comprises measuring the frequency of the oscillations, and converting the frequency into an indication of the speed of propagation of the acoustic energy in the material. This conversion can be carried out by multiplying the frequency by the distance of acoustic propagation in the material.

The simplification operation preferably includes adjusting the feedback amplification to produce sustained oscillations.

The benefit of this simplification by reaction is preferably used as another indication of the character of the material.

Acoustic energy is preferably produced in the form of a whole sinusoidal acoustic rotation wave for propagation in matter.

According to the present invention, an apparatus for ca35 characterizing materials comprises an acoustic energy generator for converting electrical energy into acoustic energy and for supplying this acoustic energy only to the material to be characterized in a first location, the acoustic energy being propagated in matter, an acoustic receiver for receiving the acoustic energy from the material at a second location and for converting the received acoustic energy into electrical signals, an amplifier connected between the receiver and the transmitter for amplifying the electrical signals from the receiver and for supply corresponding amplified electrical energy to the transmitter, the amplifier having a reaction gain sufficient to produce oscillations which indicate the characteristics of the material in which the acoustic energy is emitted, and an indicating means for indicating the character of these oscillations as an indibation of the character of this material .

This indication means preferably comprises a speed indication means for measuring the frequency of the oscillations and for converting the frequency into a speed indication by multiplying the frequency by the distance in the material between the transmitter and the receiver.

The amplifier preferably includes a gain control circuit calibrated to adjust the gain of the amplifier to produce sustained oscillations while indicating the magnitude of this gain as an additional indication of the character of the material.

The generator preferably includes means for generating sinusoidal acoustic rotation waves as a whole for propagation in the material.

The receiver preferably includes a geophone.

Other characteristics and advantages of the present invention will be highlighted in the following description, given by way of nonlimiting example, with reference to the appended drawing in which the single figure is a schematic elevation view, partially in section, showing a preferred embodiment of a device according to the present invention.

In the figure, an apparatus 10 according to the invention is intended to indicate and measure the characteristics

2534 Ô 2 6 beyond matter 12 which, in this case, takes the form of a volume of matter in the ground. Thus, the apparatus 10 is particularly intended for geological exploration. However, the apparatus of the invention can also be used for the non-destructive testing of different building materials.

As shown in section in the figure, the earth material 12 preferably comprises a first and a second bore hole 14 and the which are spaced from one another by a chosen distance D. Holes 14 and 16 can be drilled or otherwise formed in the earth material 12. An acoustic generator 18 is mounted in the first bore hole © and is suitably fixed against the side of the hole d bore 14, to provide acoustic energy to the earth material 12. The generator 18 is preferably of a known or suitable construction so that it is arranged to convert electrical energy into acoustic energy, preferably in the form of sine waves as a whole. Thus, for example, the generator 18 can be a generator of vertical rotation waves manufactured by the company Mark Products. A control circuit or a power amplifier 20 supplies an electrical supply current to the generator 18, so that it produces acoustic waves or pulses 22 which are propagated in the earth material 12.

The energy or acoustic waves 32 propagate in the material 12 and are detected by a receiver 24 which is preferably mounted in the second bore hole 16 and which is fixed against 1® on the side of the hole, in contact with earth material 12, to receive acoustic energy at a reduced or attenuated level.

The receiver 24 constitutes a transducer used to convert the acoustic energy received into electrical signals which are sent to the input of an amplifier 26 comprising a variable gain control circuit 28. The receiver 24 can be a vertical geophone.

The output of the amplifier 26 is connected to the input of the control circuit 20 to form a closed loop 30 which, preferably, also includes a filter 32. It will be noted that the order of arrangement of the amplifier 26, gain control circuit 28 and filter 32 in the closed loop. The phasing or polarity of the amplifier 26 in the closed loop is preferably such that the closed loop provides a positive feedback reaction or amplification.

The closed loop 30 may include a first transmission line 34 for passing the electrical signals from the receiver 24 to the amplifier 26, a second transmission line 36 for passing the amplified electrical signals from the amplifier 26 to the control circuit. 20, and a third transmission line 38 for passing the electric current from the control circuit 20 to the transmitter 18.

The apparatus 10 shown includes an indication means which may include an oscilloscope 40, a level measuring device 42, and a frequency measuring device 44. As shown, the oscilloscope 42 is provided with a selector switch 46, by which the input of the oscilloscope 40 can be connected to the output of the amplifier 26 or to the output of the filter 32. In this case, the level measurement device 42 is connected to the output of filter 32. The frequency measurement device 44 is also connected to the output of the filter 32. The oscilloscope 40 can be used to measure the voltage of the simplified electrical signals coming from the amplifier 26 or from the filter 32. It is also possible to use the oscilloscope 40 for indicating or measuring the frequency of the amplified electrical signals, and also for indicating the waveform of these signals.

The level measuring device 42 indicates or measures the voltage of the amplified electrical signals. The frequency of the amplified electrical signals is indicated or measured by the frequency measuring device 44.

According to the method of the present invention, the amplifier 26 disposed in the closed loop JO provides feedback amplification. The variable gain control circuit 28 is preferably adjusted so that this reaction amplification produces sustained oscillations in the closed loop 30. The frequency of these oscillations is then indicated © u measured by the oscilloscope 40 or by the frequency measuring device 44, as an indication of the characteristics of the earth material 12 in which the acoustic waves 22 are propagated between the transmitter 18 and 1 © receiver 24- The frequency of the oscillations is proportional to the speed at which the acoustic energy propagates in the earth material

- 12. The present speed can be determined or evaluated by multiplying the frequency by the distance D between the transmitter 18 and the receiver 24, in the earth material 12. The speed is deduced from the following relation- v = d / t where v = speed, d = distance, t = time and t = 1 / f, where f = frequency. We then have sv = df.

