US3193004A

US3193004A - Method for determining the position and rate of advance of a displacement front in asecondary recovery system for producing petroleum

Info

Publication number: US3193004A
Application number: US121464A
Authority: US
Inventors: James C Albright; Earl W Sutton
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1961-07-03
Filing date: 1961-07-03
Publication date: 1965-07-06
Anticipated expiration: 1982-07-06

Description

July 6, 1 J. c. ALBRIGHT ETAL ,1 93,004 VANCE ERY METHOD FOR DETERMINING THE POSITION AND RATE OF AD OF A DISPLACEMENT FRONT IN A SECONDARY RECOV SYSTEM FOR PRODUCING PETROLEUM.

2 Sheets-Sheet 1 Filed July 5. 1961 JAMES C. ALBR/GHT EARL W SUTTON ATTORNEY July 6, 1965 J. c. ALBRIGHT ETAL 3,193,004

G THE POSITION AND RATE OF AD METHOD FOR DETERMININ VANCE OF A DISPLACEMENT FRONT IN A SECONDARY RECOVERY SYSTEM FOR PRODUCING PETROLEUM Filed July 5. 1961 2 Sheets-Sheet 2 A VELOCITY METHOD 26 REFLECTION METHOD TIME R E D R o C E R O ABCDE TIME-- A VELOCITY METHOD m WN Rm 8 TLW mu WS L 5 MR AA a E F Y B 2 6 2 ||||2 6 /AV1I ulIIII VII 4 &\ VIl IIIlI-O 9 6 W A W 0 a O\ 5 w\ w.\ M m a a a A TTORNE Y United States Patent METHGD FQR DETERMINING Tl-E POSITION AND RATE OF ADVANCE (ll A DISPLACEMENT FRONT IN A SECONDARY RECOVERY SYSTEM FOR PRODUtIlNG PETROLEUM James C. Albright and Earl W. Sutton, Ponca City, Okla, assignors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware Filed July 3, 1961, Ser. No. 121,464 3 Claims. (Ql. 166-4) This invention relates to methods for recovering oil from an oil-bearing formation in the earth after it has become economically unfeasible to produce the oil by ordinary primary production techniques. More particularly, but not by way of limitation, the present invention relates to a method and apparatus for determining the location and rate of advance of a displacement front formed ahead of a displacement fluid injected into an oil-bearing formation for the purpose of displacing the oil from the point of injection toward one or more producing Wells.

The sharp increase in the costs of geophysical exploration and in the production of naturally occurring deposits of liquid fuel in recent years has given rise to a keen reappraisal of secondary recovery techniques. Such procedures as repressuring the formation with an injected gas, flooding the formation with water and other liquids, and, more recently, the process of promoting and sustaining combustion along a moving front in the formation have been used effectively to economically recover residual oil in formations which cannot be produced by direct or primary production methods. In all of the methods of secondary recovery depending upon the introduction of some type of displacement fluid to the formation for the purpose of displacing the oil contained therein toward a producing well, there is a well recognized need for production personnel to be constantly acquainted with the position of the displacement front in the oil-bearing formation, and its rate of advance in the formation. If the position and rate of movement of the displacement front can be accurately known at all times, then other factors in the procedure of secondary recovery can be more closely controlled to improve the efliciency of the production. Thus, the rate and res sure at which the displacement fluid is injected into the formation, as well as the direction of injection, are, in large part, determined by the manner in which the displacement front moves through the formation from the injection well toward the producing wells. It is also important to know the area encompassed by the moving front as it moves outwardly in all directions from the injection well.

Several procedures have been utilized in the past for determining with varying degrees of accuracy the position and rate of propagation of the displacement front in a secondary oil recovery system. One of the earliest and most elementary of the procedures utilized is that of conducting laboratory experiments to determine the approximate speed at which the injected fiuid will move through the formation under certain pressures, and then estimating the position of the front in the oil-bearing formation on the basis of the time which has elapsed since commencing the injection, and the pressure and volume of injection which have been utilized.

