CA1283201C - System for acoustically determining the quality of the cement bond in a cased borehole - Google Patents

Abstract

Abstract of the Disclosure The present invention is directed to an apparatus and method for obtaining azimuthally dependent measurements for evaluating the casing cement bond quality and, more particularly, for detecting and specifically locating channels or fissures between the casing and cement. The quality of the bond behind one or more arcs of known location on the casing is determinable by the present invention. The apparatus and method of the present invention employ a plurality of sonic transducers disposed in near contact with the casing in defined segmented locations about or around the exterior surface of a downhole tool. The transducers are arranged in a plurality of arrays, each array comprising four transducers, two transmitting and two receiving transducers, disposed along a single are about or around the exterior of the tool. The transducers are arranged so that two adjacent transducers in each array perform the same function in the array so that the bond quality between the similarly functioning and adjacent transducers is determined. The sets of arrays of transducers provide determinations in a continuous and complete loop around the casing. The casing cement bond quality is determined by measuring the attenuation of the compression wave from a pulsed acoustic wave traveling along the area of the helices. The apparatus and method of the present invention are also useful for making azimuthally dependent measurements in hard formations to detect fractures in uncased boreholes. When detecting fractures, the attenuation of the shear wave rather than the compression wave is measured.

Description

32~1 ~ack round of the Inventlon 1. Field of the Invention .
The pre~ent in~entlon generally relate~ to an apparatuq and method useful for determinin~ the quality of the cement S bond bet~een a ~all of a borehole penetratln~ an earth formatlon and a ca~lng cemented thereln. The quallty of the bond behind one or more arcs of known location on the casing ls determlned by the preaent lnrention. The pre~ent invention further comprise~ an lmproved method of ~teerin8 or directlng the acou~tlc ~ignal to maxlmlze the ~l~nal tran~itted and/or rece~ed. The pre~ent lnvention compri3es an apparatu~ and ~ethod employing a plurality of sonlc tran3ducers in deflned segmented locatlons for 3aklng the deslred measurement~.

2. Description of the Backeround In the oil and gas lndu~try, well9 are completed by ~ettin~ a string of plpe or caslng ln the borehole and filllng the annulu~ bet~een the plpe or ea~ln~ and the borehole wlth cement. Thl~ cementlng operation separates the ~arlou~ formatlon zone~, and particularly separates the productlve oil and ga3 bearing formations from non-productive formatlons ~uch a~ water-bearing formations.
Once separated by the cementing operatlon, only the de3ired oil and gas bearlng formations are perforated for production. Fallure or lncompleteness of the cement bond likely will re~ult in lncomplete aeparation bet~een the various formations. ~igration of fluids under pressure through voids or crack~ appearing in the cement bet~een the casing and borehole wall causes contamination of the fluid~
of one zone ~ith the fluids of another zone. This contamination is particularly undesirable ~here fluids from water-bearing ~trata ~igrate into fluids in a producine zone. Thi~ migration typically results in decreased productlon of hydrocarbon fluids and increased production of non-desirable flulds and contamination of equipment. The~e results often cause ~erious financlal 10~9 and may even cau~e a ~ell to become non-commerclal.
Accordingly, the de~lrablllty of accurately determlnlng 33~:0~

the quality of the cement bond between a casing and a wall of a borehole has long been a goal of those associated with the oil and gas industry. A "good bond" produces the desired separation of zones and is achieved with good adhesion, although micro-fissures or a micro-annulus may be present.
Many prior systems have been developed for investigating the bond quality between a casing and a borehole wall. Prior systems for inspecting the cement bond in wells in the oil and gas industry have included conventional acoustic cement bond systems employing acoustical energy in a variety of devices and methods. In fact, such a system is described and claimed in U.S. Patent No. 4,255,798 to Havira which employs an acoustic pulse echo technique for investigating the guality of the cement bond to a casing and the thickness of a casing located in a borehole. Further, the Havira patent lists and describes in columns 2-7 a plurality of patents and articles describing in detail many prior art systems for evaluating cement bond quality using acoustic energy.
However, these prior acoustic systems typically suffer from one or two major drawbacks. Many of these prior systems merely provide an average indication of the bond quality about the entire circumference of the casing at the measured depth within the borehole. Accordingly, the investigator using such systems cannot determine the specific azimuthal location about the borehole where a channel may be located. In fact, the investigator cannot distinguish a troublesome channel from several small fissures. Other systems which do provide azimuthal scanning measure the bond condition at Gnly specific points around the circumference and give no indication of the bond condition over the arc between those points.
Accordingly, there has been a long felt but unfulfilled need within the oil and gas industry for an effective apparatus and method useful for providing an inspection ~83~0~

~yJtem for deter~ln~ng bond quality ln ~peclflc, azlmuthally located arc~ of a cemented ca~lng.