As shown, a converter 46 is connected between the frequency measuring device 44 and a speed measuring device 48. The converter 46 can multiply the frequency by the conversion coefficient, to produce the speed, which is indicated by the speed measuring device 48. The converter 46 is preferably adjustable or programmable so that the operator can vary the conversion coefficient. The frequency measuring device 44, the converter 46 and the speed measuring device 48 can be analog or digital devices.

The gain control circuit 28 is preferably calibrated to indicate the gain coefficient which is necessary to produce sustained oscillations in the closed loop 30. This gain coefficient is an indication of the attenuation of the acoustic energy or · Mique when it is propagated in the ground material 12 between the generator 18 and the receiver 24. Thus, the coef2534026 ô

another indication of the characteristics of earth matter 12.

The filter 32 can be used to suppress or attenuate spurious or interfering signals which are picked up by the receiver 24. The filter 32 prevents these spurious signals from interfering with the precision of the measurements carried out by the device 10. These spurious signals may include low frequency vibrations of 60 Hz or 120 Hz "produced by electrical material or equipment.

When the gain or efficiency of the entire closed loop 30 exceeds unity and the proper phase relationship is present, sustained oscillations are produced in the loop. These oscillations are produced both in electrical energy and in acoustic or seismic energy 22. The gain coefficient between the generator 18 and the receiver 24 is less than unity 'due to the attenuation of the acoustic or seismic energy when it propagates in the earth material 12. To obtain one. device gain exceeding unity, the gain or amplification of the electrical device must also be greater than unity. The device gain is the product of the partial gain in the earth material 12, between the generator 18 and the receiver 24, and the amplifier gain in: the electrical device, comprising the variable gain control circuit 28, the amplifier 26, filter 32, and control circuit 20.

During operation of the apparatus 10, the amplifier has a gain which is gradually increased by the gain adjustment increase made by the variable gain control circuit 28, until the device gain is greater than one or the unit, as indicated by the production of oscillations maintained in the closed loop 30. The oscillations maintained are indicated by the oscilloscope 40 and the level measurement device 42. The noise inherent in the closed loop provides initial signals from which the sustained oscillations are established. The frequency of the maintained oscillations is a direct measure of the speed of propagation of the acoustic or seismic waves 22 in the ground material 12. The speed of propagation of the acoustic or seismic waves 22 in the ground material 12 ναό rie of approximately 300 meters / second at around 10,000 m / sec, depending on the characteristics of the soil material. Thus, the measured speed is an indication of these characteristics. The speed of propagation of electrical signals in the electrical device is much greater, approaching the speed of light which is 8 x 10 m / sec. This applies to any electrical device, including the receiver 24, the transmission line 34, the variable gain control circuit 28, the amplifier 26, the filter 32, the transmission line 36, the control 20, the transmission line 38 and the generator 18. Due to the high propagation speed of the electrical signals the propagation time of the electrical signals in the. electrical device is negligible compared to the acoustic speed.

When the gain of the closed loop is increased enough to produce sustained oscillations, the frequency of these oscillations is determined by the speed of propagation of the acoustic or seismic energy in the earth material 12 over the distance D separating the generator 18 from the receiver 24. Because of this relationship, the frequency of the oscillations is proportional to the speed of propagation of the acoustic energy. The measured frequency can be converted into speed by multiplying the measured frequency by the distance D along which the acoustic waves are propagated between the generator 18 and the receiver 24.

Measuring the speed of propagation of acoustic or seismic energy in the earth material 12 between the two bore holes 14 and 16 provides a valuable indication of the geological structure of the earth.

This geological information is necessary for most civil engineering applications, for mineral exploration, petroleum exploration, and for exploration for other sources of energy, such as oil shale. Exploration for water is also facilitated by this geological information.

The method and apparatus of the present invention can also be used to perform a non-destructive assessment of building materials, such as steel, aluminum, and concrete, to look for inclusions, cracks and other imperfections.

The present invention has the particular advantage of providing simpler and more precise measurements of the speeds of propagation of acoustic energy in the materials to be characterized. According to the present invention, the acoustic speed is determined by measuring the frequency or the period of the maintained oscillations. This frequency or this period is measured more easily and more precisely than by the traditional method for directly measuring the precise travel time of an acoustic or seismic pulse in the material to be characterized. This direct measurement of the travel time involves measuring the delay between the start of the acoustic impulse from the generator and the arrival of the acoustic impulse at the receiver. These delay measurements are difficult, particularly in the presence of a high noise level. The measurement of the frequency of the oscillations can be carried out very easily and very precisely.

The present invention has the additional advantage that the method and apparatus of the invention can easily be used to continuously control the rate of propagation of acoustic or seismic energy in the earth material. For such an application of the present invention, the sustained oscillations are produced continuously. The frequency of these oscillations is measured continuously, or at frequent intervals, and is recorded by a paper recorder 50 or the like. The recorder 50 can also record the speed. Frequency variations indicate changes in soil matter. These changes can be produced by variations in temperature, humidity or stress.

Thus, the present invention can be used to control temperature variations in the vicinity of storage means and underground disposal for radioactive materials. Furthermore, the present invention can be used to control temperature variations as one. indication of volcanic activity. In addition, the present invention can be used to control variations in stress which may precede earthquakes and it can therefore be useful for predicting possible earthquakes.

To carry out a continuous control, it is advantageous to include -in the variable gain control circuit 28 an automatic level control circuit to maintain the oscillations maintained at the desired level or amplitude.

Note that the generator. 18 and the 2U receiver are not necessarily in bore holes, but may be in contact with the material to be tested at surface locations or at any suitable location.

Various other modifications can be used without departing from the scope of the present invention as defined in the following claims.