[0001] The present invention relates to borehole logging technologies, and more particularly
to an improved acoustical transducer configuration and method for borehole logging
applications.
[0002] In the petroleum industry, several important wellbore and formation surveying and
analysis tools employ acoustical techniques. These include the Borehole Televiewer,
the Pulse Echo Tool, and the Cement Evaluation Tool, for example. With such tools,
an acoustic pulse is generated, injected or directed into the target of the survey
(depending upon the tool involved), subsequently received, and then analyzed to determine
the effects which the target environment had upon the pulse. The analysis is then
interpreted to provide a description of that environment. The steps are repeated for
thousands upon thousands of such pulses to generate a permanent record or log, typically
as a function of depth within the borehole.
[0003] In many cases, an ideal pulse would be one cycle or waveform long. In practice, real
world physical systems cannot be started and stopped quite so sharply. A common problem
is the "ring-down" of the transducer after the electrical impulse has been discontinued.
Being a physical system, the acoustical inertia of the transducer will cause it to
oscillate for a short period thereafter. This increases the pulse width, which not
only makes analysis of the subsequently received returning acoustical signal more
difficult, but can also reduce the repetition rate at which distinct pulses can be
generated.
[0004] Several known techniques for improving transducer performance include sound absorbing/damping
materials physically coupled to the transducer and electrical circuits which follow
the initial driving pulse with a reverse polarity pulse of lower magnitude to dampen
the residual oscillation in the transducer. For physical damping, which also helps
to improve the signal-to-noise ratio, it is common to mount a piezoelectric transducer
on a sound absorbing backing such as high temperature rubber impregnated with tungsten
cuttings. Such tungsten loaded rubber closely matches the acoustic impedance of the
transducer, thereby reflecting very little energy back into the transducer and accordingly
shortening the transducer ring-down time.
[0005] As far as is known, no backing has been described or is available in which all the
acoustical energy appearing on the back of the transducer disappeared into the backing.
[0006] Thus, it is an object of the invention to construct a transducer backing which appears
to be acoustically infinitely deep, or in other words, which will return no energy
to the back of the transducer.
[0007] It is a further object of the invention to construct a transducer backing which
should be inexpensive, uncomplicated, and sufficiently compact to be usable in virtually
any such borehole acoustic logging application.
[0008] Therefore the present invention provides an acoustical transducer provided with
a sound attenuating backing for decreasing the ring-down time of the transducer, comprising
a tapered backing having a face portion and a wall portion said, face portion being
acoustically coupled to the transducer, said backing having an acoustical impedance
substantially the same as the transducer, and the taper of said backing terminating
substantially in a line or point on the backing substantially opposite the face, and
comprising sound absorbing means acoustically coupled to the wall portion of said
backing.
[0009] Advantageously said backing is formed of substantially the same material as the transducer,
the backing and the transducer being both ceramic material, the transducer being a
poled ceramic and said backing being an unpoled ceramic of the same type as the transducer.
More advantageously said backing has the form of a wedge. In another advantageous
embodiment in accordance with the invention the wedge has the shape of a cone, the
cone being curved and tapered in the shape of a Wood's Horn. Furthermore the sound
absorbing means is advantageously a tungsten loaded rubber, the wall portion of the
backing being embedded in the sound absorbing means.
[0010] Furthermore the present invention provides a method for attenuating sound from the
back of such a transducer for decreasing the ring-down time of the transducer, comprising
the steps of:
- absorbing substantially all the acoustic energy from the back of such an acoustical
transducer in a tapered backing having a face portion and a wall portion, the face
portion being acoustically coupled to the transducer, the backing having an acoustical
impedance substantially the same as the transducer, and the taper of the backing terminating
substantially in a line or point on the backing substantially opposite the face; and
- attenuating the sound in the backing with a sound absorber acoustically coupled
to the wall portion of the backing.
