Patent Publication Number: US-2017350235-A1

Title: Acoustic source identification apparatus, systems, and methods

Description:
BACKGROUND 
     Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, as well as to identify downhole conditions, including the occurrence of leaks in the well bore. To obtain such measurements, acoustic sensors may be used. However, while some techniques for downhole leak detection exist, none provide a straightforward way to locate the depth, azimuthal angle, and strength of the source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of neighboring acoustic sensors in accordance with various embodiments of the invention. 
         FIG. 2  provides a side, sectioned view of an acoustic sensor array disposed in a formation in accordance with various embodiments of the invention. 
         FIG. 3  is a top view of the array shown in  FIG. 2 . 
         FIG. 4  depicts an acoustic sensor array disposed in a well, with sound pressure signal measurements transformed into a map of acoustic intensity, according to various embodiments of the invention. 
         FIG. 5  is a graph of substantially simultaneous acoustic source time history for a seven-sensor array, according to various embodiments of the invention. 
         FIG. 6  is a graph of substantially simultaneous acoustic source time histories for the seven-sensor array of  FIG. 5 , after the array of sensors has traversed the borehole relative to the source. 
         FIG. 7  is a block diagram of a logging system according to various embodiments of the invention. 
         FIG. 8  is a flow diagram illustrating methods of acoustic source identification, according to various embodiments of the invention. 
         FIG. 9  depicts an example wireline system, according to various embodiments of the invention. 
         FIG. 10  depicts an example drilling rig system, according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Conventional acoustic methods, including those used to detect leaks in a borehole, rely on the speed of sound to locate its source. However, the speed of sound (i.e., c) in an acoustic medium (e.g., oil- or water-based mud, water, etc.) can be cumbersome to measure, and may vary with frequency. To address some of these challenges, as well as others, apparatus, systems, and methods are described herein which operate to determine the acoustic pressure and vector particle velocity {right arrow over (u)} of acoustic emissions provided by the source, with the latter being extracted via differential pressure measurements. These techniques can identify an acoustic source—both as to depth and azimuthal orientation—in a manner that is independent of the speed of sound in the medium, providing a great advantage over conventional methods. The details of various embodiments will now be described. 
     Acoustic intensity is defined as the time-averaged rate of acoustic energy flow though a unit area normal to the direction of propagation of sound. If there is no net flow of acoustic energy through a given area, it will be no net acoustic intensity. 
     In many embodiments, a wireline or MWD/LWD (measurement while drilling/logging while drilling) tool is equipped with amplitude- and phase-matched acoustic sensors (e.g., hydrophones or dynamic pressure transducers), perhaps arranged as one or more multi-dimensional acoustic sensor arrays, to identify and rank acoustic sources in a downhole operational environment. When the techniques described herein are implemented, this apparatus and others can be used to identify, locate, and rank extraneous acoustic anomalies associated with flaws in a well casing, tubing flaws, and well leaks. 
     To accomplish this feat, the principle of acoustic intensity, based on obtaining acoustic pressure measurements and calculating the acoustic velocity, is employed. Because acoustic intensity provides amplitude and direction information, it is useful in locating and ranking sound sources. Therefore, the advantage of this method is that at any given well depth and azimuthal orientation, the acoustic particle velocity (a vector quantity) and the acoustic pressure can be used to obtain acoustic intensity, also a vector quantity, and/or acoustic power, to identify and rank extraneous (to the measurement tool) sources in a well. 
     The instantaneous rate at which energy flows is the product of the acoustic sound pressure [p(t)] and the acoustic particle velocity [u(t)]. When this product is integrated over a finite, selected period of time, the result is the average energy flow, which in this case is the sound field acoustic intensity. 
     Acoustic intensity is a vector quantity, having a magnitude and direction, as shown in Equation 1. 
         {right arrow over (I)}=pũ   (Eq. 1)
 
     Note that both acoustic intensity and acoustic velocity are vector quantities. By integrating pressure [p(t)] and velocity [u(t)] over a period of time T, the average acoustic intensity can be obtained, as shown in Equation 2. 
     
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       1 
                       T 
                     
                      
                     
                       
                         ∫ 
                         0 
                         T 
                       
                        
                       
                         
                           Pinst 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                         
                           u 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                         dt 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     In a free-field sound environment, where sound is not reflected, the acoustic pressure and particle velocity vary simultaneously in phase with each other. In other words, a peak in particle velocity occurs at the same time as a peak in acoustic pressure. In a relatively reverberant/diffuse field the net intensity is close to zero, because the pressure is reflected several times and travels in all directions with roughly equal magnitude and probability. Although the net acoustic intensity in a reverberant field is close to zero, in theory there is a relationship of the “one-sided” acoustic intensity to the reverberant field acoustic pressure. 
     Equation 3 shows the relationship of the acoustic intensity magnitude as a function of the acoustic pressure in an a free field environment, 
     
       
         
           
             
               
                 
                   
                      
                     I 
                      
                   
                   = 
                   
                     
                       
                         p 
                         2 
                       
                        
                       rms 
                     
                     
                       ρ 
                        
                       
                           
                       
                        
                       c 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     where p rms   2  is the RMS (Root Mean Square) pressure squared, and ρc is the product of acoustic medium density and speed of sound in the medium. 
     Equation 4 shows the relationship of the one-sided acoustic Intensity as a function of the acoustic pressure in a reverberant/diffuse field environment, 
     
       
         
           
             
               
                 
                   
                     I 
                     r 
                   
                   = 
                   
                     
                       p 
                       
                         rm 
                          
                         
                             
                         
                          
                         s 
                       
                       2 
                     
                     
                       4 
                        
                       ρ 
                        
                       
                           
                       
                        
                       c 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     where I r  is the “one-sided acoustic intensity” in a direction {right arrow over (r)} along the measurement direction. 
     In the vicinity of a down-hole leak, such as a casement leak, the product of the acoustic pressure and particle velocity are expected to increase as the source of the leak approaches the measurement sensors, and to decrease as the source retreats into the distance (especially with respect to acoustic pressure). The particle velocity can be calculated by using two adjacent acoustic pressure measurement locations (e.g., neighboring hydrophones) separated by a fixed distance Δr. By utilizing Euler&#39;s equation, the acoustic velocity can be determined, as shown in Equation 5. 
     
