CA2185412A1 - Method and apparatus for borehole acoustic reflection logging - Google Patents
Method and apparatus for borehole acoustic reflection loggingInfo
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- CA2185412A1 CA2185412A1 CA 2185412 CA2185412A CA2185412A1 CA 2185412 A1 CA2185412 A1 CA 2185412A1 CA 2185412 CA2185412 CA 2185412 CA 2185412 A CA2185412 A CA 2185412A CA 2185412 A1 CA2185412 A1 CA 2185412A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/40—Seismology; Seismic or acoustic prospecting or detecting specially adapted for well-logging
- G01V1/52—Structural details
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- Geophysics And Detection Of Objects (AREA)
- Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
- Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)
Abstract
A method of characterizing an underground formation surrounding a borehole includes the steps of generating an acoustic signal with a transmitter in a borehole which radiates into the formation surrounding the borehole so as to reflect back to the borehole from any reflecting structures, and detecting the reflected signals at a receiver in the borehole after the arrival of acoustic signals which have passed directly from the transmitter to the receiver.
An apparatus for performing this method includes a tool having an acoustic transmitter and receiver, typically with a very short spacing therebetween, and means for detecting acoustic signals which have been reflected from reflecting structures in the formation surrounding a borehole after the arrival of signal which pass directly from the transmitter to the receiver.
An apparatus for performing this method includes a tool having an acoustic transmitter and receiver, typically with a very short spacing therebetween, and means for detecting acoustic signals which have been reflected from reflecting structures in the formation surrounding a borehole after the arrival of signal which pass directly from the transmitter to the receiver.
Description
60. 1 248 METHOD AND APPARATUS FOR BOREHOLE ACOUSTIC REFLECTION
LOGGING
Field of the Invention The present application relates to a method and apparatus for use in borehole logging which involves detecting acoustic reflectors in the formation surrounding a borehole using acoustic signals generated and received in the borehole.
Background of the Invention Acoustic techniques for formation characterization are well known. All of these techniques involve transmitting an acoustic signal from a source to a receiver via the formation of interest. The wavelength of the signal can vary from very low frequencies in seismic applications through sonic frequencies to ultrasonic frequencies, depending on the particular technique used. Most borehole logging involves the measurement of the time for a sonic signal to pass substantially directly from the source to the receiver via the formation at the borehole wall. Often the received signals are filtered to remove any signal due to reflections.
Seismic surveys generally involve the use of reflected acoustic signals, typically at very low frequencies, to detect structures below the earth's surface. The source of the signals and/or the detectors are usually located at the surface. Borehole seismic techniques place one of these inside the borehole.
In borehole acoustic reflection surveys, the measurements are made with acoustictransmitters and receivers placed in the same borehole. This configuration IS best suited for recording reflected wave fields from acoustic reflectors that have small angles with the boreholes axis. For example, near-vertical fractures~ faults and salt-dome flanks are good borehole reflection targets for near-vertical wells. Near-horizontal bed boundaries, fluid contact interfaces (gas/oil or oiVwater) and stringers inside reservoirs are good targets for highly-deviated and horizontal wells.
The desired events (reflections) in borehole reflection surveys are the waves R that propagate from the borehole to the reflector in the formation and back to the borehole as _ 2 shown in Figure l. A significant portion D of the acoustic energy from the transmitter T, however, propagates directly to the receiver array. These direct waves include compressional- and shear-headwaves, tube waves (Stoneley waves), fluid and borehole modes and various casing modes if the well is cased. The direct waves may also include various tool modes that propagate along the tool body.
In acoustic logging applications some of these direct waves, e.g. the headwaves and Stoneley waves, are used to log formation properties. In reflection survey applications, however, they are unwanted. The direct waves are typically much larger than reflections.
One of the main problems in borehole reflection surveys is to ensure that the recordings of the reflected events are not masked by the direct-wave events.
Reflection imaging around a well using downhole acoustic measurements has been proposed previously. Various publications present results of using sonic measurements for reflection imaging, see for example Fortin, J. P., Rehbinder, N., and Staron, P., 1991, Reflection imaging around a well with the EVA full-waveform tool: The Log Analyst, pp.
271-278; Glangeaud, F., Vanpe, J. M., Mari, J. L., and Gavin, P., 1992, Reflection im~ging around a highly deviated well using both acoustic reflection sounding and constant offset section: 62nd Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 91-94; Hornby, B., 1989, Imaging of near-borehole structure using full-waveform sonic data: Geophysics, 54, pp. 747-757; Hornby, B., Murphy, W., Liu, H., and Hsu, K., 1992, Reservoir sonics: A North Sea case study: Geophysics, 57, pp.
146-160; and Naville, C., Rehbinder, N., and Utard, M., 1984, Study of reflected events observed on microseismograms recorded with the EVA acoustic logging system: Society of Professional Well Log Analysts, Paris Chapter (SAID), 9th Annual Symposium, paper 10.
Two approaches have been suggested to attempt to overcome the problems caused by the direct waves mentioned. The first approach is to use high frequencies. Tube waves typically dominate the direct waves, particularly at low frequencies. The tube-wave excitation function, i.e. the frequency response of the tube wave, increases very rapidly as the frequency decreases. Therefore, by using high-frequency excitation one can reduce the tube wave cont~rnin~tion and reduce the m~king of reflected waves. The second approach is to use large transmitter-receiver offsets. In open holes the most prominent direct waves are the compressional-headwaves which arrive first, followed by the shear-headwaves and the tube waves. Typically the shear-headwaves are slower than the compressional-headwaves by a factor of 1.7 or more. The tube waves are almost always slower than the shear-headwaves. Consequently, there is a time gap between the compressional- and shear-_ 3 headwave arrivals which widens with the increasing transmitter-receiver offset. All of the previous proposals have used transmitter-receiver offsets in the range from 10 to 25 ft. In these previous studies most of the reflections used for imaging have been observed with large transmitter-receiver offsets in the period between the arrivals of the two headwaves.
The previous approach to borehole reflection imaging, with large offsets and high frequencies, has applications, but the distance of reflectors from the well that can be imaged with this approach is severely limited. The first limitation is due to the use of high frequencies. The attenuation of waves in formations increases exponentially with increasing frequency. Consequently, the use of high frequencies directly limits the penetration depth of the acoustic waves away from the borehole. The second limitation is due to the limited extent of the large-offset, early-time window used in im~ging. The im~ging range away from the borehole is limited by the tr~nsmitter-receiver offsets. This is due to the fact that, in most cases, only the reflected events that arrive before the shear-headwave (and/or tube waves or other direct waves) can be reliably used in imaging reflectors. Later events coming from deeper reflectors are masked by the direct waves. Simple modeling studies indicate that when the shear-headwaves are large enough to mask the reflections, the maximum distance that can be imaged away from the well is about 0.6 to 0.8 times the transmitter-receiver offset.
