MXPA06006352A - Methods and systems for calibrating acoustic receivers - Google Patents

Abstract

A method and system for calibrating acoustic receivers (112). The method and system facilitate calibrating the acoustic receivers (112) while they are mounted to a downhole acoustic tool (102). Calibrating the acoustic receivers (112) in situ provides more accurate results than previously available. The method and system provide separate compensation factors for the acoustic receivers (112) at different frequencies and for different transmission sources. The separate compensation factors facilitate more accurate signal acquisition over a wider range of conditions.

Description

METHODS AND SYSTEMS FOR CALIBRATING ACOUSTIC RECEIVERS FIELD OF THE INVENTION The present invention relates in general terms to methods and systems for investigating underground formations using acoustic measurements performed in a well. More particularly, this invention relates to methods and systems for calibrating acoustic receivers used to collect acoustic measurements along a well. BACKGROUND OF THE INVENTION The generation and recording of acoustic waves in wells is an essential measurement used in well drilling in oil fields. Many well tools and methods are available today to take acoustic measurements. Some tools include a single source of sonic waves and two or more receivers, however, most tools now include two or more acoustic sources and many receivers placed in a set. While the acoustic tools available today are useful for providing a wide range of information about adjacent formation and well parameters, a primary use of acoustic well measurements is the estimation of the slowness in the formation of compression waves. and cutting waves. The slowness in the formation of compression waves is typically estimated using travel times acquired through a first motion detection process. In the case of a single source, a two-receiver tool suggested by the prior art, the slowness in the formation is estimated by subtracting the arrival times between two receivers and dividing by the space between the receivers. This estimate, however, is subject to inaccuracies due to tool tilt, well washes, bed limit effects, etc. Recipients and additional acoustic sources and more robust methods such as STC (Slowness-Time Consistency Analysis), among others, have been used to reduce the inaccuracies introduced by such environmental effects. Compression waves are detectable with monopole-type measurements. However, in slow formations, short waves are not detectable with monopole-type measurements. Directional or dipole type acoustic sources facilitate the detection of both compression waves and cutting waves. However, monopole and quadrupole contamination of dipole measurements is a major problem with acoustic recording tools that use sets of receivers. Acoustic receivers often have different sensitivities, and different sensitivities to the same wave result in a greater possibility of non-dipole contamination. Even receivers manufactured similarly or identically tend to report different amplitudes and different time receipts (ie, amplitude and phase mismatch). Therefore, it is usually necessary to calibrate the acoustic recording tools by detecting and correcting the amplitude and phase mismatches of the various receivers mounted on the recording tools in order to improve the estimation of the slowness and modal calculation of the background. water well. Typically, local personnel separately calibrate each individual receiver before each registration operation in an attempt to correct the amplitude and phase offset. While these calibrations can help, each receiver is calibrated before being mounted on the tool and with receivers subjected to atmospheric conditions. However, many factors can be combined to cause significant variations in sensitivity despite the usual calibration efforts. Certain factors that cause variations in sensitivity include the position and alignment of the receivers, electronic downhole characteristics, environmental factors such as pressure and temperature, and others. Normally, the receivers will be subjected to conditions very different from the conditions of surface calibration and nowadays it is difficult or impossible to take into account variations that result from the eventual placement and alignment of the receivers in the registration tool. When they operate, the receivers are housed in probes filled with oils, but during calibration they are exposed to air. Therefore, even though some receiver providers guarantee small sensitivity variations [= 5%] for receivers, individually, after mounting the receivers on an acoustic tool, the sensitivity variations are usually not within the prescribed parameters. In addition, many acoustic recording tools employ dozens of receivers or more. As the demand rises, more accurate registration data, the number of recipients used with the registration tools also rises. Therefore, the calibration of each individual receiver becomes a costly and time-consuming task, however, as discussed above, even costly and highly-available methods nowadays have limited effectiveness. The current calibration methods do not take into account many important factors, including the eventual placement of the receivers in the registration tool and the actual operating environment. The present invention has the purpose of overcoming or at least reducing the effects of one or more of the problems mentioned above. COMPENDIUM OF THE INVENTION The present invention satisfies the needs described above and others. Specifically, the present invention offers a method and system for calibrating acoustic receivers. The method and system facilitate the calibration of acoustic receivers in situ. Calibration