       
         
           
             
               
                 
                   
                     
                       ρ 
                        
                       
                         
                           ∂ 
                           u 
                         
                         
                           ∂ 
                           t 
                         
                       
                     
                     + 
                     
                       
                         ∂ 
                         p 
                       
                       
                         ∂ 
                         r 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     At a point midway between the two pressure measurement locations, the particle velocity is related to the instantaneous pressure gradient by Equation 6, 
     
       
         
           
             
               
                 
                   
                     u 
                     -&gt; 
                   
                   = 
                   
                     
                       - 
                       
                         1 
                         ρ 
                       
                     
                      
                     
                       ∫ 
                       
                         
                           
                             ∂ 
                             p 
                           
                           
                             ∂ 
                             r 
                           
                         
                          
                         dt 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     where ρ is the density of the acoustic fluid medium. The quantity inside the integral is the pressure gradient along an arbitrary path ({right arrow over (r)}). 
     The finite-difference method can be applied to Equation 6 in order to approximate the sound pressure gradient, as shown in Equation 7. 
     
       
         
           
             
               
                 
                   
                     
                       ∂ 
                       p 
                     
                     
                       ∂ 
                       r 
                     
                   
                   ≅ 
                   
                     
                       
                         p 
                         2 
                       
                       - 
                       
                         p 
                         1 
                       
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       r 
                     
                   
                   ≈ 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       p 
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       r 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     7 
                   
                   ) 
                 
               
             
           
         
       
     
     The average acoustic pressure  p  is equal to the algebraic average of the acoustic pressure between two adjacent acoustic sensors separated by a distance Δr (which in some embodiments represents the fixed distance between neighboring sensors in an array of substantially equally spaced hydrophones, or other acoustic sensors), as shown in Equation 8. 
     
       
         
           
             
               
                 
                   
                     p 
                     _ 
                   
                   = 
                   
                     
                       
                         p 
                         2 
                       
                       + 
                       
                         p 
                         1 
                       
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
     The acoustic particle velocity at any given depth can thus be readily determined using Equation 9. 
     
       
         
           
             
               
                 
                   
                     u 
                     -&gt; 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           ρ 
                         
                       
                        
                       
                         ∫ 
                         
                           
                             
                               
                                 p 
                                 2 
                               
                               - 
                               
                                 p 
                                 1 
                               
                             
                             
                               Δ 
                                
                               
                                   
                               
                                
                               r 
                             
                           
                            
                           dt 
                         
                       
                     
                     = 
                     
                       
                         - 
                         
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               p 
                             
                             _ 
                           
                           
                             p 
                              
                             
                                 
                             
                              
                             Δ 
                              
                             
                                 
                             
                              
                             r 
                           
                         
                       
                       = 
                       
                         
                           - 
                           
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 p 
                               
                               _ 
                             
                             
                               p 
                                
                               
                                   
                               
                                
                               Δ 
                                
                               
                                   
                               
                                
                               z 
                             
                           
                         
                         = 
                         
                           - 
                           
                             
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 p 
                               
                               _ 
                             
                             
                               p 
                                
                               
                                   
                               
                                
                               Δ 
                                
                               
                                   
                               
                                
                               θ 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     9 
                   
                   ) 
                 
               
             
           
         
       
     
     This initial description of how particle velocity can be determined without recourse to knowledge of the speed of sound in the associated medium sets the stage for the remainder of the discussion. 
       FIG. 1  is a schematic representation of neighboring acoustic sensors  110  in accordance with various embodiments of the invention. The acoustic sensors  110  can be set up in any of the following configurations, among others: (a) side-by-side, (b) front-to-front, (c) back-to-back. Here the sensors  110  are positioned to receive energy from a source  120 , in the form of acoustic emissions  130 . The time- and frequency-based expressions for acoustic intensity measurement and calculation, by two adjacent acoustic sensors, will now be described. 
     Acoustic Intensity can be formulated in the time domain and the frequency domain. In  FIG. 1 , a graphical depiction of the average acoustic pressure and particle velocity, is shown. 
     For example, to determine acoustic intensity in the time domain, Equation 9 can be used as the starting point. However, since 
     
       
         
           
             
               
                 
                   
                     
                       p 
                       = 
                       
                         
                           
                             p 
                             A 
                           
                           + 
                           
                             p 
                             B 
                           
                         
                         2 
                       
                     
                     , 
                     
                       
                         and 
                          
                         
                             
                         
                          
                         I 
                       
                       = 
                       
                         
                           p 
                            
                           
                               
                           
                            
                           • 
                            
                           
                               
                           
                            
                           A 
                         
                         _ 
                       
                     
                     , 
                     then 
                   
                    
                   
                     
 
                   
                    
                   
                     I 
                     = 
                     
                       
                         - 
                         
                           
                             