The large-offset approach may completely fail in cased holes and in the presence of direct tool arrivals. This is because the arrival times of the casing and tool modes can easily be as early or earlier than the formation compressional waves. Furthermore, some of these modes are very efficiently excited in narrow frequency bands with long, oscillatory wavetrains covering the time domain. When these direct waves are strongly excited they can completely mask all reflections in this large-offset early-time window.
Acoustic reflection techniques have been proposed for other uses in the borehole. US
4,916,400 proposes an ultrasonic caliper to determine the size and shape of a borehole in a drilling operation. An ultrasonic source in a drill collar sends signals to the borehole wall which are reflected and detected back at the drill collar and analyzed to determine the nature of the borehole. Other reflection techniques, including im:~ging techniques, have been proposed for use inside cased boreholes, particularly for evaluating the casing itself or the cement bond in the annulus between the casing and the formation. Examples of such techniques can be found in US 4,244,798, US 5,001,676, US 5,274,604 and EP
0549419A. Finally, US 4,289,953 proposes the use of ultrasonic backscatter from the grains in an open borehole surface to determine the grain size and distribution in the formation.
It is an object of the present invention to provide a technique for reflection acoustic logging in which the problems identified above are less significant and which will allowmeasurements to be made deeper into the formation away from the borehole.
Summary of the Invention In its broadest aspect, a method according to the invention comprises generating an acoustic signal with a transmitter in a borehole which radiates into the formation surrounding the borehole so as to reflect back to the borehole from any reflecting structures, and detecting the reflected signals at a receiver in the borehole after the arrival of acoustic signals which have passed directly from the transmitter to the receiver.
The broadest aspect of an apparatus according to the invention comprises a tool having an acoustic transmitter and receiver and means for detecting acoustic signals which have been reflected from reflecting structures in the formation surrounding a borehole after the arrival of signal which pass directly from the transmitter to the receiver.
The principle behind the present invention is to ensure that the reflected signals arrive at the detector after the direct signals from the transmitter. This is achieved by ensuring that the offset between the transmitter and receiver is relatively small so that the path for the direct signal is short compared to the path of the reflected signal, and by measuring the reflected signals after the latest time for a direct signal to reach the detector.
One particularly preferred use of the invention is the im~ging of the formation around the borehole. The reflected signals can be analyzed to identify the position of reflecting structures in the formation relative to the borehole and the positions can be represented as an image which can be used to characterize the formation.
,~ .
It is preferred to use one or more arrays of transmitters and receivers. Axial arrays provide increased reflected event amplitudes with respect to the tube waves. A7imuth~1 arrays allow the determination of the azimuthal position of reflecting bodies in the formation with respect to the borehole. With axial arrays, several arrangements are possible. All of the transmitter elements can be located next to each other and all of the detectors elements located next to 2~85412 each other to provide two juxtaposed arrays, a single transmitter array can have a pair of detector arrays on either side thereof, or vice versa, or the transmitter and receiver elements can be interlaced in a single array. It is also possible to use an array of common transmitter and receiver elements.
The frequency of the transmitted acoustic signals depends on the nature of the formation and the desired range of investigation. Frequencies can range from 100Hz or lower for long range im~ging in an attenuative medium to 20kHz or higher for short range, high resolution imaging in a non-attenuative medium.
In order to prevent part of the direct tube wave signal from propagating past the detector and reflecting back to the detector from structures inside the borehole so as to interfere with the reflected signals from the formation, it is preferred to used one or more attenuators positioned in the tool string. These are preferably of the form described in co-pending application no. 08/527,736 (incorporated herein by reference).
Brief Description of the Drawings Figure 1 shows a general schematic view of a borehole reflection imaging system;Figure 2 shows a plot of direct wave arrivals for different transmitter-receiver offsets;
Figure 3 shows a plot of reflected wave arrivals from a single reflector;
Figure 4 shows a schematic view of a borehole reflection im~ging system according to the invention; and Figures 5 (a) - (d) show different arrangements of transmitter and receiver arrays for use in the invention.
Description of the Preferred Embodiment Figure 4 shows a schematic view of a borehole acoustic reflection im~ging systemaccording to one embodiment of the present invention. A sonic reflection imaging tool 10 is shown lowered on an armored multiconductor cable 12 into a borehole 14, which can be cased or uncased, to make sonic measurements for imaging of the subsurface formation 16.
The tool 10 is provided with a transmitter array 18 and a receiver array 20 immP~ t~ly adjacent thereto. The separation of the transmitter and receiver array is arranged to be as small as possible as will be explained below. One or more tube wave attenuators 22 are provided at both ends of the tran~ll~ille~ ceiver array section to reduce interfering effects 21~5412 of reflected tube waves in the borehole. These attenuators and their function are described in detail in co-pending application no. 08/527,736, incorporated herein by reference.
The tool 10 is adapted from movement up and down borehole 14, and as the tool 10 is moved, the transmitter array 18 periodically generates a sonic signal. The generated sonic signal travels through the borehole and/or through the formation where it is reflected by underground structures, and the receivers in the receiver array 20 typically detect some energy which results from the generated signal. The mechanism for moving the tool 10 in the borehole includes the cable 12 which extends to the sheave wheel 24 at the surface of the formation, and then to a suitable drum and winch mechanism 26 which raises and lowers the tool 10 in the borehole as desired. Electrical connection between transmitter array 18 and receiver array 20 on the one hand, and the surface equipment on the other hand, is made through suitable a multi-element slipring and brush contact assembly 28 associated with the drum and winch mechanism 26. A unit 30 contains tool control and pre-processing circuits which send electrical signals to the tool 10 and receive other electrical signals (sonic logs) therefrom via cable 12 and assembly 28. The unit 30 cooperates with a depth recorder 32 which derives depth level signals from a depth measuring wheel 34 so as to associate the signals from receiver array 20 with respective depth levels in borehole 14.
The outputs of the receiver array 20, after optional pre-processing in unit 30, are sent to signal storage 36, which can also receive signals from or through depth recorder 32 so as to associate sonic receiver outputs with respective depth levels in the borehole 14. Storage 36 can store the outputs of the receiver array 20 in the form of digital sonic log measurements. Storage 36 can comprise a magnetic storage device such as a disk or tape, and/or other storage media such as semiconductor or equivalent memory circuits. The digital data can then be processed to provide an image of the underground formation surrounding the borehole. Kirchhoff-type migration of the data, such as is commonly used in seismic processing, is used to derive an image of the reflecting structures around the borehole.