techniques of the prior art calibrate the acoustic receivers before mounting them in a tool. The present invention calibrates the acoustic receivers with the receivers already mounted on the tool. Calibrating the acoustic receivers while mounted on the tool results in more accurate probing data. According to certain aspects of the invention, the methods and systems facilitate the calibration of acoustic receivers by implementing a procedure to correct and compensate for amplitude and phase mismatches between different receivers. The procedure can verify and correct an acoustic receiver response to ensure correct operation and reject non-dipole modes when dipole measurements are of interest. The procedure can include the calculation of different compensation factors for different frequency ranges and take into account different sources of acoustic transmission. According to one aspect of the invention, the amplitude and phase compensation factors for one or more individual acoustic receivers are determined from stationary measurements at low, medium and high frequencies created in an acoustic chamber housing the tool. Gross waveform signals that result from multiple tool orientations are averaged and placed in a window. Multiple frames for each acoustic transmitter and recording frequency are acquired and averaged in each tool orientation to calculate the compensation factors for one or more of the acoustic receivers. According to certain aspects of the invention, cutoff wave measurements are compensated with the low frequency compensation factors and high frequency compression wave measurements are compensated with the medium frequency and high frequency compensation factors. The calibration procedure identifies the functionality and sensitivity of each receiver, assigns the most sensitive receiver as a reference, and calculates the gain and delay factors for the remaining receivers based on differences between the reference receiver and the remaining receivers. According to some aspects of the invention, the application of the calibration process ensures a maximum level of amplitude mismatch between receivers of approximately 1.0 dB and a maximum phase mismatch between receivers of approximately 1.5 degrees. By ensuring maximum levels of mismatch between receivers, a ratio between dipole and monopole will generally be = 30 dB, which ensures the rejection of monopole, quadrupole and sextuple modes within dipole measurements. Additional advantages and novel features of the invention will be presented in the following description or may be acquired by persons skilled in the art by reading these materials or practicing the invention. The advantages of the invention can be achieved through the means mentioned in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate preferred embodiments of the present invention and form part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention. Figure 1 is an assembly view of an acoustic tool and an acoustic chamber in accordance with one embodiment of the present invention. Figure 2 is a perspective view of the acoustic chamber of Figure 1 housing the acoustic tool in accordance with the one embodiment of the present invention. Figure 3 is a diagrammatic representation of the electronics associated with the acoustic tool and the acoustic chamber in accordance with one embodiment of the present invention. Figure 4A is a wavelength box in a first receiver station of the acoustic tool before the application of a compensation factor to the receivers. Figure 4B is a waveform frame in the first receiver station of the acoustic tool after the application of compensation factors to the receivers in accordance with an embodiment of the present invention.

Figure 5 is a series of exemplary waveforms illustrating a waveform average determination method according to one embodiment of the present invention. Figure 6A is a cut-away end view of the acoustic tool housed within the acoustic chamber in a first orientation in accordance with one embodiment of the present invention. Figure 6B is a cut end list of the acoustic tool housed in the acoustic chamber in a second orientation in accordance with one embodiment of the present invention. Figure 6C is a cut-away end view of the acoustic tool housed in the acoustic chamber in a third orientation in accordance with one embodiment of the present invention. Figure 6D is a cut-away end view of the acoustic tool housed in the acoustic chamber in a fourth orientation in accordance with one embodiment of the present invention. Figures 7A-7D illustrate a window placement technique that can be applied to a waveform in accordance with one embodiment of the present invention. Figure 8A is a wave diagram illustrating the results of the application of compensation factors to mismatched low frequency receiver signals in accordance with one embodiment of the present invention. Figure 8B is a spectrum chart illustrating the results of the application of compensation factors to mismatched low frequency receiver signals in accordance with one embodiment of the present invention. Figure 9A is a wave diagram illustrating the results of the application of compensation factors to mismatched receiver signals at medium or high frequency in accordance with an embodiment of the present invention. Figure 9B is a spectrum graph illustrating the results of the application of compensation factors to mismatched receiver signals at medium or high frequency in accordance with an embodiment of the present invention. Figure 10 illustrates the calculations of average pressure and standard deviation per receiver station before and after the application of compensation factors to mismatched receiver signals according to one embodiment of the present invention.