                               p 
                               A 
                             
                             + 
                             
                               p 
                               B 
                             
                           
                           
                             2 
                              
                             ρΔ 
                              
                             
                                 
                             
                              
                             r 
                           
                         
                       
                        
                       
                         ∫ 
                         
                           
                             ( 
                             
                               
                                 p 
                                 B 
                               
                               - 
                               
                                 p 
                                 A 
                               
                             
                             ) 
                           
                            
                           
                             dt 
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     10 
                   
                   ) 
                 
               
             
           
         
       
     
     Acoustic intensity can also be determined in the frequency domain, as shown in Equation 11, 
     
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       - 
                       
                         1 
                         
                           ρωΔ 
                            
                           
                               
                           
                            
                           r 
                         
                       
                     
                      
                     Im 
                      
                     
                         
                     
                      
                     
                       G 
                       AB 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     where ω is the angular frequency, and I m G AB  is the imaginary part of the cross spectrum between the two adjacent acoustic sensors  110 . The results can be plotted on a spectral two-dimensional map as a function of frequency and amplitude, including positive and negative intensity (see  FIG. 4 ). 
       FIG. 2  provides a side, sectioned view of an acoustic sensor array  200  disposed in a formation  210  in accordance with various embodiments of the invention. The array  200  in this case comprises a number of acoustic sensors  220  attached to a downhole tool  230 . The sensors  220  may be similar to or identical to the sensors  110  shown in  FIG. 1 . Here the sensors  220  in the array  200  are located along the axial (z) direction, and the azimuthal ( 0 ) direction, so that acoustic pressure and particle velocity in multiple directions can be determined. 
     A first dimension might be along the longitudinal axis z of the tool  230 . As noted in Equation 9, the vector acoustic particle velocity in this case is: 
     
       
         
           
             
               
                 u 
                 -&gt; 
               
               = 
               
                 
                   
                     - 
                     
                       1 
                       ρ 
                     
                   
                    
                   
                     ∫ 
                     
                       
                         
                           
                             p 
                             2 
                           
                           - 
                           
                             p 
                             1 
                           
                         
                         
                           Δ 
                            
                           
                               
                           
                            
                           r 
                         
                       
                        
                       dt 
                     
                   
                 
                 = 
                 
                   - 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         p 
                       
                       _ 
                     
                     
                       p 
                        
                       
                           
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       z 
                     
                   
                 
               
             
             , 
           
         
       
     
     with the average pressure and acoustic velocity being measured or calculated along the tool or well longitudinal axis (z). 
     A second dimension might be along the circumference of the tool  230 . This is understood by referring to  FIG. 3 , which is a top view of the array  200  shown in  FIG. 2 . Again, from Equation 9, the acoustic particle velocity is: 
     
       
         
           
             
               
                 u 
                 -&gt; 
               
               = 
               
                 
                   
                     - 
                     
                       1 
                       ρ 
                     
                   
                    
                   
                     ∫ 
                     
                       
                         
                           
                             p 
                             2 
                           
                           - 
                           
                             p 
                             1 
                           
                         
                         
                           Δ 
                            
                           
                               
                           
                            
                           Θ 
                         
                       
                        
                       dt 
                     
                   
                 
                 = 
                 
                   - 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         p 
                       
                       _ 
                     
                     
                       p 
                        
                       
                           
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       Θ 
                     
                   
                 
               
             
             , 
           
         
       