The transmitter array 18 comprises seven piezoelectric monopole source elements 18' arranged side by side along the tool 10. Each elel;nent 18' is substantially the same as the monopole source used in conventional sonic logging. However, unlike the sonic measurements for compressional and shear wave speeds along the borehole, the sonic im:~ging tool needs to imagè reflectors away from the borehole. In order for waves to penetrate deeper into the formation, a high power transmitter is necessary to overcome the loss generated by the medium as well as the amplitude drop with distance due to the . 2185~1~
geometrical spreading. With the space and voltage limitations present in a borehole tool, it is difficult to realize a single high power transmitter with limited tool space. Therefore, the solution is to use an array of sources as described above to enhance the radiated acoustic power. In addition to the higher power requirement the quality of the source waveform is also very important. Because it is not possible to generate a sharp image with areverberating source. The ideal source for imaging is likely to be high power as well high fidelity which requires a compact source signature.
In order to radiate acoustic energy efficiently, the transducers 18' are designed to operate near geometrical resonances which unavoidably will ring for a long time. A damping mechanism has been introduced to stop this ringing which comprises a rubber-tungsten backing material for the acoustic signal source to provide a good impedance match as well as damping to the attached structure. The rubber-tungsten backing also prevents the additional fluid mode excitation when the transducer is immersed in the fluid. The rubber-tungsten composite comprises a butyl rubber skeleton loaded with tungsten powder. The impedance and attenuation of the backing will depend on the percentage of tungsten, the degree of compaction of the powder and the degree of vulcanization as well as adherence of the rubber onto the powder.
The packaging of the elements 18' in the array 18 can significantly alter the vibrating mode of the array source. A good packaging of the array source will insure the long term reliability of the array source and prevent additional vibrations or interference between elements beyond just the acoustic superposition of each individual elements. Soft Teflon disks are sandwiched between each transducer element to prevent electric arcing between them at the same time to isolate the vibration between two neighboring elements. There is a steel center rod to hold and guide each element along the tool. The array 18 is spring loaded at the ends to allow for the bending of the tool and thermal expansions of the elements 18'.
Then the array 18 is then enclosed in an oil filled bellows (not shown) to isolate it from the borehole fluid at the same time to provide the pressure compensation.
The potential problem with a longer source is that the radiation beam pattern may change to unfavored directions or limit the deployment of the receiver positions. A steerable array source can overcome this problem. The coherent wave front generated by the source array can be steered if one consecutively delays the firing of each element. The steering angle will depend on the amount of the delay to each source element. For example in water a thirty degrees steering away from the normal of the array will typically require a fifty microsecond delay to each array element spaced at 3.5 inches apart from center to center.
This can be achieved either by having the same power amplifier and connect a time delay device to each source element or by using several power amplifiers and each having its independent timing control.
The receiver array 20 comprises eight receiver stations spaced 6 inches apart vertically, each station having four receiver elements disposed circumferentially at 90 intervals about the tool making a total of 32 receiver elements. The signal detected at each element are recorded separately and the signals received by the array analyzed to provide the direction and distance of reflecting structures from the borehole.
The arrangement of transmitter and receiver arrays described above is currently preferred.
Other configurations which may be suitable are shown schematically in Figures 5(a) - (d).
Figure 5(a) shows a central transmitter array Ta having receiver arrays Ra disposed on either side thereof, Figure 5(b) shows the reverse situation with transmitter arrays Tb either side of a receiver array Rb, Figure 5(c) shows an array of interlaced transmitter Tc and receiver Rc elements, and Figure 5(d) shows an array of common tr~n.~mit~er and receiver elements Trd. In each of these cases, the tube wave attenuators (not shown) will be positioned at either end of the array as is shown in Figure 4.
Figure 2 shows various events that would be recorded in a typical borehole reflection survey. Horizontal axis is the transmitter-receiver offset, the vertical axis is the time, and the plot shows computed theoretical waveforms of direct waves only. The curves, marked 5 ft, 10 ft, ..., 30 ft, drawn on the waveforms represent the locations of the reflections due to reflectors parallel to the borehole axis at corresponding distances. There are two windows in this time-offset space that are not masked by the large direct waves. The large-offset early-time window is the triangular area (marked X in Figure 2) between the compressional- and shear-headwave arrivals. The small-offset late-time window is the lower right part of the plot following the borehole modes (marked Y in Figure 2). The present invention is based on the most efficient use of the small-offset window.
The present invention employs small transmitter-receiver offsets (separation of nearest tr:~n~mitter and receiver) compared to the prior art. Offset are chosen such that the reflections arrive at receivers after the direct waves. In this small-offset late-time window the reflections do not overlap with the direct waves regardless of the distance from the borehole to the reflector. Consequently the im~ging range away from the borehole is , g significantly improved. Since the tube-waves are not of concern in this window, sources with lower frequencies (as low as needed) can be used to reduce attenuation, this further increases the range. The smallest offset achievable is limited by the physical requirements of the tool and can be as small as 0.5 feet depending on the transmitter and receiver configuration used.
The large-offset window approach, as described above in relation to the prior art, severely limits the im~ging range of reflectors away from the borehole. This is due to several reasons. First, in most cases this window is tern~in~tç~l by the shear-headwave and tube waves which are typically much larger than the reflected compressional waves. For a reflector parallel to the borehole axis, the arrival time of a compressional-wave reflection is approximately given by timeref ~--~4range2 + offset2 ( 1 ) where range is the distance of the reflector from the borehole, oJj~set is the transmitter-receiver offset, and vp is the compressional-wave speed. The direct shear wave arrival times, defining the end of the imaging window, are given by timed,r ~ o~setl v5 (2) The maximum distance from the borehole, that can be imaged with the large-offsetapproach, is obtained by setting equation (1) equal to (2) and solving for range, giving rangemax ~ 0-50J~set~ 1 (3) For example, for a typical value of P = 1.73, the maximum range is given by rangem~,~ ~ 0.70Jj~set. In more general form, v5 above represents the velocity of the first domin~ting direct wave following the compressional headwave. The approach adopted by the present invention using the small-offset window does not have any maximum range limit due to any direct wave arrival because the reflections are located behind them. There is, however, a minimum range limit. As indicated in Figure 2, reflections from very-close . ~o reflectors can be masked by the ringing of the direct waves. The duration of the ringing is inversely proportional to the bandwidth of the direct wave, and it very much depends on the acquisition parameters such as the spectral output of the transmitter. Resonance frequencies of the interfering direct waves should be avoided to reduce the ringing duration. For example, for a medium with compressional velocity of 10,000 ft/sec (100 ~sec/ft), the minimum range is approximately 5 ft for 1 kHz bandwidth, and 10 ft for 500 Hz bandwidth.