In the drawings, identical reference numbers refer to similar but not necessarily identical elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrative modalities and aspects of the invention are described below. For clarity, all the features of the actual implementation are not described in this specification. It will obviously be observed that in the development of any real modality, numerous specific decisions must be taken for implementation in order to achieve the specific goals of the developers, such as compliance with system-related and business-related restrictions, which vary from one implementation to another. In addition, it will be noted that a development effort of this type can be complex and time-consuming, but nevertheless it would be a routine task for people with knowledge in the field with the benefit of this disclosure. The present invention contemplates the calibration of acoustic receivers of an acoustic tool with the acoustic receivers mounted on the acoustic tool. As mentioned above, historically, acoustic receivers have been calibrated separately from the tool and under atmospheric conditions. The accuracy of the historical calibration is compromised due to the eventual placement of the receiver in the tool and other factors can not be accurately considered separately from the tool. The present invention offers methods and systems for calibrating the acoustic receivers in situ. The methods and systems may be particularly suitable for in situ calibration of sonic recording tools used in wells. Nevertheless, the methods and systems presented here are not limited to this. The methods and systems can be applied to any calibration technique. Furthermore, even when particular methods are described below which facilitate the calibration of acoustic receivers while the acoustic receivers are mounted - in the acoustic tool, the particular methods are not limiting. Any method of calibration of acoustic receivers with the acoustic receivers mounted on the tool is contemplated within the framework of the present invention. As used in the specification and in the claims, the term "low" as used to modify a frequency means below about 7 kHz. The term "mean" refers to a frequency within a range between approximately 7 and 16 kHz. A "high" frequency refers to a frequency above about 16 kHz, or between about 16 kHz and 25 kHz. "Interstitial" refers to spaces between two or more components. The word "includes" and "has", as used in the specification, including the claims, have the same meaning as the word "comprises." Turning now to the figures, and particularly to Figure 1, there is shown an acoustic tool calibration system (100) in accordance with the principles of the present invention. The acoustic tool calibration system (100) includes an acoustic tool, for example a sonic tool (102). The sonic tool (102) can be any acoustic tool and is not limited to the configuration shown. The acoustic tool calibration system (100) also includes an acoustic chamber which, according to the embodiment of Figure 1, is a cylindrical wavelength sonic tube (104). To summarize, the sonic tube (104) is a receiver of the sonic tool (102). The sonic tube (104) may have a diameter greater than a diameter of the sonic tool (102). Accordingly, the sonic tube includes a plurality of spacers (106) placed around the sonic tool (102) in one or several axial positions to support the sonic tool (112) substantially concentric with the cylindrical sonic tube (104). The sonic tool (102) includes an electronic package (108) and a compensator frame (110) to facilitate data acquisition and calibration. The sonic tool (102) also includes one or more acoustic receivers and one or more acoustic transmitters or acoustic sources. According to the embodiment of Figure 1, the sonic tool (102) includes a set of sonic receivers (112) placed in 13 rows or stations. Each of the 13 receiver stations includes several sonic receivers arranged azimuthally (112). According to the embodiment of Figure 1, there are 8 sonic receivers (112) placed azimuthally in each of the 13 receiver stations. It will be understood by the person with knowledge in the field with the benefit of this disclosure, however, that other arrangements of sonic receptors may also be used. The set of sonic receivers (112) shown in Figure 1 is exemplary