A method which has been more recently proposed consists in transmitting an electromagnetic wave pulse from the injection well into the oil-bearing formation in the direction of the displacement front. The pulses so transmitted are reflected from the displacement front, which occurs in the form of a vertically extending interface between the displacement fluid and the oil which is being 3,l93,il4 Patented July 6, 1965 driven before it towards the production wells. The reflected signals are then received by a suitable electro magnetic wave receiver which is also located in the injection well. The travel time of the electromagnetic Wave is, of course, indicative of the distance from the injection well to the displacement front. The electromagnetic wave reflection technique has been applied to secondary recovery systems employing in-situ combustion in which a combustion front is moved through the formation from an injection well towards a producing well, and also to secondary recovery methods utilizing superheated steam as the displacement fluid.

Although the electromagnetic pulse technique works well enough in in-situ combustion and superheated steam recovery processes, it does not operate well under a frequently occurring condition in which the earth strata contains a large quantity of water. The method is also beset by difficulties arising from the substantial absorption of such pulses by the earth tending to greatly weaken their intensity prior to their arrival at the ultimate point of reception. Finally, in such reflection techniques, the difference in the characteristics of the formation on opposite sides of the vertical interface formed by the moving displacement front is frequently not suficiently pronounced to provide a strong reflected signal of sufficient amplitude to permit it to be easily distinguished from noise picked up by the receiver.

The present invention comprises a novel method and apparatus for ascertaining the location and rate of advance of a displacement front produced by continuously injecting a displacement fluid into an oil-bearing formation during a secondary recovery operation. The method is dependent upon the production of a sonic wave, and the observation of the time required for the Wave to travel through the oil-bearing formation, contacting, as it does so, the vertical interface between the displace ment fluid and the oil at the displacement'front. In a preferred embodiment of the invention, the sonic wave is originated at either a production well or the injection well and is transmitted through the formation to a listening station disposed in one of the wells located on the opposite side of the moving displacement front. The sonic wave is thus transmitted through the displacement front, and in the course of such transmission undergoes a change in its velocity as it passes from the portion of the formation which is saturated with the displacement fluid int-o the oil-bearing portion of the formation.

In a modified embodiment of the invention, the sonic Wave is originated at one of the wells and is transmitted in the direction of the displacement front. The Wave, upon contacting the vertically extending interface at the displacement front, is reflected back to the well of its origin and is there received by suitable sound receiving apparatus. The time required for the sonic wave to traverse the distance between the well from which it is transmitted and the displacement front, and to then return to the transmitting well, is, of course, a measure of the distance of the displacement front from the transmitting well.

From the methods of the present invention as thus far described, it will now be appreciated that the rate of movement of the displacement front may be determined by transmitting a series of sonic pulses separated by intervals of silence of known duration, determining the dis tance between the well from which the pulses are transmitted and the displacement front at the time of each transmission, and determining from this data the distance which the displacement front has moved during the time interval between pulse transmissions.

Since, under some conditions of operation, it may be desirable to utilize both the sonic reflection and straightthrough transmission techniques, the invention also resides apparatus utilized in performing such combined reflection and straight-through methods broadly comprises a sound transmitting device and sound receiving device suspended in at least one of the wells utilized in the secondary recovery project, and also a sound transmitting and sound receiving device located in a second well of the project, which second well is horizontally spaced from the fwell containing said first mentioned sound transmitting, and receiving means, and with said Wells being disposed on opposite sides of the moving displacement front. v

It is a major object of the present invention to provide a method and apparatus fordetermining the position and rate of advance of a displacement front of the type occurring in secondary recovery of petroleum utilizing an in: jected displacement fluid. l V

Another objects of the present invention is to provide a method of determining the position of a displacement front extending 360 around an injection well so that the configuration of the front and the area included within the front may be accurately known. V j

A further object of the present invention is-to provide a system for determining the positionand rate ofadvance of a displacement front in the secondary recovery of ,oil from an oil-bearing formation, which system may be incorporated in, and utilized with, existing secondary recovery wel-l systems. 7 V V An additional object of the present invention is to provide an apparatus for sonically determining the position,

of a displacement front in an oil-bearing formation, which apparatus is simple and relatively inexpensive in construction, easy to operate, and characterized by along and trouble-free operating life.

These objects and advantages of the present invention will be better understood, and other useful and advantageous features of the invention wil'l'become manifest, upon readingthe following disclosure in conjunction with a perusal of the accompanying drawings which illustrate my invention. I e

In the drawings: 1

FIGURE 1 is a schematic drawing illustrating 'a sec-. ondary oil recovery system in which the present invention is incorporated.