The second reason for the range limitation of the large-offset approach is the rapid decrease in reflected-wave amplitudes due to geometrical spreading and attenuation. Amplitude decrease due to geometrical spreading is approximately proportional with the total distance traveled by, or the arrival time of, the reflected event. As illustrated in Figure 2, for large offset the reflection ray paths become longer therefore amplitudes decrease. The amplitude decrease at large offsets due to attenuation could be even more significant than the geometrical spreading. The dependence of the reflected-wave amplitudes to attenuation, frequency and distance is given by amplituderef oc exp~ - ~ d ~ (4) where f is the frequency, d is the reflection ray path, and Q is a number representing attenuation properties of a medium. Q values can vary significantly, from 5 for a very attenuative medium to 100 for an ideally non-attenuative medium. The above equation shows that amplitude decrease due to attenuation increases exponentially with the propagation path length of the reflected event which increases with the offset. This equation also indicates that, using small offsets and low frequencies is particularly crucial for im~ging in slow and low-Q formations where the vpQ product can be very small. The frequencies employed can range from 100 Hz (or lower) for long-range im:~ging inattenuative medium, to 20 kHz (or higher) for short-range, high-resolution imaging in non-attenuative medium. ~
Another advantage of small transmitter-receiver offsets is that the reflections become simpler (i.e. more easily interpretable) and stronger as the offsets decrease. Point sources, in borehole fluid or clamped to the borehole wall, generate both compressional- and shear-wave radiation into the medium. Particular shapes of the radiation patterns depend on the 2t85412 medium and borehole fluid properties, but there are some properties common to all cases.
The compressional radiation is maximum in the direction perpendicular to the borehole axis (borehole normal), and it decays as the angle with the borehole normal increases. Due to reciprocity, the reception pattern of a receiver is the same as the radiation pattern of a source, therefore the combined effect in reflection measurement goes as the square of the radiation pattern. As indicated in Figure 2, reflected-wave raypaths get closer to the borehole normal at both source and receivers for smaller offsets providing amplitude gains in reflections.
In Figure 2 the arrival times of reflections are represented by only one curve per reflector for simplicity. In elastic media, such as subsurface formations, there are four reflected events associated with each reflector. Figure 3 shows such typical events due to a single reflector parallel to the borehole axis. There are four reflection events representing the combinations of incident P (compressional) or S (shear), and reflected P or S. P to P
reflections become stronger at small offsets partly due to the radiation pattern and partly due to the reflection coefficient variations with the angle of incidence on the reflector. An important observation is that the reflected events other than P to P disappear at small offsets, leaving only one strong event as the reflection from an interface. This makes the recorded waveforms at small offsets much simpler to process and interpret.
Another advantage of small-offset configuration is the application in cased boreholes, where the large-offset approach may completely fail. In cased boreholes, in addition to the direct arrivals present in open boreholes, there are casing modes that could propagate at speeds equal to or greater than the compressional-headwave. These waves could dominate the large-offset imaging window between the compressional- and shear-headwaves, masking smaller amplitude reflections. The amount of "ringing" (i.e., the time duration) of these modes depends on the condition of the cement that bonds the casing to the formation, which is usually unpredictable. The small-offset late-time im~ging window, behind the Stoneley and shear waves can be used for imaging, even in cased boreholes. There will be a minimum range (e.g., 5 to 10 ft), as described before, depending on the bandwidth of the casing modes at short offsets. ~
The signals received by each of the receiver elements are treated in essentially the same manner as signals received by elements in a seismic array and an image is produced using the same techniques. Thus an image can be produced along the borehole axis (depth axis) and azimuthally around the borehole.
LOGGING
Field of the Invention The present application relates to a method and apparatus for use in borehole logging which involves detecting acoustic reflectors in the formation surrounding a borehole using acoustic signals generated and received in the borehole.
Background of the Invention Acoustic techniques for formation characterization are well known. All of these techniques involve transmitting an acoustic signal from a source to a receiver via the formation of interest. The wavelength of the signal can vary from very low frequencies in seismic applications through sonic frequencies to ultrasonic frequencies, depending on the particular technique used. Most borehole logging involves the measurement of the time for a sonic signal to pass substantially directly from the source to the receiver via the formation at the borehole wall. Often the received signals are filtered to remove any signal due to reflections.
Seismic surveys generally involve the use of reflected acoustic signals, typically at very low frequencies, to detect structures below the earth's surface. The source of the signals and/or the detectors are usually located at the surface. Borehole seismic techniques place one of these inside the borehole.
In borehole acoustic reflection surveys, the measurements are made with acoustictransmitters and receivers placed in the same borehole. This configuration IS best suited for recording reflected wave fields from acoustic reflectors that have small angles with the boreholes axis. For example, near-vertical fractures~ faults and salt-dome flanks are good borehole reflection targets for near-vertical wells. Near-horizontal bed boundaries, fluid contact interfaces (gas/oil or oiVwater) and stringers inside reservoirs are good targets for highly-deviated and horizontal wells.
The desired events (reflections) in borehole reflection surveys are the waves R that propagate from the borehole to the reflector in the formation and back to the borehole as _ 2 shown in Figure l. A significant portion D of the acoustic energy from the transmitter T, however, propagates directly to the receiver array. These direct waves include compressional- and shear-headwaves, tube waves (Stoneley waves), fluid and borehole modes and various casing modes if the well is cased. The direct waves may also include various tool modes that propagate along the tool body.
In acoustic logging applications some of these direct waves, e.g. the headwaves and Stoneley waves, are used to log formation properties. In reflection survey applications, however, they are unwanted. The direct waves are typically much larger than reflections.
One of the main problems in borehole reflection surveys is to ensure that the recordings of the reflected events are not masked by the direct-wave events.
Reflection imaging around a well using downhole acoustic measurements has been proposed previously. Various publications present results of using sonic measurements for reflection imaging, see for example Fortin, J. P., Rehbinder, N., and Staron, P., 1991, Reflection imaging around a well with the EVA full-waveform tool: The Log Analyst, pp.
271-278; Glangeaud, F., Vanpe, J. M., Mari, J. L., and Gavin, P., 1992, Reflection im~ging around a highly deviated well using both acoustic reflection sounding and constant offset section: 62nd Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 91-94; Hornby, B., 1989, Imaging of near-borehole structure using full-waveform sonic data: Geophysics, 54, pp. 747-757; Hornby, B., Murphy, W., Liu, H., and Hsu, K., 1992, Reservoir sonics: A North Sea case study: Geophysics, 57, pp.