in nature and is used for the purpose of illustrating the principles of the invention as will be described in more detail below. According to Figure 1, the acoustic transmitter or the various acoustic transmitters comprise a first upper monopole transmitter (114) and a second lower monopole transmitter or transmitter (116). A collar (118) placed in a first end or upper end (120) facilitates the sealing of the sonic tool (102) in the sonic sound absorption tube (104). The sonic tube (104) has a generally cylindrical shape and includes a first end or open end (122) and a second end (124) closed by a stopper Before calibrating the receivers. Sonic (112), the sonic tool (102) is preferably inserted into the sonic tube (104) with the collar (118) sealing the open end (122) as shown in Figure 2. A fluid feed, such as for example a water hose, is in fluid communication with the filling line (128) of the sonic tube (104). The filling line (128) has a filling valve (130) placed there. When the filling valve (130) is open, water flows through the filling line (128) and enters the sonic tube (1049. Water fills all the interstitial spaces between the sonic tool (102) and the tube Sonic (104) In addition to the filling line (128), the sonic tube (104) can include a second line (132) with an air release valve (134) placed therein.Therefore, when the sonic tube 104 is filled with water, the air release valve (104) can be opened to allow evacuation of trapped air in the sonic sound absorption tube (104) The second line (132) can also include a pressure meter (136). ) to indicate the pressure inside the sonic tube (104) According to the embodiment of Figure 2, the sonic tube (104) is placed at an angle (a) relative to the horizontal.The angle (a) facilitates the release of the air contained in the sonic sound absorption tube (104) through s of the air release valve (134). The angle (a) is preferably approximately 30 degrees in accordance with the embodiment shown, but other angles, including an angle coincident with the horizontal can also be used. The filling line (128) is also in fluid communication with a pump (138) when a pump valve (140) is open.Therefore, the pump (138) can pressurize the sonic tube (104) when the pump valve (140) is open and the filling valve (130) and the air release valve (134) are closed.According to some embodiments, the sonic tube (104) is pressurized to at least 2. 07 MPa (300 psi) after insertion of the sonic tube (102) and the interstitial spaces are filled with water. According to other embodiments, the sound tube (104) is pressurized to approximately 3.45 MPa (500 psi). The pressurization of the sonic tube (104) to approximately 3.45 MPa (500 psi) improves the accuracy of the calibration, since the high pressure calibration environment closely resembles the eventual operating conditions of the sonic tube (102) that the local atmospheric conditions under which receivers are frequently calibrated. Referring now to Figure 3, an electrical diagram of the acoustic calibration system (100) is shown. As shown in Figure 3, a set of receivers (142) comprising the sonic receivers (112, Figure 1) and the upper and lower monopole transmitters (114, 116) are located within the sonic tube (104). A first communication cable, for example a 31-pin to 31-pin cable connector (144) extends from the sonic tool (102) and exits the sonic tube (104). The cable connector connects the sonic tool (102) to an electronic cartridge (146) and to a telemetry cartridge (148). The electronic cartridge (146) and the telemetry cartridge (148) can optionally be used to communicate between the sonic tool (102) and a surface control module, such as a computer (150). A second communication cable, for example a 31-pin to 10-pin connector cable (152) extends from the telemetry cartridge (148) to connect the sonic tool (102) to the computer (150). A 10 pin 10 pin test box (154) can be placed between the 31 pin to 10 pin connector cable (152) and the computer (150). The computer (150), the electronic cartridge (146) or the electronic package (108, Figure 1) may contain calibration processing instructions, when executed, automatically calibrate one or more sonic receivers (112, Figure 1). The processing instructions may comprise calibration methods, some of which are described with additional details below.