FIGURE 2 is a plot constructed from data obtained using the method of the presentinvention. The plot illus-' trates the advance of thedisplacement front from the injection well toward the production wells with sage of time. a

7 FIGURE 3. schematicallyillustrates a plurality of rethe. pas-,

ment fluid .is contained in areservoir 17 The displacement fluid is pumped from the reservoir 17 'by pump 18 into a the bore of 'the injection well and is injected into the oil-bearing formation 16from the injection well. As the displacement fluid' moves radiallyoutward in the oil-beara One 'or more production wells 26 extending from the surface 14 into the oil-bearing formation 16 and horizontally' spaced some. distance from the injection well 10 are provided in-accordance with secondary recovery procedureswell understood in the art. It will furtheribe un derstood that, though but a single production well 26 is illustratedin FIG. 1,'the common practice in secondary IBCOVEIYpI'OCdHI'ESflSdQ provide a plurality of production wells in horizontally spaced relation from a centrally located injection well so that as theoil is displaced radially through .360 from the, injection wellyit may be simultajr neously produced at a number of producing well locations. As. has been previously explained, in order to control the efficiency of the production from such wells, it is highly desirable that: operating personnel be appraised of, the. position and progress of the displacement front throughthe oil-bearing formation 1 6. Thepresentinventionpermits the position and rate of advance of the displacement front20 to be constantly known, thereby Vastly improving the efficiency with which such secondary recovery operations may be conducted.

Referring again to FIG. '1, a suitable sound" transmit- 1 ting device 28 and sound receiving device 30 are sus-' pende'd'inthe injection well 10 by "means of a suitable cable 32.,. f-Oneor, more-of the productionwells 26 may also eontain-suchsound transmitting and receiving devices. The. soundtransmitting and receiving devices 28 p, and 30, respectively, .are positioned in the bores of the wells inhorizontal alignment with the oil-producing formation 16 and are of thetype in which an electrical signal isconverted to a sonic signal and vice versa. In the preferred embodiment illustrated in'FIG. l, a suitable recording instrument, 34 is electrically connected to the transmitter'2 8 and. receiver 30 disposed in the injection well '10, and to, the receiver 30 located in production Well 26. Thus, the, travel time of sonic waves 36 transmittedby the transmitter 28 in the injection well 10and "reflected by. the displacement front 20 to receiver 30 in the'i njectionwell may be constantly recorded, as may the travel time of sonic, waves 38'which are transmitted from the injectionwell transmitter 2-8 through the interface of corder chart traces as they appear'when the method of.

transmitting sound waves between wells located on opposite sides of the displacement frontis utilized.

FIGURE 4 schematically illustrates a plurality of recorder chart traces as they appear when'the reflected sound wave method of front location is employed.

FIGURE 5 is a schematic illustrationof a secondary recovery system in which a multiple trace recording instrument is utilized to simultaneously record the travel times of sound waves traveling from an injection well to a plurality of surrounding production wells. 7 1

FIGURE 6 illustrates a multiple trace recorder chart upon which a plurality of traces have been recorded in correspondence to the travel times of sound waves transmitted from the injection well through a displacement front of the configuration shown in FIG. 5 to severalsproduction Wells located as shown in FIG. 5.

Referring now to the drawings in detail, and particularly to FIG. 1, reference'character 10 designates'an injection well extending from the, surface 14 of the earth into an oil-bearing formation designated generally by ref-- .erence character 16. At the surface 14 of the ground adjacent the injection well 10, a supply of a suitable displacethe production well 2.6, s

thedisplacement front 20 to the receiver 30 located in l A second recording instrument40 is electrically connected to the transmitter 28' and receiver 30 located in the production well 26., The. recording instrument 40 may thus be utilized to record the travel timeof sonic back to thereceivers3 0 of the production well.

duction well 26 to the displacement front 20 and reflected From the arrangement of the. apparatus depicted in FIG. 1, it will be apparent thatfollowing transmission of a sound from either of the transmitters 28 located in either the injection Well'10 .or the production we1l 26, a sound wave is refiectedfrom the casing of the respective Well and travels, for all practical purposes, directly to the receiverlocated in thesame well as that from. which the sound is transmittedf. The. time required for these direct Sound Waves, designated by reference character 44 in FIG. 1, to travel between transmitter and receiver is very small comparedto' the'travel time of the sound waves 36, 38

and i2 which are transmitted to and through the disbe suitably connected to each of the transmitters 28 and receivers 30 to record the travel times of the several sonic waves transmitted and received in the manner described above.