     
     with the average pressure and acoustic velocity being measured or calculated along the tool or well azimuthal direction (θ). Taken together,  FIGS. 2 and 3  demonstrate how various embodiments of an acoustic array  200 , when used in conjunction with the techniques described herein, provide the ability to identify and rank acoustic sources in different directions and dimensions with respect to the body of the tool  230 . 
     Arrays of sensors  220  can be formed in various ways. For example, the arrays may comprise linear arrays, or plane arrays. Linear arrays may be added together to form planar arrays, and planar arrays may be wrapped around a cylinder to form cylindrical arrays (e.g., the array  200  shown in  FIGS. 2-3 ). 
     Acoustic radiation measurements in many embodiments are made between each sensor and its nearest neighbor, that is, between “neighboring” sensors. However, acoustic radiation measurements can also be made between a sensor and other sensors in an array, such that the distance between the sensors is fixed, albeit interrupted by the existence of other sensors. These measurement are said to be conducted between “non-neighboring” sensors. For example, in some embodiments a linear array may include ten sensors spaced approximately equidistant from each other along the direction of the array, so that in some applications measurements are made between sensors 1-2, 2-3, 3-4, 4-5, etc. (i.e., between neighboring sensors). However, in some embodiments, measurements are made between sensors 1-3, 3-5, 5-7, etc. (i.e., between non-neighboring sensors: sensors having another sensor separating them, so that they are no longer neighbors). This latter approach may be suitable when lower frequencies are used, or when a course mapping resolution is sufficient for the desired image quality. 
       FIG. 4  depicts an acoustic sensor array  200  disposed in a well  440 , with sound pressure signal measurements  444  transformed into a map  448  of acoustic intensity, according to various embodiments of the invention. Here the tool  230  is lowered into first position (Depth #1) and sound pressure time histories (represented by the measurements  444 ) are recorded. All acoustic pressure measurements are taken by the array  200  substantially simultaneously, essentially freezing the measurements at the instant in time when measurement occurs. The tool  230  is then lowered to the next position (Depth #2) and the measurement process is repeated. The acoustic intensity is then determined via the formulas described earlier, and can be plotted on a spectral map  448 . The map  448  shows the location and strength (closer/stronger sources of acoustic emission are shown by darker patterns on the map  448 ) of each extraneous acoustic source. 
     In some embodiments, the array  200  is configured to cover a cylindrical surface (e.g., the tool  230 ) so that a color surface map projection indicating the direction and magnitude of the acoustic emission intensity can be readily observed. The sensor array  200  can then be moved upward or downward along the z axis (longitudinally within the borehole  440 ), and the process of measuring and calculating the acoustic intensity can be repeated as often as desired, to provide varying degrees of resolution. 
     The raw data obtained from the sensors  220  (e.g., acoustic radiation, in the form of acoustic pressure) can be processed in real time, or post-processed to determine acoustic intensity. At each depth in the borehole, the sensor array  200  can thus be used to measure acoustic intensity in the axial (z) and circumferential (azimuthal θ) directions simultaneously, without rotating the array  200 . 
     The measurement process can be repeated at different depths to log information over the entire area of interest in the borehole  440 . The downhole background noise (e.g., noise due to acoustic sources on or in the tool) can also be measured, prior to initiating the source location process. Noise that is extraneous to the tool acoustic sources may then be located and classified as an indication of a leak in the well, and used to direct corrective action by the drilling control/steering apparatus, if desired. 
       FIG. 5  is a graph  500  of substantially simultaneous acoustic source time histories for a seven-sensor array, according to various embodiments of the invention. The graph  500  can be considered similar to or identical to the graph of measurements  444  shown in  FIG. 4 . 
     Here substantially simultaneous time histories of the acoustic source  510  and seven sensors (e.g., hydrophones)  521 - 527  that together form a linear array are shown. In this figure it can be seen that the stationary acoustic source  510  is approximately aligned with the middle of the array—at sensor  524 . 
       FIG. 6  is a graph  600  of substantially simultaneous acoustic source time histories for the seven-sensor array of  FIG. 5 , after the array of sensors  521 - 527  has traversed the borehole relative to the source  510 . Here it can be seen that the array of sensors  521 - 527  has been moved downward in the borehole, so that the stationary acoustic source  510  is now located approximately between sensor  524  and sensor  525 . It should be noted that the time lag between the source  510  and the array sensors  521 - 527  depends on how far the sensors  521 - 527  are from the source  510 , but since the time-averaged acoustic intensity is used to locate the source  510  in various embodiments, this does not affect the results. Thus, still further embodiments may be realized. 
     For example,  FIG. 7  is a block diagram of a logging system  700  according to various embodiments of the invention. Referring now to  FIGS. 1-7  it can be seen that the logging system  700  includes the apparatus illustrated in  FIGS. 1-4 . Here the sensors  220  are included in a down hole tool  230  and arranged to form an array  200 , which may comprise a linear, planar, or cylindrical array, or some combination of these. A multiplexer and/or receivers  704  can be used to acquire signals from the sensors  220 , as a measure of acoustic pressure. 
     Referring now to  FIGS. 1-4 and 7 , it can be seen that the processing unit  702  can couple to the tool  230  to obtain acoustic pressure measurements from the sensors  220  in the array  200 . In some embodiments, a logging system  700  comprises one or more tools  230 . The processing unit  702  may be part of a surface workstation or attached to the downhole tool  230 , or disposed otherwise downhole. 
     The logging system  700  can include a controller  725 , other electronic apparatus  765 , and a communications unit  740 . The controller  725  and the processing unit  702  can be fabricated to operate one or more components of the apparatus to acquire measurement data, such as acoustic radiation measurements, in the form of acoustic pressure. 
     Electronic apparatus  765  (e.g., acoustic sensors, electromagnetic sensors, etc.) can be used in conjunction with the controller  725  to perform tasks associated with taking acoustic and resistivity measurements downhole. The communications unit  740  can include downhole communications in a drilling operation. Such downhole communications can include a telemetry system. 
     The logging system  700  can also include a bus  727  to provide common electrical signal paths between the components of the logging system  700 . The bus  727  can include an address bus, a data bus, and a control bus, each independently configured. The bus  727  can also use common conductive lines for providing one or more of address, data, or control, the use of which can be regulated by the controller  725 . 