~83~01 Summary of the_Inventlon The pre3ent inventlon provide~ a new and lmproved app~ratus and method for deter~inln~ the quallty of a bond bet~een the wall of a borehole penetratlng an earth formatlon and a caaing cemented thereln. The acoustlc logging tool and meth~d sf the pre~ent lnventlon proYlde an indlcation of the bond quallty behind an arc of the caslng deflned by two clrcumferentially aeparated point~ at a kno~n depth and orlentatlon ~lthln the borehole. The apparatus and oethod of the present inventlon pro~ide informatlon on the Sntegrlty o~ the bond between the ca~lng and the cement a~ a function of azlmuth about the caalng perlphery.
In a slmple embodloent, an acoustlc logglng tool userul ~ith the pre~ent inventlon comprl~es a sonde lncludlng mean~
for ~ecurln~ the sonde to a ~irellne. D~sposed perlpherally about or around the exterlor ~urface o~ the sonde are a plurallty of tran~ducer~ for tran3mlttln~ and recelvlng acou~tlc pulse ~ignal~ lnto a ca~lng cemented ln a - borehole. The transducers are arranged and coup~ed to approprlate electrical clrcultry to proYlde a means for operatlng the transducer~ ln a plurality of arrays. Each array compri~e~ four transducers dlsposed along a 3ingle arc de~ined by one or more hellces extendlng about or around the exterlor of the ~onde. Each array ~omprlses flrst and Qecond transducer~ capable of transmittlng and first and second transducers capable of recelvlng an acoustlc pulse slgnal. Flnally, the transducers of each array are dlsposed 90 that the transducer3 of an adJacent pair of transducers ln each array perform the ~ame function ln the array. The preferred tran~ducer pattern in the array~ is Tl-R1-R2-T2.
Other acceptable transducer patterns ln the arrays lnclude Rl-Tl-T2-R2 and T1-T2-R1-R2- These array~ permit a determinatlon of the bond quallty bet~een the adjacent transducera performlng the same functlon ln the array. The arrays are preferably dlspoaed along the pltch of one or more helice~ along the axl~ of the tool.
The pre~ent $nvent$on further provlde3 a ne~ and lmproved tranqducer confl~uration for tran~lttln~ oaxlmum atrength acou~tlc ~lgnals and recelvlng mlnlmum ~tren~th acoustlc slgnalq to lmprove the detectlon limlt~ for a bond quallty tool. The improved conflguratlon lc u~eful ln the apparatu~ and method of the present lnventlon for deter~lnlng She quallty of a bond bet~een the wall of a bore~ole penetratlng an earth formatlon and a ca~lng cemented therein.
The lmproved tranaducer configuration of the preaent lnventlon provlde3 a means for maxlmlzlnR the ~ignal tran~mltted or recelved along the arc of lnvesti~ation about tbe ~urroundln6 caslng or borehole. In the lmprovement of the present lnvention, each transducer comprl e at leaDt t~o aequentlally operated transducer ele~ent~. In the preferred embodlment, each tran~ducer comprl~es a pair o~
~quare transdu~er elements placed along the axl~ of the tool or borehole, haYlng dlmenslons one-half the acoustic ~a~e length and dlspo~ed 30 that the edBe~ of the square are perpendlcular to the lire of tran~mlssion along the pitch o~
the hellx and mo~t preferably at 45 angles to the tool axl~. The indivldual element~ of theae transducers are sequentlally operated in order to maximize the slgnal ~trength transmitted along the pltch of the helix, most preferably at +45 to the tool or casing axls and dlrected toward the receivlng transducers. Use of transducer element~ of thi~ improved conflguratlon maxlmlzes the slgnal -~trength along the arc of investigation and improves the accuracy of bond quality determination.
The logging tool further comprise~ means for generating a first acoustic pulse 31gnal from the flr3t transmltting transducer and a second acoustic pul~e ~ignal from the second transmltting tranaducer and means for measuring the maximum amplitude of a portion of the first algnal recelved at each of the first and 3econd receiving transducers and the maximum amplitude of a portion of the second qignal received at each of the flrst and second receiving transducera. Finally, the tool comprises means for determlnlng the tlme for either of the acou~tic pulse ~;~832 slgnalQ to travel bet~een the adJacent palr Or slmllarly functlonlng transducers ~hlch deflne the ~lr~t and second clrcumferentlally separated polnt~ between whlch the bond quallty ~9 determined.
In electrlcal communlcatlon with the aboYe ~easuring and determlning ~ean~ 1 mean~ for computlng the rate of attenuatlon of the acoustic pulse sl~nal~ between the adJacent palr of sl~ilarly functlonlng transducera ba~ed on the ~easured maximum amplltude~ and travel tlme. In ~ore preferred embodl~ent~, the transducer~ are equldl tantly spaced about an elongated hou~ing ha~lng a longltudlnal axla. In an exemplar~ conflguratlon, fiv~ tran~ducer~
equldlstantly ~paced about the houslng in a ~lngle plane perpendlcular to the longltudinal axls of the hou3ing produce fl~e dlfferent tran~ducer arrays pro~ldlng measurementa of bond quallty behind each of fiYe aegments of 72 eacht 8ecause o~ geo~etry problems encountered ln dlJpoJlng the transducer~ about the housing, partlcularly where smaller mea~ure~ent arcs are deslred, more preferred embodiments pro~lde for helical dlsposltlon of the transducers. In a presently preferred embodlment, nlne transducers are dlsposed equidi~tantly along a alngle hellx of 480 providlng meaaurement about alx ~egment~ each of 60. Alternative preferred embodlments lnclude twelve transducers dlspoced ln two hellces of 91x transducers each, the two hellces belng dlsplaced 180 about the housing.
Another alternative embodlment comprl~es twelve transducer~
dlsposed ln three helices of four tranaducers each, each hellx belng dlsplaced 120 about the houslng. AdJacent transducers are prefer~bly separated by a distance at least as great a~ 3~ ~here ~ ls the ~avelength of the acoustlc pul~e ~lgnal~ of the system or by a dlstance at lea~t as great aa 12th ~here th ia the thickne~ of the ca~ing of the borehole.
A system ln accord ~lth the present lnventlon further lnclude~ means for dl~playing the rate of attenuatlon and for comparlng the computed attenuatlon ~lth an attenuation lndlcatlve of a good bond.

T~e ~ethod Or the pre ent lnventlon 1~ u~eful for detectin~ the quallty of 8 cement bond behind a wall of a borehole penetrating an earth formatlon. The method comprlses the atepa of tran~mittlng a flrst acoustlc compre~alon ~ave through the ca~ln6 alon~ an arc deflned by flrst and aecond clrcu~ferentially separated polnts 30 that the ~ave arrlves at the flr~t polnt before arrivin6 ~t the second point. The lopro~ement of the preaent lnvention comprl~e-q enhanclng the strength of the tran3mltted and recelved slgnal by sequentlally activatlng a palr of tranflducer e1ement~ co~prlJlng each tran~ducer. The transducer eleoents each lnclude a ~tralght, leadlng face perpendlcular to the arc of lnvestlgation on a llne to~ard the receiver. Sequentlal actlYatlon of theae element~ wlth approprlate tlme dela~ maxlmlze~ the transmltted and recelved signal ~trength. The maxioum amplltude of the wave ls ~ea~ured at both the ~lr~t and second polnts together ~lt~ the tioe for the ~a~e to travel bet~een the flrst and second poln~. The method further comprl~e~ tranqmittlng a ~econd acouatlc compre~slon wave throu3h the casing and along the arc and measuring the maxlmum amplitude of the ~econd wave recelved at each of the flrst and aecond polntq. The method further compri~e~ computlng the rate of attenuation for the acou3tic compres~lon wave between the fir~t and ~econd polnta from the four mea~ured amplltudea and the measured travel time and displaylng the rate of attenuation a~ an indlcation of the quality of the cement bond. Alternatively, or additionally, the computed attenuation i~ compared to the attenuation for a good bond or a bond of known quality. If the depth and orientatlon uithin the borehole ia not previou~ly known, the method further compri~es mea~u~ing the azimuth of the borehole and the depth within the borehole.
The apparatua and method of the preaent invention provlde a meana for accurately determlning the quallty of the cement bond behind an arc of the caaing ln a cemented borehole. The apparatua and method of the pre~ent lnventlon proYlde a mean~ for precl~ely locating channela, fia3urea 3~
. 9 and other lmperfectlon~ ln the cement behlnd the ca3ing.
Ihe3e and other merltorlou~ features and advantagec of the present lnventlon will be more fully appreclated from the followln~ detalled deacriptlon and clalms.