146-160; and Naville, C., Rehbinder, N., and Utard, M., 1984, Study of reflected events observed on microseismograms recorded with the EVA acoustic logging system: Society of Professional Well Log Analysts, Paris Chapter (SAID), 9th Annual Symposium, paper 10.
Two approaches have been suggested to attempt to overcome the problems caused by the direct waves mentioned. The first approach is to use high frequencies. Tube waves typically dominate the direct waves, particularly at low frequencies. The tube-wave excitation function, i.e. the frequency response of the tube wave, increases very rapidly as the frequency decreases. Therefore, by using high-frequency excitation one can reduce the tube wave cont~rnin~tion and reduce the m~king of reflected waves. The second approach is to use large transmitter-receiver offsets. In open holes the most prominent direct waves are the compressional-headwaves which arrive first, followed by the shear-headwaves and the tube waves. Typically the shear-headwaves are slower than the compressional-headwaves by a factor of 1.7 or more. The tube waves are almost always slower than the shear-headwaves. Consequently, there is a time gap between the compressional- and shear-_ 3 headwave arrivals which widens with the increasing transmitter-receiver offset. All of the previous proposals have used transmitter-receiver offsets in the range from 10 to 25 ft. In these previous studies most of the reflections used for imaging have been observed with large transmitter-receiver offsets in the period between the arrivals of the two headwaves.
The previous approach to borehole reflection imaging, with large offsets and high frequencies, has applications, but the distance of reflectors from the well that can be imaged with this approach is severely limited. The first limitation is due to the use of high frequencies. The attenuation of waves in formations increases exponentially with increasing frequency. Consequently, the use of high frequencies directly limits the penetration depth of the acoustic waves away from the borehole. The second limitation is due to the limited extent of the large-offset, early-time window used in im~ging. The im~ging range away from the borehole is limited by the tr~nsmitter-receiver offsets. This is due to the fact that, in most cases, only the reflected events that arrive before the shear-headwave (and/or tube waves or other direct waves) can be reliably used in imaging reflectors. Later events coming from deeper reflectors are masked by the direct waves. Simple modeling studies indicate that when the shear-headwaves are large enough to mask the reflections, the maximum distance that can be imaged away from the well is about 0.6 to 0.8 times the transmitter-receiver offset.
The large-offset approach may completely fail in cased holes and in the presence of direct tool arrivals. This is because the arrival times of the casing and tool modes can easily be as early or earlier than the formation compressional waves. Furthermore, some of these modes are very efficiently excited in narrow frequency bands with long, oscillatory wavetrains covering the time domain. When these direct waves are strongly excited they can completely mask all reflections in this large-offset early-time window.
Acoustic reflection techniques have been proposed for other uses in the borehole. US
4,916,400 proposes an ultrasonic caliper to determine the size and shape of a borehole in a drilling operation. An ultrasonic source in a drill collar sends signals to the borehole wall which are reflected and detected back at the drill collar and analyzed to determine the nature of the borehole. Other reflection techniques, including im:~ging techniques, have been proposed for use inside cased boreholes, particularly for evaluating the casing itself or the cement bond in the annulus between the casing and the formation. Examples of such techniques can be found in US 4,244,798, US 5,001,676, US 5,274,604 and EP
0549419A. Finally, US 4,289,953 proposes the use of ultrasonic backscatter from the grains in an open borehole surface to determine the grain size and distribution in the formation.
It is an object of the present invention to provide a technique for reflection acoustic logging in which the problems identified above are less significant and which will allowmeasurements to be made deeper into the formation away from the borehole.
Summary of the Invention In its broadest aspect, a method according to the invention comprises generating an acoustic signal with a transmitter in a borehole which radiates into the formation surrounding the borehole so as to reflect back to the borehole from any reflecting structures, and detecting the reflected signals at a receiver in the borehole after the arrival of acoustic signals which have passed directly from the transmitter to the receiver.
The broadest aspect of an apparatus according to the invention comprises a tool having an acoustic transmitter and receiver and means for detecting acoustic signals which have been reflected from reflecting structures in the formation surrounding a borehole after the arrival of signal which pass directly from the transmitter to the receiver.
The principle behind the present invention is to ensure that the reflected signals arrive at the detector after the direct signals from the transmitter. This is achieved by ensuring that the offset between the transmitter and receiver is relatively small so that the path for the direct signal is short compared to the path of the reflected signal, and by measuring the reflected signals after the latest time for a direct signal to reach the detector.
One particularly preferred use of the invention is the im~ging of the formation around the borehole. The reflected signals can be analyzed to identify the position of reflecting structures in the formation relative to the borehole and the positions can be represented as an image which can be used to characterize the formation.
,~ .
It is preferred to use one or more arrays of transmitters and receivers. Axial arrays provide increased reflected event amplitudes with respect to the tube waves. A7imuth~1 arrays allow the determination of the azimuthal position of reflecting bodies in the formation with respect to the borehole. With axial arrays, several arrangements are possible. All of the transmitter elements can be located next to each other and all of the detectors elements located next to 2~85412 each other to provide two juxtaposed arrays, a single transmitter array can have a pair of detector arrays on either side thereof, or vice versa, or the transmitter and receiver elements can be interlaced in a single array. It is also possible to use an array of common transmitter and receiver elements.
The frequency of the transmitted acoustic signals depends on the nature of the formation and the desired range of investigation. Frequencies can range from 100Hz or lower for long range im~ging in an attenuative medium to 20kHz or higher for short range, high resolution imaging in a non-attenuative medium.
In order to prevent part of the direct tube wave signal from propagating past the detector and reflecting back to the detector from structures inside the borehole so as to interfere with the reflected signals from the formation, it is preferred to used one or more attenuators positioned in the tool string. These are preferably of the form described in co-pending application no. 08/527,736 (incorporated herein by reference).
Brief Description of the Drawings Figure 1 shows a general schematic view of a borehole reflection imaging system;Figure 2 shows a plot of direct wave arrivals for different transmitter-receiver offsets;
Figure 3 shows a plot of reflected wave arrivals from a single reflector;
Figure 4 shows a schematic view of a borehole reflection im~ging system according to the invention; and Figures 5 (a) - (d) show different arrangements of transmitter and receiver arrays for use in the invention.