Referring again to Figure 1, according to the principles of the present invention, the sonic receivers (112) are mounted on the sonic tool (102) the sonic tool is inserted into the sonic sound absorption tube (104), and the sonic receivers (102) are calibrated. A uniform sound pressure field is generated in order to facilitate the calibration of the sonic receivers (112). To better calibrate the sonic receivers (112), a uniform sound pressure field can be generated by pressurizing the sonic tube (104) in accordance with what is described above. Then, acoustic waves are generated in the sonic sound absorption tube (104) and measured through each of the various sonic receivers. (112). With reference to Figure 4A, each of the eight azimuth-located receptors (112, indicated as R1-R8 in Figure 4A) at any receiver station can measure a waveform. The waveform can be generated by one of the upper or lower monopole transmitters (114, 116). Figure 4A represents a simple waveform received separately by each of the receivers (R1-R8) before any calibration. As shown in the figure, the amplitude measured by each of the receivers (R1-R8) varies for the same wave generated by the transmitter by up to 20%. In addition, the wave reception time for each receiver (R1-R8) varies by up to 60 μs. For example, with a gain offset of 2 dB, and phase mismatch of 20 degrees, the ratio between dipole and monopole will be well below 30 dB. Therefore, without calibration, dipole measurements will probably be contaminated by non-dipole modes. Studies have shown that a ratio between dipole and monopole of at least 30 dB guarantees rejection in non-dipole modes. further, a ratio between dipole and monopole of at least 30 dB is ensured if the gain mismatch is corrected to less than approximately 1.0 dB and if the phase mismatch is corrected to less than approximately 1.5 degrees. Accordingly, according to a method of the present invention, the receivers (Rl-R8) at each receiver station are calibrated for a gain and phase mismatch not greater than 1.0 dB and 1.5 degrees, respectively. To calibrate the gain and phase mismatch to no more than 1.0 dB and 1.5 degrees, an algorithm generates compensation factors for one or more receivers in one or more frequency conditions. When the appropriate compensation factors are applied to the waveforms generated by the receivers (R1-R8) shown in Figure 4A, each of the corrected measured waveforms is within 1.0 dB and 1.5 degrees as shown in FIG. Figure 4B. According to typical calibration techniques, the sonic receivers are not only calibrated while they are separated from their associated sonic tools, but they are usually calibrated at a frequency and based on only one source of acoustic transmitter. However, the sonic receivers often have different sensitivities to different transmitters and different frequencies, therefore, according to certain embodiments of the present invention, the sonic receivers (112, Figure 1) are calibrated with multiple gain and phase compensation factors. For example, according to certain aspects of the present invention, one or more of the sonic receivers (112) are calibrated with a gain and phase compensation factor for three different frequencies generated from two different transmitters. Accordingly, one or more sonic receivers (112) can have six gain compensation factors and six phase compensation factors. The six facts of gain and phase compensation are generated from the six possible different combinations of two transmitters that separately generate three different frequencies at different intervals. However, a smaller number of compensation factors or additional compensation factors based on different numbers of transmitters and frequency combinations can also be calculated. However, according to one embodiment of the present invention, six gain and phase compensation factors are calculated for one or more of the sonic receivers. The three gain and phase compensation factors calculated for each of the two different transmitters are then preferably averaged to provide a total of three gain and phase factors, one for each of the three frequencies. The two transmitters may include the upper and lower monopole transmitters (114, 116) described above, and the three different frequencies may include a low frequency, a medium frequency and a high frequency. An algorithm for calculating the gain and phase compensation factors for calibrating the sonic receivers (112) is described below with reference to Figures 5-9. In order to avoid calibration of individual sonic receivers (112, Figure 1) based on measurements that may not be representative of the true sensitivity of the receiver, several waveforms are averaged for each sonic receiver (112). In addition, since the orientation of the sonic tool (102, Figure 1), may not be exactly concentric with the sonic tube (104, Figure 1), waveforms can be generated in various rotational orientations for the sonic tool (102) within the sonic sound absorption tube (104, Figure 1) and can be included in an average determination procedure. An example of averaging method is shown in Figure 5. Figure 5 illustrates various forms of raw waves received by a sonic receiver (112, Figure 1) located at each of the 13 receiver stations. The gross waveforms are designated in general terms in (156) and divided into frames. The tables are divided by tool orientation. According to Figure 6A, the sonic tool (102) is arranged in the sonic tube (104) in a first orientation shown. A first column (158) of the graph shown in Figure 5 corresponds to measurements taken by the receivers (112) in a first orientation of Figure 6A. The first column (158) also represents waveforms measured by the receivers (112, Figure 1) as waves generated at a single frequency by one of the sonic transmitters, which, for discussion purposes, is a low frequency generated by the lower monopole transmitter (116, Figure 1). Several waveform frames are measured by the receivers (112, Figure 1) to create a representative sample of receiver sensitivity. For example, according to certain modalities, at least 30 frames of shape are measured, and according to the modality shown in Figure 5, 60 frames of waveform are measured. The various waveform frames represented by the first column (158) are averaged in an average determination row (160) to create an average waveform (162) for the receivers (112, Figure 1) in the first orientation of sonic tool shown in Figure 6A. Similarly, a second column (164) of the graph shown in Figure 5 corresponds to measurements taken by the receivers (112, Figure 1) according to the same parameters of the first column (158), but with the sonic tool (112) placed in a second orientation as shown in Figure 6B. The waveforms of the second column (164) are also averaged to create a second average waveform (166). The third column and the fourth column (168, 172) follow the same pattern as the first column and the second column (158, 164), but in a third and fourth sonic tool orientations (102) shown in the Figures 6C and 6D, respectively. Accordingly, the waveforms of the third column and the fourth column (168, 172) are each averaged to create a third waveform and a fourth averaged waveform (170, 174). It will be understood by part of the persons with knowledge in the matter with the benefit of this disclosure that however, any number of columns can be created including only one column, and each column can correspond to a different orientation of sonic tool (102). However, in accordance with Figures 6A-6D, there are four tool orientations, rotationally displaced between them at approximately 90 degrees. When the four averaged waveforms (162, 166, 170, 174) have been created (or any other number), the averaged waveforms can be averaged themselves to create a master average waveform (176). The average master waveform (176) is a very accurate average of the waves detected by one of the sonic receivers in each of the three receiver stations. The average master waveform (176), however, is only representative of a sonic receiver in each receiver station and in the first set of parameters mentioned above (the lower monopole transmitter (116) generating low frequency waves). Other figures similar to Figure 5 are created for other parameters and other sonic receivers. For example, as mentioned above, according to one aspect of the present invention there are 5 additional figures similar to Figure 5: one for each of medium and high frequencies generated by a lower monopole transmitter (116), and one each for each low, medium and high frequency generated by the upper monopole transmitter (114). In addition, additional figures similar to Figure 5 (groups of 6 figures according to the present embodiment that define 6 transmitter / frequency combinations, are created for each azimuth-arranged sonic receiver (112) located in each of the 13 stations of receiver shown in Figure 1. The average master waveform (176) (and every other master waveform created) can optionally be placed in window to eliminate all modulations except the strongest wave modulations as shows in Figures 7A-7B, According to Figure 7A, low frequency master averaged waveforms (176) are filtered by low pass and then each master averaged waveform (176) is linearly interpolated to a sampling of 1 μs. A rectangular window (178) is generated for the interpolated waveforms as shown in Figure 7A for each receiver station.The rectangular windows (178) are then applied to the average master waveform (176). For example, the rectangular window (178) for the first receiver station as shown in Figure 7B is applied to the average master waveform (176) associated with a receiver (Rl) as shown in Figure 7C to create a window waveform as shown in Figure 7D. This window positioning procedure can then be applied to all generated master average waveforms (176). According to the sonic tool (102) shown in Figure 1, the result would be 104 waveforms in individual windows (13 receiver stations with 8 azimuth sonic receivers in each station) for each of the six transmitter / frequency combinations. As shown in Figure 7D, only the three most notable wave measurements remain after the window placement procedure: a first valley (El), a first peak (E2) and a second valley (E3). It will be understood, however, that according to certain modalities other window training techniques are used and according to other modalities there is no window training. For example, low frequency master averaged waveforms (176) may be formed in window according to the method described above, while medium frequency and high frequency master averaged waveforms may be formed in windows in order to eliminate all the valleys, except the first valley (El) as shown later in Figures 9A-9B. In addition, high frequency and medium frequency master averaged waveforms can be filtered by high pass before any window placement. When the window waveforms have been generated (or, if there is no window), the average master waveforms (176) for each sonic receiver (112), and each transmitter / frequency combination, the waveforms in individual windows they are examined in each receiver station to determine which sonic receiver in each receiver station has the highest sensitivity for each of the transmitter / frequency combinations. According to certain embodiments, the sonic receiver at each receiver station that is determined to have the highest sensitivity is then assigned