-320~

~rlef De~crlption Or the Dra~lnR~
Other feature~ and intended advantages of the pre~entlnYentlon wlll be more readlly apparent by the references to the followln~ detalled descrlptlon ln connectlon ~lth the 5 a~cQmpanying drawlngs, ~hereln:
Flg. 1 ls an illustratlon of a wlrellne acoustlc logging tool ln accord wlth the pre~ent lnventlon dlsposed wlthln a cemented caslng in a borehole;
Flg. lA ls a crosY-sectlonal lllustration of an acoustlc transducer useful ln an acou~tlc logglng tool in accord ~lth the present lnventlon;
FlB. 2 1~ an lllustratlon o~ a pro~ectlon oS a transducer pattern of a trlple helix, each hellx of 180 dlsplaced 120 about an acou~tlc logging tool ln accord ~lth the present lnvention;
Fig. 3 18 an lllustratlon of a portion o~ the exterlor o~ an acouJtlc lo~glng tool ln accord wlth the present lnvent~on and ~hlch ~ould produce the tran~du~er array pattern o~ Flg. 2;
Fig. 4 is a cross-sectlonal illu tratlon of the acoustic logging tool of Flg. 3 through the llne 4-4;
Flg. 5 is a cros~-3ectional illustratlon of the acou3tlc logglng tool of Fig. 3 through the llne 5-5;
Flg. 6 is an lllustratlon of a proJectlon of a transducer pattern of a single hellx of 4800 about an acouatic logglng tool in accord wlth the present lnventlon;
Fig. 7 is an lllustratlon of a proJection of a transducer pattern of two hellces, each hellx of 300 and dlsplaced 180 about an acoustlc logglng tool in accord with the present lnvention;
Fig. 8 19 a side lllustratlon of a presently preferred embodlment of a wlrellne acoustlc logging tool in accord with the ~resent inventlon;
Flg. 9 1~ a cro~s-sectlonal illustration of the acoustlc logglng tool of Flg. 8 through the llne 9-9;
Flg. lO ls an lllustratlon of an acoustic transducer pad of the wlrellne logglng tool of Fl~. 8;
Fl~. ll 1~ an lllustratlon of a portlon of an acou~tlc loggln~ tool ha~ing a directlonal transducer configured ln accord ~ith the present lnventlon;
Fl~. 12 1~ a block dlagram of the ma~or eomponent~ and clrcult~ of an aeous'tlc lo~glng tool ln accord ~lth the present lnventlon;
Fl~. 13 19 a 6raphical lllustratlon of the sttenuation ob3erved ln a te~t ca~lng havln~ a cement vold in an arc approxlmately 120 ~lde and centered at the de lgnated posltlon zero;
Fig. 14 i~ a graphlcal lllu~tration of the attenuatlon obser~ed ln z test ca3in~ haYlng a cement vold ln an arc approxlmately 120 ~lde and centered at the designated po~ltlon zero;
Fi6. 15 ia a graphlcal illu~tratlon of the attenuatlon observed ln a test caJing ha~lng a cement vold ln an arc approxlmately 17.6 wlde and centered at the de~lgnated position zero;
F~g. 16 1B a grap~cal lllu~tratlon of the attenuatlon ob~erved ln a te~t casln~ havlng a ce~ent ~old ln an arc approxlmately 17.6 ~lde and centered at the deal~nated posltlon zero; and Flg. 17 la a graphical illustratlon of an output signal for an acou~tlc logging tool ln accord wlth the present inventlon produclng result3 ror 91x sectors formlng a clreumference aboul; the casing and lllustratlng varlous bond characteristics~
Whlle the invention will be descrlbed ln connectlon with the presently preferred embodlment, it ~111 be understood that lt 19 not intended to llmlt the lnventlon to thls embodlment. On the contrary, lt is intended to cover all alternatlves~ modlflcatlons and equivalents as may be 1ncluded ln the 3plrlt of the inventlon as de~lned ln the appended clalms.

.
. -, ~3~
-~2-Detalled De~cri tlon of the Preferred Embodlment . . . P . . _ _ The pre~ent lnventlon i8 dlrected to an spparatu~ and method useful for determlnin~ thc quallty of a bond between a ~all of a borehole penetratlng an earth formatlon and a caalng cemented thereln. Thl~ determlnatlon 1J made between two clrcumferentlally ~eparated polnt~ on an arc of the calln~, The apparatu~ and method of the pre~ent lnvent~on employ~ a plural~ty Or transducers for transmlttlng and recel~lng an acou~tlc pul~e co~pre~ion waYe through the ca~lng between the t~o clrcum~erentlally ~eparated polntA.
Meaaure~ent of the ~axl~um a~plltude of a selec~ed portlon of the 31gnal recel~ed by tran~ducer3 located at the two separated polnt~ and the tra~el t10e therebetween for each of t~o such acou~tlc pul~e co~pre~slon waves permlt~
lS calculatlon of the rate of attenuatlon of the acoustlc pulae slgnal bet~een the t~o polnt~ and comparlson ~lth the attenuatlon bet~een simllar polnts ~or a good bond or a bond of known quallty.
Flg. l lllustrates an acou~tlc logglng tool lO attached at 12 to a w~rellne cable 24 pas~lng o~er a sheave 26 to conventlonal ~urface wlrellne equlpment (not illustrated).
Dlspo ed about the surface of the tool lO are a plurallty o~
tranaducer pad~ 30 whlch include tran~ducer element~ 40 whlch function a~ either transmitters or receivera for an acou~tic pulse slgnal. In electrlcal communlcatlon wlth tran~ducer elements 30 i3 appropriate electrical circultry 20 to control operat~on of the transducer3 and receive and process information to provide signal amplltudes, travel time and rate of attenuation for determination Or bond quallty. Data la displayed on conventlonal recorder 22 or other approprlate means.
The acoustlc logging tool lO la lllustrated dlsposed ln a caslng 84 cemented ~ith cement 86 to the wall of a borehole 82 in an earth formatlon 80. The tranaducer pad~
30 proJect from the surface 14 Or the tool lO and may be forced lnto contact wlth the caslng B4 by spring 46 actlng on the rear thereor. Upaet rlng~ ll or other centrallzer~
known to those of ordlnary ~klll ln the art malntaln the i~832(~1 tool 10 centrally disposed within the casing 84.
Fig. lA illustrates in greater detail an exemplary transducer pad 30 for use in a tool 10 in accord with the present invention. Transducer pad 30 comprises a transducer element 40 comprising a thin disk shaped member of an appro-priate ceramic piezoe-ectric material, e.g., barium titanate, iead zirconate titanate or the like, centrally disposed within cylindrical support member 32. A large backing member 41 comprising a poor acoustical transmission medium such as tungsten loaded epoxy or the like is disposed directly behind the transducer element 40. These members are disposed and cushioned in an acoustically absorbent material 42 comprising porous rubber and the like for acoustically isolating the transducer element from the surface 14 of the tool 10. The cylindrical support 32 includes a protective member 34 of a protective material such as teflon or the like to protect the transducer element 40 from contact with the interior surface of the casing 84.
The cylindrical support member 32 ends interiorly with flange 36 having surface 38 for abutting contact with surface 18 between concentric mounting bores 16 and 17 for limiting the radial travel of the transducer pad 30. A
spring 46 maintains the transducer pad 30 against the interior surface of the casing 84 or fully extended with ~5 flange 36 stopped at surface 18. A similar acoustic trans-ceiver pad is described in U.S. Patent No. 3,811,529 at Fig. 4. Electrical contacts 44 extend from the transducer element 40 to the control circuits 20 through a central bore 48 through the tool 11.
Fig. 11 illustrates a transducer pad in accord with the improved configuration of the present invention~ Each transducer pad 30 comprises a pair of transducer elements 40a and 40b operated sequentially as either sequential transmitters or sequential receivers. The transducer com-prises transducer elements 40a and 40bl centered at a and b,respectively, and preferably disposed along the axis of the tool 10 or the casing 84 ! both illustrated by line .
:

. . .

~'28320~

n The center-to-center dl~tance bet~een tran~ducer ele~ent~ 40a and 40b 1~ dl. The tran~ducer element~ ~Oa and 40b are con~lgured and ha~e centera poaltlon~d JO that propcrly tl0ed actl~atlon of the ~lementfi produce~
S conatructive interference of the acou~tic pul~e~ along She arc of nve~tlgatlon and destructlve lnterference ln other dlrectlona. When ~o dl~po~ed and act~vated, tran~ducer ele~entJ 40a and 40b produce lmproved "steered~ ~lgnalA
~long the arc of lnve~tlgatlon. When dl~posed ~long a llne parallel to the tool axl~, as tllu~trated ln Flg. 11, the sl~nsla may eaaily be ~teered~ along th~ path of ~oth thc le~t and right ~eliceJ. If dlnpo~ed 210ng a linc not parallel to thc tool axl~ teerlng~ along both left and rl~ht h~llces 1~ ~tlll po~Jlble after the proper delay for lS each dlrectlon i~ calculated. Although more tlfflcult, calculatlon o~ thl~ delay 1~ ~lthln the ~klll of tho~e in th~ art.
In the preferred embodlment, transducer ele~ent~ 40a and 40b each lnclude a sub~tantlally llnear, leadlng edge dl~po~ed perpendlcular to the arc of ln~e~tigation and to a llne connectlng the transducers/recelvers of the array. In the ~o~t preferred embodlment, the tran~ducer elementa are aquare ~ith slde~ one-half the wa~elength of the acou~tlc signal to be tran~mltted ~o that the center~ a and b and the leadlng faces are each ~eparated by a dl~tance one-half the acou~tic pul~e wa~relength along thl~ arc. Properly tlmed, sequentlal actl~atlon of tran~ducer elements 40a and 40b ln thl~ mo3t preferred embodlment results ln maxlmlzatlon of the slgnal ~tren8th at angle ~ , also the pitch of the helix and the angle of the arc of lnYestlgation in Flg. Il. In thl~ mo~t preferred embodl~ent, where the pltch of the hellx of the tranJducer configuratlon 1~ 45 and the angle between the tool axl~ and the llne connectlng the center3 a and b of the tran~ducer element~ 40a and 40b i~ zero, thi~
3S conflguration maxlmlze~ the ~lgnal ~trength at +45 to produce lmproved result~. The flring of the tranAducer element close~t to the recelver~, ln the lllu~tratlon tran~ducer element 40a for recelvers 34 or tranaduc~r lV~83~1 ele~cnt 40b for recelver~ (not illustrated) located above tran~ducer pad 30, l~ delayed by a ti~e factor td = d2/V~
~here d2 l~ the dlAtance ~long the arc Or in~eJtigatiOn bet~een perpendlcular~ dropped from the centers a and b of the transducer elementJ. Speciflcally, td = d1 c Vl!~
~here td = the tlme delay bet~een the firlng of tran~ducer ele~ent~ 40a and 40b;
d1 5 the center-to-center dl~tance bet~een trannducer elemcntn 40a and 40b;
; V~ ~ the ~peed of the acoustl~ ~a~e in the arc of determlnatlon in the canlng or format1on; and lS 9 = the angle bet~een th~ llne connecting the center~
a and b o~ the tran~ducer elem~nta 40a and 40b and the arc Or ln~estlgatlon.
F~g. 12 ~llu~trat~s ln ~che~atlc and block dlagra~ rorm the ~aJor clrcuit~ and ~unctlon~ of thc present lnventlon.
Selectlon o~ the tran~mltters to be actlvated as each tranlmltter palr 1~ achle~ed through transmltter selector mean~ 50 ~lth signal actlvation controlled by transmitter flrlng means 52~ Selectlon of the palr of receiver~ to be actlYated ln con~unctlon ~lth each actl~ated tran~mltter 19 achleved through channel recelver ~elector 60. The recelved slgnal 18 proces3ed through galn control ampllfler 62 to recelver amplltude mea~urement mean~ 64 and gated tlmlng detector 66. Input and output to and from the~e circult control devlces l~ controlled by controller and data acqul~ltlon ~ystem 68. Flnally, slgnal~ may be transmltted to and froo the controller and data acquisltion ~ystem 68 over conventlonal telemetry ~yste~ 70. The above-described control clrcuitry employ~ conventional clrcuits and subclrcuit.~ ~ell kno~n to tho~e skllled ln the arS and, accordlngly, need not be further detalled hereln.
In lt~ t~o most prlmltlve embodiment~, an acou~tlc logglng tool u~eful ~lth the preJent lnventlon lnclude~ fl~e or ~lx tranAducers sym~etrlcally located ln a ~lngle plane :