     The bus  727  can include instrumentation for a communication network. The bus  727  can be configured such that the components of the logging system  700  are distributed. Such distribution can be arranged between downhole components such as the apparatus shown in  FIGS. 1-4, and 7 , and components that can be disposed on the surface of a well. Alternatively, several of these components can be co-located, such as on one or more collars of a drill string or on a wireline structure. 
     In various embodiments, the logging system  700  includes peripheral devices that can include displays  755 , additional storage memory, or other control devices that may operate in conjunction with the controller  725  or the processing unit  702 . The display  755  can display measurements, calculated data, and diagnostic information based on the signals generated according to embodiments described above. The display  755  can also be used to display one or more maps  448  of acoustic intensity, and/or time history graphs  500 ,  600 , similar to or identical to what is illustrated in  FIGS. 4-6 . 
     In an embodiment, the controller  725  can be fabricated to include one or more processors. The display  755  can be fabricated or programmed to operate with instructions stored in the processing unit  702  (for example in the memory  706 ) to implement a user interface to manage the operation of components distributed within the logging system  700 . This type of user interface can be operated in conjunction with the communications unit  740  and the bus  727 . Various components of the logging system  700  can be integrated into the associated housing (e.g., the tool  230 ) such that processing identical to or similar to the methods discussed with respect to various embodiments herein can be performed downhole. 
     In various embodiments, a non-transitory machine-readable storage device can include instructions stored thereon, which, when performed by a machine, cause the machine to become a customized, particular machine that performs operations comprising one or more activities similar to or identical to those described with respect to the methods and techniques described herein. A machine-readable storage device, herein, is a physical device that stores information (e.g., instructions, data), which when stored, alters the physical structure of the device. Examples of machine-readable storage devices include, but are not limited to, memory  706  in the form of read only memory (ROM), random access memory (RAM), a magnetic disk storage device, an optical storage device, a flash memory, and other electronic, magnetic, or optical memory devices, including combinations thereof. 
     The physical structure of stored instructions may thus be operated on by one or more processors such as, for example, the processing unit  702 . Operating on these physical structures can cause the machine to engage in activities according to methods described herein. Operating on the instructions can cause the processing unit  702  to store associated data or other data in the memory  706 . The memory  706  can store the results of measurements of formation parameters or parameters of the system  700 , to include gain parameters, calibration constants, acoustic source identification data, acoustic source location data, etc. The memory  706  can store a log of the acoustic pressure measurements and acoustic intensity determined by the system  700 . The memory  706  therefore may include a database, for example a relational database. 
       FIG. 8  is a flow diagram illustrating methods  811  of acoustic source identification, according to various embodiments of the invention. The methods  811  described herein are with reference to hardware circuitry, reception, measurements, and calculation, etc. shown in  FIGS. 1-7 . Some operations of the methods  811  can be performed in whole or in part by the processing unit  702  or controller  725  ( FIG. 7 ), although many embodiments are not limited thereto. 
     In some embodiments, a method  811  comprises measuring acoustic radiation at block  829 , and then locating the source at block  831 . For the purposes of this document, a device or sensor that is located “in a geological formation” means the device/sensor is disposed at some location having coordinates within the geological formation, whether the location is inside a borehole, or outside of the borehole—within the physical structure of the Earth. Many variations may thus be realized. 
     For example, the method  811  may be carried out as part of a drilling operation, or a wireline logging operation. Thus, in some embodiments a method  811  may begin at block  821  with measuring background noise using one or more sensors in an array of sensors. In this way, noise that is germane to the measuring apparatus (e.g., a downhole tool) can be measured, to reduce or eliminate the effect of this noise on borehole measurements, perhaps through a subtractive process. 
     Neighboring (directly adjacent) sensors can be used to make measurements for higher-frequency, or fine resolution imaging. Non-neighboring sensors (i.e., sensors separated from each other along a substantially straight line, or along an azimuthal arc, by at least one other sensor) can be used to make measurements for lower-frequency, or coarse resolution imaging. Thus, in some embodiments, the method  811  may comprise, at block  825 , selecting sensors in an array as one pair of multiple pairs of neighboring sensors in the array. In some embodiments, the activity at block  825  may comprise selecting sensors in the array as one pair of multiple pairs of non-neighboring sensors in the array. 
     The method  811  may continue on to block  829  to include substantially simultaneously measuring acoustic radiation from a first source in a geological formation, at each of at least two sensors in an array, over a selected time period to provide first acoustic pressure measurements. For the purposes of this document, “substantially simultaneously” means that the acoustic radiation data acquired as part of this activity overlaps in the time domain, such that some portion of the waveforms that can be constructed from the data are seen to exist at the same time. 
     Measurements of acoustic radiation can be made in multiple directions, to provide an image. Thus, the activity of substantially simultaneously measuring the acoustic radiation at block  829  may comprise substantially simultaneously measuring acoustic radiation from the first source at each of at least three sensors in an array, over the selected time period and along at least two substantially orthogonal axes (e.g., the vertical axis z, and azimuthal axis θ), to provide the first acoustic pressure measurements. 
     The method  811  may continue on to block  831  to include locating the first source at a location in the geological formation with respect to the sensors by determining acoustic intensity associated with the first source. The determined acoustic intensity can be based on the first acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors and the first source. 
     Acoustic intensity can be determined as a function of acoustic pressure measurements, and the acoustic particle velocity. Thus, the acoustic intensity associated with the first source can be determined as a function of an average of the first acoustic pressure measurements over the selected time period, and an acoustic particle velocity. 
     A variety of information that results from making acoustic radiation measurements can be recorded in a log. Thus, the method  811  may comprise, at block  833 , logging the first acoustic pressure measurements or the acoustic intensity at a variety of depths in a borehole in the geological formation. 