The manner in which the recorded times of sonic signals may be utilized in practicing the present invention is best illustrated in FIGS. 2 through 5. FIG. 3 depicts a plurality of recorder chart traces, A through E, each ofwhich indicates the time interval elapsing between the instant of transmission of a sonic signal 38 from the injection well and the instant of its reception at the producing well 26-hence, the time required for the sonic wave to travel through the oil-bearing formation 16 between the two wells. The direct path of travel 44 between the transmitter 28 and receiver 30 located in the injection well 10 is relatively short compared to the distance between the injection and producing wells, so that the pip 50, designating the initial pulse pip, produced on the chart 52 by the reception of the directly transmitted signal by the receiver 30 in theinjection well 10 may be taken as a reference point corresponding to the time in which the signal is transmitted. When the sound wave 38 is received by the receiver 30 in the production well 26, an electrical signal activates the recording instrument 34 to produce a second pip 54 on the chart. The second pip 54 will be spaced from the initial pulse pip by a distance which is directly proportional to the time required for the sound wave to travel through the oil-bearing formation 16 between the injection well 10 and the producing Well 26.

Referring now to FIG. 3, the chart trace at the top of'the figure represents the indicia which are recorded on the chart 52 of the recording instrument 34 when a sound wave is transmitted between the injection well 10 and the producing well 26 prior to injection of a displacement fluid into the formation 16. The space separating the initial pulse pip 50 from the received signal pip 54 corresponds to the travel time of a sound wave 38 through the untreated formation. After the displacement medium is injected into the formation, the travel time of a sound wave 38 between the injection and displacement wells will decrease if the velocity of sound is greater in the fluid which is injected than it is in the oil-bearing portion 24 of the formation 16 ahead of the displacement front. Conversely, if the velocity of the sound wave in the injected fluid is less than its velocity in the oil saturated portion 24 of the formation 16, the travel time of the sound between injection well 10 and the production well 26 will be increased.

FIG. 3 illustrates the pattern of the chart traces when water is used as an injection fluid. As the displacement front moves outwardly from the injection well 10 toward the surrounding production wells, the travel times of sound waves 38 progressively decrease by virtue of their greater velocity in the flooded portion 22 of the formation 16 than in the unfloodezl portion 24 ahead of the front 20. Thus, on the trace marked A, the

distance separating pips

50 and 54 on the chart 52 is less than the distance which separates these pips on the uppermost trace which represents the travel time of the sound wave 38 prior to the injection of the water. Trace A is recorded a known time interval, which may also be designated A,

after the water is injected. After time interval B has elapsed following the recordation of trace A, another sonic wave 38 is transmitted from the transmitter 28% the injection well 10 and is received at the receiver 30 in the production well 26. The travel time of this sound wave is recorded on trace B of FIG. 3, and is less than the travel time recorded on trace-A, since, in the time interval B, the injection fluid has advanced further toward the production well 26. The procedure of transmitting, receiving and recording the travel times of sound waves transmitted intermittently after known time intervals is repeated to yield traces C, D and E of FIG. 3. The advance of the displacement front with the passage of time 6 is reflected in the decreasing travel times of the sonic waves as indicated by traces A through B.

After the travel time of the sound wave 38 through the formation 16 at any given time after injection is determined from the recorder chart 52, the distance of the displacement front 20 from the injection well 10 at that time may then be determined if the velocity of the sound in both the treated and

untreated portions

22 and 24, respectively, of the formation is known. The latter parameters may be determined in any suitable manner, such as from tables or from measurement at the location of the secondary recovery operation. For example, the uppermost trace in FIG. 3 may be used to obtain the travel time between injection well 10 and production well 26 before the displacement fluid is injected into the formation 16, and since the distance between these wells is known, the velocity of the sound in the untreated portion 24 of the formation 16 may be calculated. The velocity of sound in the displacement fluid may also be measured. With total travel time between transmitting and receiving wells known, as well as the velocity of the sound in the formation on opposite sides of the displacement front 20, the distance of the displacement front from the injection well may be calculated by use of the equation:

D is the distance from the injection well to the displacement front;

T is the total travel time of the sound Wave from the injection well to the production well;

S is the distance from the injection well to the production Well;

V is the velocity of sound in the oil saturated portion of the formation ahead of the displacement front; and

V is the velocity of sound in the portion of the formation lying between the injection well and the displace ment front, that is, the flooded portion of the formation in FIG. 1.