Description of the Preferred Embodiment Figure 4 shows a schematic view of a borehole acoustic reflection im~ging systemaccording to one embodiment of the present invention. A sonic reflection imaging tool 10 is shown lowered on an armored multiconductor cable 12 into a borehole 14, which can be cased or uncased, to make sonic measurements for imaging of the subsurface formation 16.
The tool 10 is provided with a transmitter array 18 and a receiver array 20 immP~ t~ly adjacent thereto. The separation of the transmitter and receiver array is arranged to be as small as possible as will be explained below. One or more tube wave attenuators 22 are provided at both ends of the tran~ll~ille~ ceiver array section to reduce interfering effects 21~5412 of reflected tube waves in the borehole. These attenuators and their function are described in detail in co-pending application no. 08/527,736, incorporated herein by reference.
The tool 10 is adapted from movement up and down borehole 14, and as the tool 10 is moved, the transmitter array 18 periodically generates a sonic signal. The generated sonic signal travels through the borehole and/or through the formation where it is reflected by underground structures, and the receivers in the receiver array 20 typically detect some energy which results from the generated signal. The mechanism for moving the tool 10 in the borehole includes the cable 12 which extends to the sheave wheel 24 at the surface of the formation, and then to a suitable drum and winch mechanism 26 which raises and lowers the tool 10 in the borehole as desired. Electrical connection between transmitter array 18 and receiver array 20 on the one hand, and the surface equipment on the other hand, is made through suitable a multi-element slipring and brush contact assembly 28 associated with the drum and winch mechanism 26. A unit 30 contains tool control and pre-processing circuits which send electrical signals to the tool 10 and receive other electrical signals (sonic logs) therefrom via cable 12 and assembly 28. The unit 30 cooperates with a depth recorder 32 which derives depth level signals from a depth measuring wheel 34 so as to associate the signals from receiver array 20 with respective depth levels in borehole 14.
The outputs of the receiver array 20, after optional pre-processing in unit 30, are sent to signal storage 36, which can also receive signals from or through depth recorder 32 so as to associate sonic receiver outputs with respective depth levels in the borehole 14. Storage 36 can store the outputs of the receiver array 20 in the form of digital sonic log measurements. Storage 36 can comprise a magnetic storage device such as a disk or tape, and/or other storage media such as semiconductor or equivalent memory circuits. The digital data can then be processed to provide an image of the underground formation surrounding the borehole. Kirchhoff-type migration of the data, such as is commonly used in seismic processing, is used to derive an image of the reflecting structures around the borehole.
The transmitter array 18 comprises seven piezoelectric monopole source elements 18' arranged side by side along the tool 10. Each elel;nent 18' is substantially the same as the monopole source used in conventional sonic logging. However, unlike the sonic measurements for compressional and shear wave speeds along the borehole, the sonic im:~ging tool needs to imagè reflectors away from the borehole. In order for waves to penetrate deeper into the formation, a high power transmitter is necessary to overcome the loss generated by the medium as well as the amplitude drop with distance due to the . 2185~1~
geometrical spreading. With the space and voltage limitations present in a borehole tool, it is difficult to realize a single high power transmitter with limited tool space. Therefore, the solution is to use an array of sources as described above to enhance the radiated acoustic power. In addition to the higher power requirement the quality of the source waveform is also very important. Because it is not possible to generate a sharp image with areverberating source. The ideal source for imaging is likely to be high power as well high fidelity which requires a compact source signature.
In order to radiate acoustic energy efficiently, the transducers 18' are designed to operate near geometrical resonances which unavoidably will ring for a long time. A damping mechanism has been introduced to stop this ringing which comprises a rubber-tungsten backing material for the acoustic signal source to provide a good impedance match as well as damping to the attached structure. The rubber-tungsten backing also prevents the additional fluid mode excitation when the transducer is immersed in the fluid. The rubber-tungsten composite comprises a butyl rubber skeleton loaded with tungsten powder. The impedance and attenuation of the backing will depend on the percentage of tungsten, the degree of compaction of the powder and the degree of vulcanization as well as adherence of the rubber onto the powder.
The packaging of the elements 18' in the array 18 can significantly alter the vibrating mode of the array source. A good packaging of the array source will insure the long term reliability of the array source and prevent additional vibrations or interference between elements beyond just the acoustic superposition of each individual elements. Soft Teflon disks are sandwiched between each transducer element to prevent electric arcing between them at the same time to isolate the vibration between two neighboring elements. There is a steel center rod to hold and guide each element along the tool. The array 18 is spring loaded at the ends to allow for the bending of the tool and thermal expansions of the elements 18'.
Then the array 18 is then enclosed in an oil filled bellows (not shown) to isolate it from the borehole fluid at the same time to provide the pressure compensation.
The potential problem with a longer source is that the radiation beam pattern may change to unfavored directions or limit the deployment of the receiver positions. A steerable array source can overcome this problem. The coherent wave front generated by the source array can be steered if one consecutively delays the firing of each element. The steering angle will depend on the amount of the delay to each source element. For example in water a thirty degrees steering away from the normal of the array will typically require a fifty microsecond delay to each array element spaced at 3.5 inches apart from center to center.
This can be achieved either by having the same power amplifier and connect a time delay device to each source element or by using several power amplifiers and each having its independent timing control.
The receiver array 20 comprises eight receiver stations spaced 6 inches apart vertically, each station having four receiver elements disposed circumferentially at 90 intervals about the tool making a total of 32 receiver elements. The signal detected at each element are recorded separately and the signals received by the array analyzed to provide the direction and distance of reflecting structures from the borehole.
The arrangement of transmitter and receiver arrays described above is currently preferred.
Other configurations which may be suitable are shown schematically in Figures 5(a) - (d).
Figure 5(a) shows a central transmitter array Ta having receiver arrays Ra disposed on either side thereof, Figure 5(b) shows the reverse situation with transmitter arrays Tb either side of a receiver array Rb, Figure 5(c) shows an array of interlaced transmitter Tc and receiver Rc elements, and Figure 5(d) shows an array of common tr~n.~mit~er and receiver elements Trd. In each of these cases, the tube wave attenuators (not shown) will be positioned at either end of the array as is shown in Figure 4.
Figure 2 shows various events that would be recorded in a typical borehole reflection survey. Horizontal axis is the transmitter-receiver offset, the vertical axis is the time, and the plot shows computed theoretical waveforms of direct waves only. The curves, marked 5 ft, 10 ft, ..., 30 ft, drawn on the waveforms represent the locations of the reflections due to reflectors parallel to the borehole axis at corresponding distances. There are two windows in this time-offset space that are not masked by the large direct waves. The large-offset early-time window is the triangular area (marked X in Figure 2) between the compressional- and shear-headwave arrivals. The small-offset late-time window is the lower right part of the plot following the borehole modes (marked Y in Figure 2). The present invention is based on the most efficient use of the small-offset window.