as a reference receiver and has an associated reference waveform. However, the assignment of a reference receiver and associated waveform is not limited to the most sensitive receiver. Our choices can also be made based on any criteria. Accordingly, a window waveform for a receiver at each receiver station is assigned as a reference for each transmitter / frequency combination. Each window waveform that is not assigned as a reference waveform is then compared to the reference waveform to generate a gain and phase compensation factor that corresponds to each non-reference receiver for a transmitter / frequency combination Dadaist. For example, for discussion purposes, we contemplate that the window waveform shown in Figure 7 is selected as a reference waveform and correspond to a first sonic receiver located at a first receiver station. The reference waveform for the first sonic receiver is transferred to the graph shown in FIG. 8A, which also includes an unadjusted window waveform associated with a second sonic receiver located at the first receiver station. The differences between the unbalanced window waveform and the reference waveform are analyzed in order to calculate a gain and phase compensation factor for the second sonic receiver. According to certain embodiments, for low frequencies, the gain factor is calculated by dividing a difference between a first peak (E2R) of the reference waveform and a second valley (E3R) of the reference waveform by a difference between a first peak (E2C) of the unbalanced window waveform and a second valley (E3C) of the unbalanced window waveform as follows: 3S7 * JS * ÍS 's? Gain Factor S > Ur '^ JSsc (1) The factor of compensation of delay or phase for corrections of low frequency is calculated as a difference between a time of the first peak (E2R) of the reference waveform and a time of a first peak (E2C) ) of an unbalanced window waveform in the following manner: Delay_factor (%) * S l where TE2R and TE2C are the moments when E2R and E2C are at a maximum. If the gain and delay compensation factor have been calculated separately at a particular frequency or in a particular group of frequencies for each of several multiple transmitters, preferably the gain and delay factors are averaged in order to provide a factor of gain and single delay for a given frequency or a given set of frequencies. For example, if a first gain factor (gMLLF) for a particular receiver is calculated based on a low frequency generated by the lowest monopole transmitter (116, Figure 1), and a second gain factor (gMULF) is also calculated at a low frequency but generated by the upper monopole transmitter (114, Figure 1), an average of the two gain factors is assigned as a factor of low frequency gain for the particular receiver. Therefore we have: Y for an acoustic tool with two transmitters. Figure 8A illustrates an application of the calculated gain and delay factors U? KF *? I - *, J to the unbalanced window waveform as a compensated waveform. As shown, the compensated waveform is much closer to the reference waveform which is the desired result. Both the first receiver and the second receiver "see" the same waves, and therefore it is desirable that each of the receivers measure and report approximately the same waveforms. The formation properties (such as information slowness) estimated from the acoustic waveforms are more accurate when the waveforms are more precise and the waveforms are more accurate after the calibration of the receivers according to the described method above. Figure 8B is a spectrum graph and illustrates a reference waveform, a mismatched window waveform, and the waveform compensated as a function of frequency. The graph shows, for example, an amplitude mismatch of 0.35 dB at 1.2 kHz, which will result in a ratio between dipole and monopole of > 30 dB and therefore guarantees the rejection of non-dipole modes. A similar or identical compensation control calculation in accordance with the principles described above is performed for each of the non-reference receivers and for each desired transmitter / frequency combination. In addition, each of the calculated compensation factors can be normalized to a maximum value of 1.0 in order to avoid saturation of the electronic attachments of the sonic tool. However, the calculations of compensation factor can be modified from equations (1) and (2) for certain frequency / transmitter combinations. For example, in the case of medium frequency and high frequency corrections, the gain factor of each non-reference receiver can be calculated by dividing an amplitude value of the reference waveform in a first valley (EIR) by the value of amplitude of the unbalanced window waveform in its first valley (E? C) in the following manner: Factor-of? MMCia mm) "~ r ® In addition, according to certain embodiments, the delay or phase compensation factor for medium frequency and high frequency corrections is calculated as a difference between a first valley time (EiR) of the reference waveform and a first peak time (E? C) of the unbalanced window waveform in the following way: Delay factor Figures 9A and 9B illustrate a real example of the time and frequency domains, respectively, of the application of compensation factors calculated in accordance with equations (3) and (4) to a receiver subjected to high frequency parameters. According to Figure 9B, the amplitude mismatch is 0.5 dB at 14 kHz, and the amplitude mismatch at higher frequencies such as the frequencies associated with cement junction record measurements (from 20 kHz to 25 kHz) is even lower than 0.5 dB. By applying the principles described here to calibrate acoustic receivers in situ, the acoustic measurements of different