     The array of sensors can be moved longitudinally within a borehole, to make multiple sets of measurements. Thus, the method  811  may continue on to block  837  to include moving the array within a borehole, from a first elevation to a second elevation, wherein the first acoustic pressure measurements are made at the first elevation. 
     The activities of at blocks  829  and  831  may be repeated. Indeed, any one or more of the activities shown in the method  811  may be repeated as desired. Thus, the method  811  may further include repeating the measuring (at block  829 ) and the locating (at block  831 ) at the second elevation to obtain second acoustic pressure measurements, and so on. 
     Other sources of extraneous acoustic emission can be located in the same way as the first source. Thus, the method  811  may comprise locating these other sources at block  839 , using data acquired in any number of locations in the borehole, perhaps using a map similar to or identical to the map  448  shown in  FIG. 4 . 
     Multiple sources can be ranked against each other, based on relative acoustic signal strength, distance, or direction. Thus, the method  811  may comprise, at block  841 , ranking multiple sources, including the first source, based on relative acoustic signal strength between the first source and other sources included in the multiple sources. 
     As noted previously, acoustic sources can be mapped. Mapping can occur with respect to location, acoustic pressure, acoustic velocity, acoustic intensity, or acoustic power versus frequency, among other source properties. Colors in the map can be used to indicate the relative magnitude of property values. Therefore some embodiments of the method  811  comprise, at block  845 , displaying a color map of at least one of the location or the acoustic intensity in the geological formation in a visible format. 
     Multiple sets of measurements can be made to better define the initially-determined location of the source, perhaps as an average of two or more determined locations in space. Thus, the method  811 , as part of the activity at block  831 , or as a completely separate activity at block  849 , may comprise identifying a revised version of the location and acoustic strength of the first source, based on the first acoustic pressure measurements and the acoustic pressure measurements acquired at second or subsequent locations. 
     It should be noted that the methods described herein do not have to be executed in the order described, or in any particular order, unless explicitly specified as such. Moreover, various activities described with respect to the methods identified herein can be executed in iterative, serial, or parallel fashion. Information, including parameters, commands, operands, and other data, can be sent and received in the form of one or more carrier waves. 
     Upon reading and comprehending the content of this disclosure, one of ordinary skill in the art will understand the manner in which a software program can be launched from a computer-readable medium in a computer-based system to execute the functions defined in the software program. One of ordinary skill in the art will further understand the various programming languages that may be employed to create one or more software programs designed to implement and perform the methods disclosed herein. For example, the programs may be structured in an object-orientated format using an object-oriented language such as Java or C#. In another example, the programs can be structured in a procedure-orientated format using a procedural language, such as assembly or C. The software components may communicate using any of a number of mechanisms well known to those of ordinary skill in the art, such as application program interfaces or interprocess communication techniques, including remote procedure calls. The teachings of various embodiments are not limited to any particular programming language or environment. Thus, other embodiments may be realized. 
     For example,  FIG. 9  depicts an example wireline system  964 , according to various embodiments of the invention.  FIG. 10  depicts an example drilling rig system  1064 , according to various embodiments of the invention. Either of the systems in  FIG. 9  and  FIG. 10  are operable to control an acoustic measurement apparatus, such as an array  200  attached to a tool  230  (see  FIG. 4 ), to conduct measurements in a wellbore, and to locate acoustic sources. Thus, the systems  964 ,  1064  may comprise portions of a wireline logging tool body  970  (e.g., as a more particular implementation of a tool  230 ) as part of a wireline logging operation, or of a downhole tool  1024  (e.g., a drilling operations tool) as part of a downhole drilling operation. The downhole tool  1024  may be similar to or identical to the tool  230  described in other figures in more detail. 
     Returning now to  FIG. 9 , a well during wireline logging operations can be seen. In this case, a drilling platform  986  is equipped with a derrick  988  that supports a hoist  990 . 
     Drilling oil and gas wells is commonly carried out using a string of drill pipes connected together so as to form a drilling string that is lowered through a rotary table  910  into a wellbore or borehole  912 . Here it is assumed that the drilling string has been temporarily removed from the borehole  912  to allow a wireline logging tool body  970 , such as a probe or sonde, to be lowered by wireline or logging cable  974  into the borehole  912 . Typically, the wireline logging tool body  970  is lowered to the bottom of the region of interest and subsequently pulled upward at a substantially constant speed. 
     During the upward trip, at a series of depths the instruments (e.g., the apparatus and systems shown in  FIGS. 2-4, and 7 ) included in the tool body  970  may be used to perform measurements on the subsurface geological formations adjacent the borehole  912  (and the tool body  970 , which can serve as a housing for various components of the system  700 ). The measurement data can be communicated to a surface logging facility  992  for storage, processing, and analysis. The logging facility  992  may be provided with electronic equipment for various types of signal processing, which may be implemented by any one or more of the components of the system  700 . Similar formation evaluation and downhole condition data may be gathered and analyzed during drilling operations (e.g., during LWD operations, and by extension, sampling while drilling). 
     In some embodiments, the tool body  970  comprises a system  700  for obtaining and analyzing acoustic radiation measurements in a borehole  912 . The tool is suspended in the wellbore by a wireline cable  974  that connects the tool to a surface control unit (e.g., comprising a workstation  954 , which can also include a display). The tool may be deployed in the borehole  912  on coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable deployment technique. 
     Turning now to  FIG. 10 , it can be seen how a system  1064  may also form a portion of a drilling rig  1002  located at the surface  1004  of a well  1006 . The drilling rig  1002  may provide support for a drill string  1008 . The drill string  1008  may operate to penetrate the rotary table  910  for drilling the borehole  912  through the subsurface formations  914 . The drill string  1008  may include a Kelly  1016 , drill pipe  1018 , and a bottom hole assembly  1020 , perhaps located at the lower portion of the drill pipe  1018 . 