The present invention employs small transmitter-receiver offsets (separation of nearest tr:~n~mitter and receiver) compared to the prior art. Offset are chosen such that the reflections arrive at receivers after the direct waves. In this small-offset late-time window the reflections do not overlap with the direct waves regardless of the distance from the borehole to the reflector. Consequently the im~ging range away from the borehole is , g significantly improved. Since the tube-waves are not of concern in this window, sources with lower frequencies (as low as needed) can be used to reduce attenuation, this further increases the range. The smallest offset achievable is limited by the physical requirements of the tool and can be as small as 0.5 feet depending on the transmitter and receiver configuration used.
The large-offset window approach, as described above in relation to the prior art, severely limits the im~ging range of reflectors away from the borehole. This is due to several reasons. First, in most cases this window is tern~in~tç~l by the shear-headwave and tube waves which are typically much larger than the reflected compressional waves. For a reflector parallel to the borehole axis, the arrival time of a compressional-wave reflection is approximately given by timeref ~--~4range2 + offset2 ( 1 ) where range is the distance of the reflector from the borehole, oJj~set is the transmitter-receiver offset, and vp is the compressional-wave speed. The direct shear wave arrival times, defining the end of the imaging window, are given by timed,r ~ o~setl v5 (2) The maximum distance from the borehole, that can be imaged with the large-offsetapproach, is obtained by setting equation (1) equal to (2) and solving for range, giving rangemax ~ 0-50J~set~ 1 (3) For example, for a typical value of P = 1.73, the maximum range is given by rangem~,~ ~ 0.70Jj~set. In more general form, v5 above represents the velocity of the first domin~ting direct wave following the compressional headwave. The approach adopted by the present invention using the small-offset window does not have any maximum range limit due to any direct wave arrival because the reflections are located behind them. There is, however, a minimum range limit. As indicated in Figure 2, reflections from very-close . ~o reflectors can be masked by the ringing of the direct waves. The duration of the ringing is inversely proportional to the bandwidth of the direct wave, and it very much depends on the acquisition parameters such as the spectral output of the transmitter. Resonance frequencies of the interfering direct waves should be avoided to reduce the ringing duration. For example, for a medium with compressional velocity of 10,000 ft/sec (100 ~sec/ft), the minimum range is approximately 5 ft for 1 kHz bandwidth, and 10 ft for 500 Hz bandwidth.
The second reason for the range limitation of the large-offset approach is the rapid decrease in reflected-wave amplitudes due to geometrical spreading and attenuation. Amplitude decrease due to geometrical spreading is approximately proportional with the total distance traveled by, or the arrival time of, the reflected event. As illustrated in Figure 2, for large offset the reflection ray paths become longer therefore amplitudes decrease. The amplitude decrease at large offsets due to attenuation could be even more significant than the geometrical spreading. The dependence of the reflected-wave amplitudes to attenuation, frequency and distance is given by amplituderef oc exp~ - ~ d ~ (4) where f is the frequency, d is the reflection ray path, and Q is a number representing attenuation properties of a medium. Q values can vary significantly, from 5 for a very attenuative medium to 100 for an ideally non-attenuative medium. The above equation shows that amplitude decrease due to attenuation increases exponentially with the propagation path length of the reflected event which increases with the offset. This equation also indicates that, using small offsets and low frequencies is particularly crucial for im~ging in slow and low-Q formations where the vpQ product can be very small. The frequencies employed can range from 100 Hz (or lower) for long-range im:~ging inattenuative medium, to 20 kHz (or higher) for short-range, high-resolution imaging in non-attenuative medium. ~
Another advantage of small transmitter-receiver offsets is that the reflections become simpler (i.e. more easily interpretable) and stronger as the offsets decrease. Point sources, in borehole fluid or clamped to the borehole wall, generate both compressional- and shear-wave radiation into the medium. Particular shapes of the radiation patterns depend on the 2t85412 medium and borehole fluid properties, but there are some properties common to all cases.
The compressional radiation is maximum in the direction perpendicular to the borehole axis (borehole normal), and it decays as the angle with the borehole normal increases. Due to reciprocity, the reception pattern of a receiver is the same as the radiation pattern of a source, therefore the combined effect in reflection measurement goes as the square of the radiation pattern. As indicated in Figure 2, reflected-wave raypaths get closer to the borehole normal at both source and receivers for smaller offsets providing amplitude gains in reflections.
In Figure 2 the arrival times of reflections are represented by only one curve per reflector for simplicity. In elastic media, such as subsurface formations, there are four reflected events associated with each reflector. Figure 3 shows such typical events due to a single reflector parallel to the borehole axis. There are four reflection events representing the combinations of incident P (compressional) or S (shear), and reflected P or S. P to P
reflections become stronger at small offsets partly due to the radiation pattern and partly due to the reflection coefficient variations with the angle of incidence on the reflector. An important observation is that the reflected events other than P to P disappear at small offsets, leaving only one strong event as the reflection from an interface. This makes the recorded waveforms at small offsets much simpler to process and interpret.
Another advantage of small-offset configuration is the application in cased boreholes, where the large-offset approach may completely fail. In cased boreholes, in addition to the direct arrivals present in open boreholes, there are casing modes that could propagate at speeds equal to or greater than the compressional-headwave. These waves could dominate the large-offset imaging window between the compressional- and shear-headwaves, masking smaller amplitude reflections. The amount of "ringing" (i.e., the time duration) of these modes depends on the condition of the cement that bonds the casing to the formation, which is usually unpredictable. The small-offset late-time im~ging window, behind the Stoneley and shear waves can be used for imaging, even in cased boreholes. There will be a minimum range (e.g., 5 to 10 ft), as described before, depending on the bandwidth of the casing modes at short offsets. ~
The signals received by each of the receiver elements are treated in essentially the same manner as signals received by elements in a seismic array and an image is produced using the same techniques. Thus an image can be produced along the borehole axis (depth axis) and azimuthally around the borehole.
Claims (19)
1 A method of characterizing an underground formation surrounding a borehole, comprising:
a) transmitting an acoustic signal from a transmitter location in the borehole so as to radiate, at least in part, into the formation and reflect back to the borehole from reflecting structures therein;
b) detecting, at a receiver location, arrivals of reflected acoustic signals occurring after arrivals of acoustic signals which have passed substantially directly from the transmitter location to the receiver location; and c) using the detected arrivals of reflected signals to characterize the formation.
a) transmitting an acoustic signal from a transmitter location in the borehole so as to radiate, at least in part, into the formation and reflect back to the borehole from reflecting structures therein;
b) detecting, at a receiver location, arrivals of reflected acoustic signals occurring after arrivals of acoustic signals which have passed substantially directly from the transmitter location to the receiver location; and c) using the detected arrivals of reflected signals to characterize the formation.