8~

perpendlcular to the axi~ of the tool lO and dl~po~ed equldl~tantly about it~ ~urface 14 at the vertlceJ of a pentagon or a hexagon~ re~pectlvely. It could alAo be ~ald that the~e tran~ducer~ are locaked along the arc of hellx of 360 and havin~ a plSch of zero. A penta~onal conflguratlon pro~ides flve array~ of four equldl~tantly dl~poaed tran~ducer~ for pro~lding informatlon concernln~ each of flve ~ectors comprl~ln~ 72 about the clrcumference of the ca~lng. In the hexagonal conflguratlon, slx tran~ducer array~ provide slmllar lnfor~ation concernlng each of ~lx equal 60O ~ector~ dl~posed about the circuofercnce.
Ho~e~er, in re~llt~, ~u~ planar dlstrl~utlon o~ tranaducer~
1B dlfficult to achle~e becauae of tho cro~dlng re~ultlng from attemptlng to plac~ fi~e or 91X tran~ducers ln a ~in~le plane ln a tool ~hose dlameter is re~trlcted for paJsage through a borehole. ~ccordlngly, Appllcant~ ha~e devl~ed se~eral preferred eobodloent~ for dlJpoJltlon of a ~lnimal plurallty of tran~ducera to produce the d~lred result~.
Flg. 6 relate~ to one such conflguratlon uherein nlne transducer~ are dispo~ed equldlstantly along a ~ingle hellx of 480 about the longltudlnal axi~ of the tool lO. Flg. 6 lllustrate~ the array pattern whlch ~ould be produced by 3uch a conflguration. ~here the central palr of any four adJacent transducer~ functlons as recelvers and the exterior transducers function a~ transmltter~, the tool whlch would produce the patte!rn ln FlR. 6 would provlde informatlon concernlng the bond quality between tran.qducers T2 and T3 ~ith the array comprlaing transducers T1-T4. Slmllarly, in~ormation concernlng the bond quallty bet~een transducers T3 and T4 ~ould be obtained employlng the array comprisin6 transducers T2-T5 and 90 on untll lnformatlon concernln~ the bond quallty between tran~ducers T7 and T8 18 provided by the array comprlsing tran~ducers T6-Tg. Tho~e ~kllted in the art will readlly comprehend that other conflguratlon~t 3S e.~., the RTTR and TTRR configuratlons lllustrated ln the su~mary may be employed ln place of the ~ust described TRRT
configuratlon to provld~ lnfor~atlon concernlng each seetor about the clrcumference o~ the caJlng. Further, tho~e ~X~3~20~

~klll~d in the art ~ill appreclate the tran~ducer~ need not be ~paced apart by 60o and that cloJer ~pacln~ Or a greater nu~ber of tran~ducer~ along a hellx of great~r pltch ~ould permlt determlnation of bond quallty ln approprlately narro~er arc~ about the clrcu~ference of the caslng.
Ho~ever, it ~hould be noted that the di~tance bet~een ad3acent tran~ducer~ ~hould be ~aintalned preferably at leaAt a~ great a9 ~ where ~ la ~he wavelength of the acou~tlc ~lgnal~ ln order to produce ~atl~factory re~ultJ.
Addltionally, becau~e the ~avelen6th chould not exceed 4th ~here th ia the thickneJJ Or the caa~n~ the apa~ing bet~een adJacent tranAducera ahould not be 1~B~ than 12th. Becau~e th for conventlonal caJlrg 18 one~fourth lneh to one-half lnch, the ~pacing bet~een adJacent transducer~ ~hould typlcally not be less than slx incheJ.
Flg. 7 lllustrate~ an alternati~e tran~ducer conSlguratlon e~ploying SwelYe tranJducera T1-T12 dl~posed ln t~o hellce~ of 300 each about the Jur~ace of a tool lO. The tran~ducerJ are uJed both a~ trans~lttera and recel~ers. The hellc~e~ are displaced 180 from one another about the surface 14 of the tool lO~ Measurements are made slmllar to tho~e dlscussed in relatlon to the alngle helix ha~ing nlne transducer~. In this conflguratlon, the bond quallty bet~een T2 and T3, T3 and T4, and T4 and T5 1~
determlned employlng the transducer~ T1-T6 of the flrst hellx and the bond quallty bet~een T8 and Tg, Tg and Tlo, and Tlo and Tll ls obtalned employlng the transducers T7-T12 of the second hellx. In thls conflguratlon, t~o tran~ducers are mounted dlametrlcally ln each of 9iX planes perpendlcular to the axl~ of the tool lO to provlde lnformatlon concernlng each Or ~lx equldistant segments about the caqing ln a ~horter vertlcal dlstance.
Flgs. 2-5 lllu~trate a conflguratlon of t~elve transducers dl~po~ed ln three hellce~ each of l80 and dl~placed by 120~ about the surface of a tool lO. Such a confi~uratlon actually produce~ four tran~ducer~ lylng on a lln61e arc Or each of three left and three rlght helice~
such that each hellx 1A employed to determlne the bond l ~B320i quallty ln one Or ~lx ~ector~ about a surroundlng ca~lng.
ThlA conflgt~ratlon provldes lnfor~ation restricted wlthln a narro~ vertlcal range d. In thl~ conflguratlon, each array comprl~e~ the t~o recelver~ and two tranRmltt~r~ found along S each diagonal llne of Fig. 2. The mea~uremen~s are made between adJacent recel~er~ a~ lllustrated by the ~olld lines of Flg. 2.
A test deYlce ha~lng tran~mlSter~ T1 and T2 and recelver~ Rl and R2, ln the pattern TRRT, wa~ con~tructed ~hereln the tranaducer~ ~ere confl~ured about a 180 hellx, each ~eparated by 600 and ~lth a ~ertlcal dlJtance d of 6 lnches ln accord wlth ~he conf'lguratlon ~shown ln Fl~. 2 and 3. The axe~ of the t~o tranemlttero ~ere at 180~ ~lth re~pe~t to each o~her and the mea~urlng arc Or the array 18 the arc bet~een ~he t~o recet~ers R1 a~d R2.
In u~e, Tl transmltted an acoustlc pulse ~lgnal In the ~urroundlng ca~in~ produ~lng a ~lgnal flr~t at R 1 and later at R2. The maximu~ aaplitud~ of the ~lgnal reee~v~d flrJt at R1 and later at R2 uaJ mearured and recorded. T2 ~as then employed to tranqmit a ~lmilar signal whlch was recel~ed and mea~ured fir~t at R2 and then at Rl. Further, the tlme for the si~nal to tra~el bet~een Rl and R2 wa~
measured for elSher one or both of the acou~tic co~pre~sion waves. The attenuatlon rate i~ determined by the ~5 equation:

(~llA22 ) 20 log A12A21 al2 = ... __ V~ (t12 + t21 - t11 - t22 where 12 = the attenuatlon rate between Rl and R2;
V~ , the speed Or the acoustlc wave in the ca~lng;
Al~ _ the amplitude of the .~lgnal recelved at R
from tranl-ltter Ti where i = 1 or 2 and J _ I or 2; and tlJ = the time at ~hlch the ~lgnal from Tl 1 lX8~2~

recelved at RJ ~here 1 - l or 2 and J = l or 2.
The above device wa~ te~ted in a Jlmulated borehole ~hereln caslng was cement~d ~ith hydrollte to a ~lmulated borehole ~all. The ca~lng ~a~ 9-3~4~ OD conventlonal ca~lng havln~ a ~all thickne~ of .462 lnches or .22 where was the ~avelength of th~ acou~tic pul~e ~lgnal. A vold repre~entative of a 120 unbonded ~ector ~a~ produced about a ~ero reference polnt. Thl~ model waA placed in a water fllled tank and meaDurement~ were made in an area encoopa~lng the unbonded Jector and 60 on either slde thereof. Data wa~ obtalned at lO~ lnter~al~ around the pipe pcrlphery and attenuatlon ~or the ma%lmu~ Jignal 81 and the aeconda~y rlgnal ~2 plotted to produce the result~
lllustrated ln Flg~ and 13, reJpecti~ely. The ~trlklng feature of these Cur~eJ 1~ the Jymmetrlc attalnment of mlnlmal attenuation at pointJ corresponding to the vold or an arc o~ the plpe haYlng bad bond quallty.
A ai~ilar test ~aJ undertaken ~ith a te~t annulu~
ha~ing a Yold representatl~e Or a 17.6 vertlcal channel at reference point zero, All other condltion~ were ldentlcal. The re~ult~ of the measured attenuatlon are lllu~trated ln Flg~. 15 and 16 ~howlng a ~l~niflcant mea~urable attenuatlon dlfference most pronounced at the narrow vert$cal channel.
The lncorporatlon of such a device ln a wlrellne acou~tlc logging tool enploylng a conventlonal ~trip chart recorder could re~ult ln ea~lly readable output ~uch as that lllu~trated ln FLg. 17. Conventional depth and azimuth devlces 13 would provide the operator with the depth of each measurement and the aziouthal locatlon of each of sectors l-2, 2-3 through 6-l. Such a devlce can ea~lly lndlcate whether the caslng and cement have formed a good bond about the entlre perlphery o~ the caslng or whether a bad bond exists ln one or more of the mea~ured sect~r~ thereabout.
The tool lllu~trated ln Flg~. 8-lO 19 the pre~ently preferred embodlment of a wlrellne tool ln accord ~lSh the pre~ent lnventlon. Tool llO lnclude~ t~elve adJu~tably po~ltlonable transducer pads 130 dlspo~ed in the pad ~8320~

conflgurat~on lllustrated ln Fi8. 2. Tool 110 lncludeA slx ~d~ustable arm~, each arm includln~ two tranaducer pads 130 Jeparsted by a ~pacer bar 128. Each arm further Include~ an upper arm 124 hingedly afflxed to the central body portlon S 120 of the tool llO at ltJ upper end and hlngedly a ffl%ed at 134 to the upper tran~ducer pad 130a. A ~lmllar lower arm 126 1~ hlngedly afflxed at lt~ lo~er end to the central tool portlon 120 and hlngedly afflxed at ltJ upper end to the bottom of lo~er pad 130b. Elther one or both Or arm~ 124 and 126 are slldable alon~ central portion 120 of the tool 110 to per~lt a ~lldabl~ ring an~ motor (not rho~n) to puah ! and pull the ar~s along the central portlon 120 o~ th~ ~ool 110 to exSend or retra~t, respectl~ely, the tran~ducer padr 130. In the retracted posltlon, th2 pads 130 are pulled 15 flush agalnst the central tool portlon 120 withln ln~ets 122 to provlde a tool more easlly pulled through the cas~ng.
Exten~lon of the aros carrylng tranaducer p~d~ 130 place~
the tran3ducer elements 140a and 140b mounted ~lthln an abaorbent materlal 142 to be brough~ lnto near contact wlth the inner wall of the ca~lng to be te~ted.
The forego~ng descriptlon of the invention has been directed ln prlmary part to a partlcular preferred embodlment and method ln accordance wlth the requirements of the patent AtatUtes and for purposes of explanation and lllustratlon. It wlll be apparent, however, to those skllled ln the art that many modlflcatlon~ and changes ln the speclfically descrlbed apparatus and method may be made ~lthout departlng froo the ~cope and Aplrit of the lnventlon. For example, Appllcants ha~e illustrated and descrlbed a devlce and method employlng a plurallty of transducers each having two transducer elements equld1~tantly spaced along one or more hellce~. However, those skilled ln the art would be aware that a plurallty of transducer elements spaced and properly sequentlally actlvated may be eoployed to further maximl2e the dlrectlonal slgnal strength. Appllcants belleve the dlsclosed apparatu~ and method provlde the mo3t compact, efflclent and economlcal devlce ln accord wlth the present lnventlon. However, thoae ~kllled ln the art ~lll appreclate that other configuration~ havlne the descrlbed characterlJtlc~ may produce the ~ame re~ult~. Therefore, the lnventlon i~ not re~trlcted to the partlcular form of constructlon and method lllustrated and descrlbed, but cover~ all modlflcatlon~ ~hich may fall ~lthln the ~cope of the following clalm~.
It 18 Appllcant~' lntentlon ln the follo~lng cla1ms to cover such modlflcatlons and varlation~ a~ fall ~lthln the true ~plrlt and ~cope of the lnventlon.

Claims (47)

1. An acoustic logging tool useful for determining the quality of a bond between a wall of a borehole penetrating an earth formation and a casing cemented therein in several adjacent sectors around the casing, comprising:
a sonde, including means for securing said sonde to a wireline;
a plurality of pads extendable from said sonde, each said pad having an exterior surface for contacting said casing surrounding said sonde;
a plurality of transducers for transmitting and receiving acoustic pulse signals into said casing, said plurality of transducers disposed in said exterior surfaces of said plurality of pads;
means for selecting a plurality of arrays of transducers, each array comprising four transducers disposed on an arc projected through at least four of said pads, two said transducers functioning as transmitters and two functioning as receivers; and means for determining the bond quality in each said sector between two adjacent transducers in each said array employing the amplitude of the compressional acoustic pulse signals received by said two receivers from said two transmitters.