     The bottom hole assembly  1020  may include drill collars  1022 , a downhole tool  1024 , and a drill bit  1026 . The drill bit  1026  may operate to create the borehole  912  by penetrating the surface  1004  and the subsurface formations  1014 . The downhole tool  1024  may comprise any of a number of different types of tools including MWD tools, LWD tools, and others. 
     During drilling operations, the drill string  1008  (perhaps including the Kelly  1016 , the drill pipe  1018 , and the bottom hole assembly  1020 ) may be rotated by the rotary table  910 . Although not shown, in addition to, or alternatively, the bottom hole assembly  1020  may also be rotated by a motor (e.g., a mud motor) that is located downhole. The drill collars  1022  may be used to add weight to the drill bit  1026 . The drill collars  1022  may also operate to stiffen the bottom hole assembly  1020 , allowing the bottom hole assembly  1020  to transfer the added weight to the drill bit  1026 , and in turn, to assist the drill bit  1026  in penetrating the surface  1004  and subsurface formations  1014 . 
     During drilling operations, a mud pump  1032  may pump drilling fluid (sometimes known by those of ordinary skill in the art as “drilling mud”) from a mud pit  1034  through a hose  1036  into the drill pipe  1018  and down to the drill bit  1026 . The drilling fluid can flow out from the drill bit  1026  and be returned to the surface  1004  through an annular area  1040  between the drill pipe  1018  and the sides of the borehole  912 . The drilling fluid may then be returned to the mud pit  1034 , where such fluid is filtered. In some embodiments, the drilling fluid can be used to cool the drill bit  1026 , as well as to provide lubrication for the drill bit  1026  during drilling operations. Additionally, the drilling fluid may be used to remove subsurface formation cuttings created by operating the drill bit  1026 . 
     Thus, it may be seen that in some embodiments, the systems  964 ,  1064  may include a drill collar  1022 , a downhole tool  1024 , and/or a wireline logging tool body  970  to house one or more components of the system  700  in  FIG. 7 , which may also be attached to or housed by the tool  1024  or the tool body  970 , to be constructed and operated as described previously. 
     Thus, for the purposes of this document, the term “housing” may include any one or more of a tool  230 , a drill collar  1022 , a downhole tool  1024 , or a wireline logging tool body  970 , all having an outer wall that is shared among a number of components. The housing can be used to enclose or attach to magnetometers, sensors, fluid sampling devices, pressure measurement devices, transceiver antennae, transmitters, receivers, acquisition and processing logic, and data acquisition systems. The tool  1024  may comprise a downhole tool, such as an LWD tool or MWD tool. The wireline tool body  970  may comprise a wireline logging tool, including a probe or sonde, for example, coupled to a logging cable  974 . Many embodiments may thus be realized. 
     For example, referring now to  FIGS. 1-10 , it can be seen that an apparatus may comprise two sensors  220  and a location processor, perhaps taking the form of a processing unit  702 . In some embodiments, the apparatus may comprise at least two sensors  220  forming a first portion of an array  200 , with a location processor (e.g., processing unit  702 ) coupled to the at least two sensors. The processing unit may be operative to substantially simultaneously measure acoustic radiation from a source in a geological formation in which the array  200  is disposed, at the at least two sensors  220 , over a selected time period to provide acoustic pressure measurements. The processing unit may further operate to locate the source at a location in the geological formation with respect to the sensors  220  by determining acoustic intensity associated with the source, based on the acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors  220  and the source. 
     A linear, planar, or cylindrical array of sensors can be formed. Thus, the apparatus may comprise additional sensors (that is, in addition to the two recited previously) forming a second portion of the array and arranged longitudinally along a housing. A linear array, or single line of a cylindrical array would comprise two possible implementations of this configuration. 
     A circumferential arrangement of linear sensor arrays can also be formed. Thus, the apparatus might include additional sensors forming a second portion of the array and arranged azimuthally around a cylindrical housing. The cylindrical housing may comprise one of a wireline tool housing, or a downhole tool housing coupled to a drill string, among others. Still more embodiments may be realized. 
     For example, referring to  FIGS. 4, 7, and 9-10 , it can be seen that a system  700 ,  964 ,  1064  may comprise a downhole tool housing having a longitudinal axis; and sensors, such as hydrophones, attached to the housing, to operate as described previously. Thus, a system  700 ,  964 ,  1064  may include a display  755  to illustrate the location and intensity of acoustic sources. 
     In some embodiments, a system  700 ,  964 ,  1064  comprises hydrophone sensors  220  forming a cylindrical array  200  and a location processor (e.g., a processing unit  702 ) coupled to the sensors, which is operative to substantially simultaneously measure acoustic radiation from a source in a geological formation in which the array is disposed, at the sensors, over a selected time period to provide acoustic pressure measurements, and to locate the source at a location in the geological formation with respect to the sensors by determining acoustic intensity associated with the source, based on the acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors and the source. The system  700 ,  964 ,  1064  may further comprise a display  755  to display a map  448  of the acoustic intensity and the location. 
     The sensors can be spaced approximately equidistant from each other, along one or more axes. Thus, the sensors  220  may be disposed in a central portion of the array  200  spaced approximately equally apart from each other along at least two axes (e.g., the z and U axes). 
     A multiplexer can be used to selectively switch sensors in and out of the measurement configuration. Thus, the system  700 ,  964 ,  1064  may comprise a multiplexer  704  to enable selecting non-neighboring ones of the sensors  220  to obtain acoustic pressure measurements. 
     Leakage into the borehole can be detected by the processor, resulting in an alarm to initiate corrective action. Thus, the system  700 ,  964 ,  1064  may comprise an alarm  760  coupled to the location processor (e.g., processing unit  702 ) to indicate fluid leakage into a borehole in the geological formation. 
     The system may include a memory to log measurements, and the calculated results of measurements. Thus, the system  700 ,  964 ,  1064  may comprise a memory  706  coupled to the location processor (e.g., the processing unit  702 ), wherein the memory  706  is used to store a log of the acoustic intensity corresponding to the location of the source(s). 