2 A method as claimed in claim 1, wherein the detected arrivals of reflected signals are used to identify positions of the reflecting structures in the formation around the borehole.
3 A method as claimed in claim 1, wherein the step of detecting arrivals of reflected signals comprises detecting compressional waves.
4 A method as claimed in claim 1, wherein the step of detecting reflected signals comprises detecting signals at a plurality of radial positions in the borehole so as to allow the direction of reflecting structures to be determined.
5 A method as claimed in claim 1, wherein the transmitter location and the receiver location are substantially adjacent to each other.
6 A method as claimed in claim 1, wherein the step of transmitting the acoustic signal comprises transmitting signals from an array of transmitters at the transmitter location.
7 A method as claimed in claim 1, the step of detecting the reflected signals comprises detecting the signals at an array of detectors at the receiver location.
8 A method as claimed in claim 1, wherein the detected arrivals of reflected signals are used to generate an image of the formation.
9 A method as claimed in claim 1, wherein the transmitter location and the receiver location are at substantially the same position in the borehole.
10 Apparatus for characterizing an underground formation surrounding a borehole, comprising:
a) a tool body;
b) an acoustic transmitter located in the tool body;
c) an acoustic receiver located in the tool body spaced from the transmitter by a small distance such that the time for an acoustic signal to travel from the transmitter to the receiver is less than the time for a corresponding acoustic signal to pass from the transmitter into the formation, reflect from a structure in the formation and return to the receiver.
a) a tool body;
b) an acoustic transmitter located in the tool body;
c) an acoustic receiver located in the tool body spaced from the transmitter by a small distance such that the time for an acoustic signal to travel from the transmitter to the receiver is less than the time for a corresponding acoustic signal to pass from the transmitter into the formation, reflect from a structure in the formation and return to the receiver.
11 Apparatus as claimed in claim 10, wherein the transmitter comprises an array of transmitting elements.
12 Apparatus as claimed in claim 10, wherein the receiver comprises an array of receiving elements.
13 Apparatus as claimed in claim 10, further comprising at least one tube wave attenuator connected to the tool body.
14 Apparatus as claimed in claim 13, wherein the transmitter and receiver are disposed between a pair of tube wave attenuators.
15 Apparatus as claimed in claim 11, wherein the elements of the array are operated as a phased array to transmit a signal into the formation in a predetermined direction.
16 Apparatus as claimed in claim 12, wherein the elements of the array are operated as a phased array so as to receive signals from a predetermined direction.
17 Apparatus as claimed in claim 12, wherein the elements are disposed about an axis of the tool body.
18 A method of imaging an underground formation surrounding a borehole, comprising:
a) transmitting an acoustic signal from a transmitter location in the borehole so as to radiate, at least in part, into the formation and reflect back to the borehole from reflecting structures therein;
b) detecting, at a receiver location, arrivals of reflected acoustic signals occurring after arrivals of acoustic signals which have passed substantially directly from the transmitter location to the receiver location; and c) repeating steps a) and b) at a plurality of depths along the borehole and using the detected arrivals of reflected signals from each depth to generate an image of reflecting structures in the formation surrounding the borehole.
a) transmitting an acoustic signal from a transmitter location in the borehole so as to radiate, at least in part, into the formation and reflect back to the borehole from reflecting structures therein;
b) detecting, at a receiver location, arrivals of reflected acoustic signals occurring after arrivals of acoustic signals which have passed substantially directly from the transmitter location to the receiver location; and c) repeating steps a) and b) at a plurality of depths along the borehole and using the detected arrivals of reflected signals from each depth to generate an image of reflecting structures in the formation surrounding the borehole.
19 A method as claimed in claim 18, wherein step a) and b) are performed using a tool comprising arrays of transmitter and receiver elements.
A method as claimed in claim 18, wherein arrivals of direct and reflected signals are detected and only reflected signals are used to generate the image.
A method as claimed in claim 18, wherein arrivals of direct and reflected signals are detected and only reflected signals are used to generate the image.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US52773595A | 1995-09-13 | 1995-09-13 | |
| US08/527,735 | 1995-09-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2185412A1 true CA2185412A1 (en) | 1997-03-14 |
Family
ID=24102717
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2185412 Abandoned CA2185412A1 (en) | 1995-09-13 | 1996-09-12 | Method and apparatus for borehole acoustic reflection logging |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH09133775A (en) |
| CA (1) | CA2185412A1 (en) |
| GB (1) | GB2305245B (en) |
| NO (1) | NO963770L (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112065362A (en) * | 2020-09-24 | 2020-12-11 | 东北石油大学 | Anti-interference type natural potential logging device and method |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1348954A1 (en) * | 2002-03-28 | 2003-10-01 | Services Petroliers Schlumberger | Apparatus and method for acoustically investigating a borehole by using a phased array sensor |
| GB2459091B (en) * | 2008-04-07 | 2012-05-23 | Thales Holdings Uk Plc | Method and system for acoustic imaging |
| US8009510B2 (en) * | 2008-10-23 | 2011-08-30 | Schlumberger Technology Corporation | Two way check shot and reverse VSP while drilling |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4962489A (en) * | 1989-03-31 | 1990-10-09 | Mobil Oil Corporation | Acoustic borehole logging |
| US5521337A (en) * | 1994-09-28 | 1996-05-28 | Exxon Production Research Company | Seismic profiling tool with variable source/receiver spacer |
-
1996
- 1996-09-09 NO NO963770A patent/NO963770L/en not_active Application Discontinuation
- 1996-09-11 GB GB9618990A patent/GB2305245B/en not_active Expired - Fee Related
- 1996-09-12 CA CA 2185412 patent/CA2185412A1/en not_active Abandoned
- 1996-09-13 JP JP24343296A patent/JPH09133775A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112065362A (en) * | 2020-09-24 | 2020-12-11 | 东北石油大学 | Anti-interference type natural potential logging device and method |
Also Published As
| Publication number | Publication date |
|---|---|
| NO963770D0 (en) | 1996-09-09 |
| GB9618990D0 (en) | 1996-10-23 |
| NO963770L (en) | 1997-03-14 |
| JPH09133775A (en) | 1997-05-20 |
| GB2305245A (en) | 1997-04-02 |
| GB2305245B (en) | 1997-12-03 |
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