2. The acoustic logging tool of Claim 1 further comprising means for determining the travel time of said compressional acoustic pulse signals between said adjacent transducers.

3. The acoustic logging tool of Claim 2 further comprising means for determining the position within said borehole of each sector defined by said adjacent transducers.

4. The acoustic logging tool of Claim 3 further comprising means for centralizing said sonde within said borehole.

5. The acoustic logging tool of Claim 4 further comprising means for transmitting data from said tool to a remote location at the surface and means for displaying an output indicative of the bond quality in each said sector.

6. The acoustic logging tool of Claim 1 wherein adjacent transducers are separated by a distance at least as great as 3.lambda. where .lambda. is the wavelength of said compressional acoustic pulse signal.

7. The acoustic logging tool of Claim 6 wherein adjacent transducers are separated by a distance at least as great as 12th where th is the thickness of said casing.

8. The acoustic logging tool of Claim 1 wherein the wavelength of said compressional acoustic pulse signal is at least as great as 4th where th is the thickness of said casing.

9. The acoustic logging tool of Claim 1 wherein each said array comprises first and second transmitters T1 and T2 and first and second receivers R1 and R2 disposed in an order so that at least one pair of like functioning transducers are adjacent.

10. The acoustic logging tool of Claim 1 wherein each said transducer comprises first and second transducer elements and means for activating sequentially said first and second transducer elements to maximize the transmitted acoustic pulse signal along said arc.

11. The acoustic logging tool of Claim 10 wherein said first and second transducer elements of each transducer are disposed with centers parallel to the axis of said sonde.

12. The acoustic logging tool of Claim 11 wherein said centers of said first and second transducer elements of each transducer are separated by a distance d and further comprising means for delaying the sequential activation of said elements by a delay time td = (d/V?) cos .theta. where V? is the acoustic velocity of said compressional acoustic pulse signal in said casing and .theta. is the angle between said sonde axis and said arc.

13. An acoustic logging tool useful for determining the presence of fractures in a borehole penetrating an earth formation, comprising:
a sonde, including means for securing said sonde to a wireline;
a plurality of pads extendable from said sonde, each said pad having an exterior surface for contacting a wall of said borehole surrounding said sonde;
a plurality of transducers for transmitting and receiving acoustic pulse signals into said object, said plurality of transducers disposed in said exterior surfaces of said plurality of pads;
means for selecting a plurality of arrays of transducers, each array comprising four transducers disposed on an arc projected through at least four of said pads, two said transducers functioning as transmitters and two functioning as receivers; and means for determining the presence of fractures in each said sector between two adjacent transducers in each said array employing the amplitude of the shear acoustic pulse signals received by said two receivers from said two transmitters.

14. A method for detecting the quality of a cement bond between a wall of a borehole penetrating an earth formation and a casing cemented therein, said bond quality detected behind an arc of said cemented casing defined by first and second circumferentially separated points, comprising:
transmitting an acoustic compression wave through said casing along said arc;
measuring the amplitude of said acoustic compression wave received at each of two receivers, said two receivers defining said first and second circumferentially separated points; and determining the cement bond quality from said measured amplitudes.

15. The method of Claim 14 further comprising measuring the time for said acoustic compression wave to travel between said adjacent receivers.

16. The method of Claim 15 comprising determining said bond quality by determining the rate of attenuation of said compression wave between said receivers.

17. The method of Claim 16 further comprising displaying said rate of attenuation as an indication of the quality of the cement bond along the arc defined by said first and second circumferentially separated points.

18. The method of Claim 14 further comprising determining the location within said borehole of said arc between said circumferentially separated points.

19. The method of Claim 18 wherein determining said position comprises determining the depth of said arc within said borehole and determining the azimuthal location of said arc within said borehole.

20. A method for detecting the presence of fractures in a wall of a borehole penetrating an earth formation in an arc of said borehole wall defined by first and second circumferentially separated points, comprising:
transmitting an acoustic shear wave through said wall of said borehole. along said arc;
measuring the amplitude of said acoustic shear wave received at each of two receivers, said two receivers defining said first and second circumferentially separated points;
measuring the time for said acoustic shear wave to travel between said receivers; and detecting the presence of fractures between said first and second points from said measured amplitudes and said measured time by determining the rate of attenuation of said shear wave between said receivers.

21. An acoustic logging tool useful for determining the quality of a bond between a wall of a borehole penetrating an earth formation and a casing cemented therein behind an arc of said casing defined by two circumferentially separated points Ri and Rj, comprising:
an elongated housing, including means for securing said housing to a wireline;
nine transducers for transmitting and receiving acoustic pulse signals, said transducers disposed equidistantly about the circumference of said housing in a single helix of 480°;
means for operating said transducers in six different arrays, each said array comprising four transducers disposed along a single arc of said helix, two transducers Ti and Tj capable of transmitting and two different transducers Ri and Rj capable of receiving an acoustic pulse signal, said transducers disposed so that the distance between Ti and Ri is less than the distance between Ti and Rj and the distance between Tj and Rj is less than the distance between Tj and Ri;
means for generating a first acoustic pulse signal from each said transducer Ti and a second acoustic pulse signal from each said transducer Tj;
means for measuring the amplitude Aii and Aij of a selected portion of said first acoustic pulse signal received at said transducers Ri and Rj and the amplitude Ajj and Aji of a selected portion of said second acoustic pulse signal received at said transducers Rj and Ri;
means for determining the time .DELTA. t for either of said acoustic pulse signals to travel between transducers Ri and Rj;
and;
means for determining the rate of attenuation Aij of said compression wave between said receivers Ri, Rj from said measured amplitudes Aii, Aij, Aji, Ajj and said travel time .DELTA.t.

22. An acoustic logging tool useful for determining the quality of a bond between a wall of a borehole penetrating an earth formation and a casing cemented therein behind an arc of said casing defined by first and second circumferentially separated points, comprising:
a housing, including means for securing said housing to a wireline;
transducers for transmitting and receiving acoustic pulse signals into an object surrounding said housing, said transducers disposed about the outer surface of said housing;
means for operating said transducers in a plurality of different arrays, each said array comprising four transducers disposed along a single arc about the circumference of said housing, each said array comprising first and second transducers capable of transmitting and first and second transducers capable of receiving an acoustic pulse signal, said transducers disposed so that an adjacent pair of transducers in said array perform the same function in said array;
mean for generating a first acoustic pulse signal from said first transmitting transducer and a second acoustic pulse signal from said second transmitting transducer;
means for measuring the amplitude of a selected portion of said first acoustic pulse signal received at each of said first and second receiving transducers and the amplitude of a selected portion of said second acoustic pulse signal received at each of said first and second receiving transducers; and means for determining the time for either of said acoustic pulse signals to travel between said adjacent pair of transducers.