     Any of the above components, for example the elements of the systems  700 ,  964 ,  1064 , may all be characterized as “modules” herein. Such modules may include hardware circuitry, and/or a processor and/or memory circuits, software program modules and objects, and/or firmware, and combinations thereof, as desired by the architect of the systems  700 ,  964 ,  1064  and as appropriate for particular implementations of various embodiments. For example, in some embodiments, such modules may be included in an apparatus and/or system operation simulation package, such as a software electrical signal simulation package, an acoustic signal propagation package, a power usage and distribution simulation package, a power/heat dissipation simulation package, a measured radiation simulation package, and/or a combination of software and hardware used to simulate the operation of various potential embodiments. 
     It should also be understood that the apparatus and systems of various embodiments can be used in applications other than for logging operations, and thus, various embodiments are not to be so limited. The illustrations of the systems  700 ,  964 ,  1064  are intended to provide a general understanding of the structure of various embodiments, and they are not intended to serve as a complete description of all the elements and features of apparatus and systems that might make use of the structures described herein. 
     In summary, using the apparatus, systems, and methods disclosed herein may utilize the principle of acoustic intensity to locate and identify downhole acoustic sources, which are extraneous to the tool. The capability to determine the axial and azimuthal location of a downhole acoustic source is novel and unique to the field, allowing operators to direct their maintenance efforts more economically and effectively when addressing well leaks. These advantages can significantly enhance the value of the services provided by an operation/exploration company, helping to reduce costs and increase customer satisfaction. A list of additional possible embodiments is now presented. 
     In some embodiments, an apparatus comprises sensors forming an array, and one or more location processors coupled to the sensors. The location processors are operative to substantially simultaneously measure acoustic radiation from a source in a geological formation in which the array is disposed, at the sensors, over a selected time period. The processors are thus operative to provide acoustic pressure measurements, and to locate the source at a location in the geological formation with respect to the sensors by determining acoustic intensity associated with the source, based on the acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors and the source. 
     The array may be divided into portions, and sensors may be arranged within the portions linearly (e.g., longitudinally along a housing, or azimuthally around the housing), in a plane, or as a cylinder. The sensors may be disposed approximately equally spaced apart from each other, at any portion in the array, along one or more axes. The array may be attached to a housing, such as a wireline tool housing, or a downhole tool housing coupled to a drill string. 
     In some embodiments, a system comprises sensors, including hydrophones, forming an array, such as a linear, planar, or cylindrical array. The system may further comprise a location processor coupled to the sensors, which is operative to substantially simultaneously measure acoustic radiation from a source in a geological formation in which the array is disposed, at the sensors, over a selected time period to provide acoustic pressure measurements. The location processor may further operate to locate the source at a location in the geological formation with respect to the sensors by determining acoustic intensity associated with the source, based on the acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors and the source. 
     The system may comprise one or more displays to display acquired data, calculated data, and one or more maps of the acoustic intensity and the location of various sources that have been located. One or more memories may be coupled to the location processor, and used to store a log of the acoustic intensity and other properties corresponding to the location of the source(s). 
     The system may comprise receivers and multiplexers coupled to the sensors, perhaps to enable the selection of neighboring and non-neighboring ones of the sensors to obtain the acoustic pressure measurements. Selection can be based on resolution requirements, storage limitations, or telemetry limitations, among others. 
     Alarms may be activated to warn of various conditions downhole. Thus, one or more alarms may be coupled to the location processor to indicate fluid leakage into a borehole in the geological formation. 
     Methods may comprise a wide variety of activities, including substantially simultaneously measuring acoustic radiation from a first source in a geological formation, at each of at least two sensors in an array, over a selected time period to provide first acoustic pressure measurements. Further activities may comprise locating the first source at a location in the geological formation with respect to the sensors by determining acoustic intensity associated with the first source, based on the first acoustic pressure measurements, independent of medium sound velocity in one or more media disposed between the sensors and the first source. 
     In some embodiments, methods include ranking multiple sources, including the first source, based on relative acoustic signal strength between the first source and other sources included in the multiple sources, as well as displaying a map, including a grayscale or color map of the acoustic intensity, or some other source property, with respect to the location in a geological formation in a visible format. 
     The acoustic intensity associated with the first source may be determined as a function of an average of the first acoustic pressure measurements over the selected time period, and an acoustic particle velocity. Substantially simultaneously measuring the acoustic radiation may include substantially simultaneously measuring acoustic radiation from the first source at each of at least three sensors in the array, over the selected time period and along at least two substantially orthogonal axes, to provide the first acoustic pressure measurements. 
     In some embodiments, methods include moving the array within a borehole, from a first elevation to a second elevation, wherein the first acoustic pressure measurements are made at the first elevation; and repeating the activities of measuring and the locating at the second elevation to obtain second acoustic pressure measurements. In this way, the activity of locating may comprise identifying a revised version of the location and acoustic strength of the first source based on the first acoustic pressure measurements and the second acoustic pressure measurements. 
     In some embodiments, activities include measuring background noise using at least one of the at least two sensors in the array, perhaps prior to making measurements. In addition, perhaps also prior to making measurements, sensors in an array may be selected as individual pairs, out of multiple pairs of neighboring sensors in the array. Similarly, sensors may be selected as individual pairs, out of multiple pairs of non-neighboring sensors in the array. In each case, the pairs of sensors may be selected to make acoustic radiation measurements, and locate acoustic emission sources. 
     Any data acquired may be logged. Thus, some embodiments include the activity of logging acoustic pressure measurements, acoustic intensity, and other information at a variety of depths in a borehole in the geological formation. Many more embodiments may be realized, but have not been explicitly listed here in the interest of brevity. 
     The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     In the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.