The distance, D, between the injection well and the displacement front as determined by the above equation, is, of course, the distance measured along a line extending between the injection well 10 and one of the production wells 26. Similar calculations can be made for the distance to the front as measured along lines extending between the injection well and the

other production wells

56, 58 and 60, so that a plot of the displacement front may be made similar to that shown in FIG. 2. In such a plot, the locations of the front 20 at given times will be indicated by the concentric lines designated by letters A through E, with the distances between each line and the injection well 10 corresponding to the distances between the

pips

50 and 54 on the several traces A through E portrayed in FIG. 3. It will be apparent that instead of transmitting the sound wave from the injection well 10 to the

production wells

26, 56, 58 and 60, waves could be transmitted from the production wells to the injection well. It will also be understood that the rate of advance of the displacement front 20 may be determined by calculating the distance it has moved in the time between the transmissions of succeeding sound waves.

FIG. 4 illustrates several chart traces of the type recorded when the sound wave reflection method is utilized to determine the position of the displacement front. Thus, at a time A after the displacement fluid has been injected into the oil-bearing formation, a sonic wave 36 is transmitted from the injection well 10 through the formation 16 toward the moving displacement front 20. As ex plained above, the sound wave is almost immediately directly received by the receiver 31 in the injection well it) and its reception is indicated by the initial pulse pip eneaooa 50 on chart trace A. The wave 36 then travels outwardly to the displacement front 29 and areflected sound wave is returned to the receiver 30 in the injection well 10 as I a result of the differentdensities of the fluid on opposite sides of the vertically extending interface formed at the displacement front 20. When the reflected sound wave is received by the receiver 30 in the injection well Iii, a pip 62 is inscribed on chart trace A. As in the change in velocity method of front location depicted in FIG. 3, the travel time of the sound wave in thereflection method is proportional to the distance on the chart which separates the initial pulse pip 50 from'the pip 62 representing the.

reception of the sound wave. However, as FIG.V4 clearly shows, the advance of the displacement front toward the production well 26 is indicated by an increase in the time required for the sound Wave to travel from the injection well 10 to the front and be reflected back to the injection Well. Thus, since-traces A throughE in FIG. 4 represent the transmission and reception of sound waves transmitted intermittently and after successively longer periods of time following the initial injection of the displacement fluid, the distance separating the initial pulse pip 50 from the pip 62 inscribed on the chart upon reception of 'the reflected signal is increasingly large as' traces A through E are recorded. 7 a 7 When the reflection method is utilized to determine the location of the displacement front, the distance from the injection well 10 to the front 20 may be calculated using From the data thus derived fromthe reflection method of displacement front location, a plot of the front movement identical to that shown in FIG. 2 may be obtained. Of, course, an advantage resides in the reflection method which is not shared by the change in velocity method in that the location of more points along the displacement front may be obtained,'thus making the plotof the displacement front curve between its intersections with the lines extending between injection and production wells more accurate. On the other hand, under some circumstances which may be occasionally encountered in secondary recovery operations, the reflected sound wave may be very weak sothatbetter results may be obtained using the change in velocity method. It is' thus desirable to provide the apparatus depicted in FIG. 1 so that either or both methods may be practiced. It will be apparent, of course, that although the reflection method has been described as it is applied to a sonic wave trans-- V ings may be obtained corresponding to the travelitim'es, of sound waves between the :injection well '10 and a plurality of

production wells

1, 2,, 3 and 4. Of course, a plurality of recorders may be utilized instead of a multiple trace type, it being only necessary to provide recording equipment capable of simultaneously recording a separate trace foreach of the production wells. The change in velocity method of I displacement front location is utilized with such a recorder arrangement, and

8 when a multiple trace recorder is utilized, the chart and data recorded thereon appear as shown in FIG. 6. By referring to FIG; 6, it may be seen that the displacement front has progressed farthest toward production well.2 and least toward productionwell 3, as indicated by the total travel time distancesbetween the two pips 5t) and 62 on each trace. The front has progressed approximately the same distance toward

production wells

1 and 4. With this information at hand'in theform of specific distances from the injection well 10 to the front 20 as measured along lines extending between the injection well and the several production wells, the form of the front and its relationship to the injection and production wells at any given time may be plotted, and also its rate of advance toward any of the production wells may be calculated. t e