23. The apparatus of Claim 22 further comprising:
means for computing the rate of attenuation of said acoustic pulse signals between said adjacent pair of transducers from said measured amplitudes and said travel time.

24. The apparatus of Claim 23 further comprising means for displaying said rate of attenuation.

25. The apparatus of Claim 23 further comprising means for comparing said computed attenuation with an attenuation indicative of a good cement bond between said casing and said earth formation.

26. The apparatus of Claim 22 wherein adjacent transducers on said arc are separated by a distance at least as great as 3 .lambda. where .lambda. is the wavelength of said acoustic pulse signals.

27. The apparatus of Claim 26 wherein adjacent transducers on said arc are separated by a distance at least as great as 12th where th is the thickness of said casing.

28. The apparatus of Claim 27 wherein the wavelength of said acoustic pulse signals is at least as great as 4th.

29. The apparatus of Claim 22 comprising at least six transducers equidistantly disposed about the circumference of said housing.

30. The apparatus of Claim 29 wherein said transducers are disposed helically about said housing.

31. The apparatus of Claim 29 comprising nine transducers disposed about said housing in a single helix.

32. The apparatus of Claim 29 comprising twelve transducers disposed in two helices of six transducers each about said housing wherein said transducers are disposed in six planes perpendicular to the axis of said housing, each plane including a single transducer for each helix disposed diametrically to each other.

33. The apparatus of Claim 29 comprising twelve transducers disposed in three helices of four transducers each about said housing wherein said transducers are disposed in four planes perpendicular to the axis of said housing, each plane including a single transducer for each helix disposed 120° from each other.

34. The apparatus of Claim 22 further comprising means for extending each of said transducers radially from said surface of said housing.

35. The apparatus of Claim 22 wherein said transducers are separated on said arc by less than 90°.

36. The apparatus of Claim 22 further comprising means for determining the azimuth of said borehole.

37. The apparatus of Claim 22 comprising a sufficient number of arrays of transducers to permit determining of a bond quality for each of a plurality of arcs, said plurality of arcs forming in combination a circumference of said casing.

38. A method for detecting the quality of a cement bond between a wall of a borehole penetrating an earth formation and a casing cemented therein, said bond quality detected behind an arc of a cemented casing defined by two circumferentially separated points Ri and Rj on said well casing, comprising:
disposing at a known depth and orientation within said cased borehole a tool having at least one array of four transducers disposed along a single arc on the surface of said tool, two transducers Ti and Tj capable of transmitting and two different transducers Ri and Rj capable of receiving an acoustic pulse signal, said transducers disposed so that the distance between Ti and Ri is less than the distance between Ti and Rj and the distance between Tj and Rj is less than the distance between Tj and Ri;
generating from said tool an acoustic pulse signal at a frequency and location which produces a first acoustic compression wave transmitted through said casing from a point Ti on said well casing so that said first compression wave arrives at Ri prior to arriving at Rj;
measuring the amplitude Aii of a selected portion of said first acoustic compression wave in said casing received at said point Ri and the amplitude Aij of a selected portion of said first acoustic compression wave received at said point Rj;
generating from said tool an acoustic pulse signal at said frequency and a location which produces a second acoustic compression wave transmitted through said casing from a point Tj on said well casing so that said second compression wave arrives at Rj prior to arriving at Ri;
measuring the amplitude Ajj of a selected portion of said second acoustic compression wave in said casing received at said point Rj and the amplitude Aji of a selected portion of said first acoustic compression wave received at said point Ri;
determining the time .DELTA.t for either of said acoustic compression waves to travel between points Ri and Rj;
computing the rate of attenuation .alpha.ij for said acoustic compression wave between said points Ri and Rj on said casing from said measured amplitudes and travel time of said acoustic compression waves between said points Ri and Rj; and displaying said rate of attenuation as an indicator of the quality of the cement bond behind said arc between points Ri and Rj.

39. The method of Claim 38 comprising computing said rate of attenuation from the equation where V? is the speed of said wave in said casing.

40. The method of Claim 38 further comprising comparing said rate of attenuation with a known rate of attenuation for a bond of known quality.

41. The method of Claim 38 for detecting the quality of the cement bond in each of a plurality of circumferential segments defined by points Ri and Rj, said segments combining to form a complete circumference of said casing, comprising:
repeating the steps of Claim 38 for each said segment forming said complete circumference of said casing.

42. The method of Claim 38 further comprising measuring the azimuth of said borehole at said known depth in order to precisely locate points Ri and Rj on said casing.

43. The method of Claim 42 further comprising measuring the depth of said points Ri and Rj in said borehole.

44. A method for detecting the quality of a cement bond between a wall of a borehole penetrating an earth formation and a casing cemented therein, said bond quality detected behind an arc of said cemented casing defined by first and second circumferentially separated points, comprising:
transmitting a first acoustic compression wave through said casing alone said arc, said first acoustic compression wave arriving at said first point before arriving at said second point;
measuring the amplitude of a selected portion of said first acoustic compression wave in said casing at said first point and at said second point;
transmitting a second acoustic compression wave through said casing along said arc, said second acoustic compression wave arriving at one of said first or second points before arriving at the other of said points;
measuring the amplitude of a selected portion of said second acoustic compression wave in said casing at said first point and at said second point;
determining the time for one of said acoustic compression waves to travel between said first and second points;
computing the rate of attenuation for said acoustic compression wave between said first and second points on said casing from said four measured amplitudes and said measured travel time; and displaying said rate of attenuation as an indication of the quality of the cement bond behind said arc between said first and second points.

45. The method of Claim 44 for detecting the quality of the cement bond in each of a plurality of arcs which in combination form a circumference of said casing, each arc defined by first and second circumferentially separated points, comprising:
repeating the steps of Claim 44 for each said arc to provide an indication of the quality of the cement bond behind each arc of said casing.

46. The method of Claim 45 further comprising measuring the azimuth of said borehole at said circumference to precisely locate said arc on said casing.

47. The method of Claim 46 further comprising measuring the depth of said circumference in said borehole.