receivers of the same wave become much more uniform. Accordingly, by applying the principles described herein, acoustic receivers can be calibrated while mounted on an acoustic tool by inserting the acoustic tool into an acoustic chamber, averaging the waveforms received by each of the acoustic receivers for create an average waveform associated with each of the acoustic receivers, assigning one or more average waveforms as reference waveforms, calculating compensation factors for one or more of the non-reference receivers by measuring the differences between the non-reference waveforms and the reference waveforms, and applying the compensation factors to the non-reference receivers. The average waveforms can be placed in a window, if desired, and the averages can be calculated according to multiple sonic tool orientations. Various compensation factors can be calculated for one or more of the sonic receivers, including gain and phase compensation factors, for different frequency ranges and different transmission sources. After calibration of the sonic receivers by applying the calculated compensation factors, it may be desirable to verify the effectiveness of the compensation factors. Therefore, according to certain methods, standard and standardized deviations before and after correction of the single receivers are calculated and plotted. Figure 10 illustrates a real statistical analysis of a mean and standard deviation of acoustic pressure measurements by a receiver before and after the calibration of the receiver. As shown, the standard deviation of the calibrated receivers is significantly lower than the standard deviation of the uncalibrated receivers, which means more accurate measurements using the compensation factors. In addition, the effectiveness of the calibration can be verified according to certain methods by using the data acquired from the receivers to find the values of pipeline and Stoneley slowness through STC processing. The values generated from the acquired data can then be compared with the predicted values by medical analysis to check the accuracy. While the Figures and the description are specific to a sonic tool with multiple receivers in each of multiple stations, the principles described herein can be used for any acoustic tool having a set of acoustic receivers. For example, similar compensation factors can be calculated for a set of axial receivers having only one receiver at each receiver station by applying known wave attenuation rates through known fluids and measuring the intervals between acoustic receivers. The preferred embodiments were selected and described in order to better explain the principles of the invention and its practical application. The above description is intended to allow other persons with knowledge in the art to better use the invention in various modalities and with various modifications suitable for the particular use contemplated. It is understood that the scope of the invention is defined by the appended claims.

Claims (11)

1. CLAIMS 1. A method for calibrating one or more individual acoustic receivers mounted on an acoustic tool for well sounding, comprising: inserting the tool into an acoustic chamber; generate acoustic waves in the acoustic chamber; receive acoustic waves with the receivers to calibrate one or more of the acoustic receivers mounted on the acoustic tool. The method according to claim 1, further comprising: averaging the waveforms received by each of a plurality of acoustic receivers to create an average waveform associated with each of the plurality of acoustic receivers; assign an average waveform as a reference waveform; calculate compensation factors for one or several of the plurality of receivers. 3. The method according to claim 2, wherein the calculation comprises the measurement of differences between the reference waveform and one or more of the remaining average waveforms. 4. The method according to claim 2, wherein the calculation comprises the calculation of three gain compensation factors and three delay compensation factors for each of the several receivers, one for each of the following ranges: Low frequency, medium frequency range and high frequency range. The method according to claim 2, wherein the generation further comprises the generation of acoustic waves in each of at least two different axially rotated positions. The method according to claim 5, wherein the at least two different axially rotated positions comprise four positions displaced by approximately 90 degrees. The method according to claim 2, further comprising calculating the compensation factors for each of the several receivers except for a reference receiver. 8. A system for calibrating one or more individual acoustic receivers mounted on an acoustic tool, comprising: at least one source and several receivers mounted on the acoustic tool; an acoustic chamber that receives the acoustic tool; several spacers placed around the acoustic tool to support the acoustic tool substantially concentrically with respect to the acoustic chamber; The acoustic tool is configured to calibrate each of the various receivers while the receivers are mounted on the acoustic tool. 9. The system according to claim 8, further comprising multiple receiver stations spaced axially along the acoustic tool, wherein each of the several receiver stations comprises several azimuth-positioned receivers. The system according to claim 8, wherein the acoustic chamber is pressurized to at least 2.07 MPa (300 psi). The system according to claim 8, wherein the acoustic tool comprises an upper monopole source and a lower monopole source with the receivers located between the upper monopole source and the lower monopole source.