It will be apparent to petroleum engineers and others concerned with 'petroleum production by secondary re- 1 covery techniques. that this ability torconstantly follow the movements of the displacement front will be an invaluable and in properly controlling .such factors as the rate and direction of injection of the displacement fluid so that production efllciency can be improved. The method herein disclosed forlocating the displacement front, while being described. as it may be applied to secondary recovery using water asthe displacement fluid,

have been described with'a certain degree of particularity,

certain elements of the apparatus and steps of the method may bealtered in ways apparent to those who possess ordinary. skill in petroleum production procedures. Insofar as such alternatives involve only the use of elemerits and steps which are the'practical equivalent of those disclosed'herein, they are considered to fall within the.

spirit and scope of the invention as defined by the following claims. 7 7 a We'claim:

v 1. In a secondary recoverysystem for recovering oil from an oil-bearing formation of the type characterized by an injection well and at least one producing well,

the method of determining the location of a displacement front which comprises; p

(a) injecting a displacement fluid into said oil-bearing formation from said injection well ,to displace the oil in said formation ahead of a moving interface formed between said fluid and the oil;

(b) transmitting a sound wave through said formation to said interface from apoint in saidformation horizontally spaced from said interface on one side thereof;

(c) detecting said sound wave at a listening station horizontally spaced from said interface and on the opposite side thereof; and

(d) registering the travel time of said, sound 'wave through the formation from its point of transmission to said listening station whereby the position of said interface in said formation may be determined.

oil from an oil-bearing steps of: I Y I v (a) drilling an injection well and-at least one production well frointhe surface to said formation with said production wells horizontally spaced from said injection well and from each other; (b) supporting a sound transmitting device in one 0 said wells at the level of said formation; (c) supporting a sound receiving device in one of said wells at the level of said formation; a 7

formation which comprises the (d) injecting a displacement fluid into said oilbearing formation from said injection well to displace the oil in said formation ahead of a moving interface between said fluid and the oil;-

(e) transmitting a sound wave from said sound transmitting device through said formation through said interface;

(f) detecting said sound wave at said receiving device located in one of said wells on the opposite side of said interface from the transmitting device so that the registered travel time of said sound wave is the time required for said wave to travel from said transmitting device through said interface to said receiving device; and

(g) registering the travel time of said sound wave through the formation from said sound transmitting device to said receiving device whereby the position of said interface in said formation may be determined.

References Cited by the Examiner UNITED STATES PATENTS Athy et al. 181S3 Beers 181--0.5 Behning 1664 McDonald 181-53 Dyes 166--9 CHARLES E. OCONNELL, Primary Examiner.

Claims

1. IN A SECONDARY RECOVERY SYSTEM FOR RECOVERING OIL FROM AN OILBEARING FORMATION OF THE TYPE CHARACTERIZED BY AN INJECTION WELL AND AT LEAST ONE PRODUCING WELL, THE METHOD OF DETERMINING THE LOCATION OF A DISPLACEMENT FRONT WHICH COMPRISES: (A) INJECTING A DISPLACEMENT FLUID INTO SAID OIL-BEARING FORMATION FROM SAID INJECTION WELL TO DISPLACE THE OIL IN SAID FORMATION AHEAD OF A MOVING INTERFACE FORMED BETWEEN SAID FLUID AND TH E OIL; (B) TRANSMITTING A SOUND WAVE THROUGH SAID FORMATION TO SAID INTERFACE FROM A POINT IN SAID FORMATION HORIZONTALLY SPACED FROM SAID INTERFACE ON ONE SIDE THEREOF; (C) DETECTING SAID SOUND WAVE AT A LISTENING STATION HORIZONTALLY SPACED FROM SAID INTERFACE AND ON THE OPPOSITE SIDE THEREOF; AND (D) REGISTERING THE TRAVEL TIME OF SAID SOUND WAVE THROUGH THE FORMATION FROM ITS POINT OF TRANSMISSION TO SAID LISTENING STATION WHEREBY THE POSITION OF SAID INTERFACE IN SAID FORMATION MAY BE DETERMINED.