[0011] It is noticed that although a Wood's Horn type device has not heretofore been used
in acoustics, historically it is known from and has been used in optics. Its function
is to reduce extraneous reflections in an optical device by trapping, attenuating,
and ultimately extinguishing any light which enters it. The optical Wood's Horn is
typically made of glass, coated on the inside with carbon black, and otherwise hollow.
However, the sound attenuating backing for borehole logging applications is significantly
different in several respects. First, it is not hollow. Secondly, the sound absorption
does not all take place at the back wall of the Wood's Horn backing. Instead, by essentially
matching the impedances at the back wall interface, the largest portion of the acoustical
energy at each encounter with the wall is encouraged to exit the Horn into a highly
attenuating medium on the outside thereof. Thus, the Horn is preferably embedded in
tungsten loaded rubber sound absorbing material, and the Wood's Horn itself is either
a solid or liquid of the desired shape located within the sound absorbing material.
[0012] Thus, rubber impregnated with tungsten cuttings has been used, for example, in prior
art borehole televiewers to absorb the sound on the back of a piezoelectric acoustical
transducer. In the prior art, the transducer was simply attached to a piece of the
sound absorbing rubber of about the same size as the transducer itself. While acoustical
energy received by the sound absorbing rubber from the back of the transducer was
quickly attenuated in the rubber, the acoustic mismatch between the transducer and
backing would cause some of the energy to reflect back and forth within the transducer,
and in so doing extended the transducer ring-down time.
[0013] Advantageously said transducer and said method are suited to the widest possible
utilization in acoustical type borehole logging applications.
[0014] These and other objects and advantages of the invention will be apparent from the
following description, the accompanying drawings, and the appended claims.
Fig. 1 is a somewhat figurative illustration showing a borehole televiewer type logging
tool which incorporates a sound attenuating backing according to the present invention,
and located within a borehole within an earth formation;
Fig. 2 is a vertical sectional view through the transducer and backing combination
shown generally in phantom in Fig. 1;
Fig. 3 is a cross-sectional view taken generally on line III-III in Fig. 2; and
Fig. 4 is a graphical illustration showing representative reflection/attenuation paths
within the sound attenuating backing.
[0015] With reference to the drawings, a sound attenuating backing for decreasing the ring-down
time of an acoustical transducer in borehole logging applications, and a method therefor
according the present invention, will be described. Fig. 1 shows, somewhat figuratively,
a borehole televiewer 10 suspended in a borehole 14 penetrating earth formations 16.
A motor 17 within the televiewer 10 drives a shaft 18 which in turn rotates a cylindrical
transducer assembly 20 located on the bottom of the borehole televiewer 10.
[0016] As shown in greater detail in Figs. 2 and 3, the transducer assembly 20 includes
a transducer 25 to which thin front and rear metallic electrodes 26 and 27 are attached
in conventional fashion. In the illustrated preferred embodiment, the transducer is
a 1.0 inch (2.5 cm) diameter, 1 megahertz transducer of poled lead metaniobate ceramic,
available from Keramos, Inc., Indianapolis, Indiana under the name Kezite K-81 (registered
trademark). The transducer has a mechanical Q measured in the thickness mode which
is less than 15.
[0017] Attached to the rear electrode 27 of transducer 25 and thus acoustically coupled
thereto is a backing 30 shaped in the form of a Wood's Horn. Backing 30 has a face
31 in contact with electrode 27 and a tapered, curvilinear wall portion 32 extending
rearwardly from the face 31 and ultimately terminating in a point 33 substantially
opposite the face 31. Transducer 25 and the backing wall 32 are, in turn, embedded
in a rubber sound absorber 35. Backing 30 is also made from Kezite K-81 ceramic but
is unpoled (i.e., does not have piezoelectric properties).
[0018] It will be immediately seen that there are several favorable phenomena at work here.
First, the acoustic properties of the backing 30 and the transducer 25 are as closely
matched as possible. The Wood's Horn shape of the backing wall 32 reduces multiple
scattering in the backing 30 as much as possible. Additionally, the rubber sound absorber
35 is a high temperature tungsten loaded rubber which also has an acoustical impedance
nearly identical to that of the transducer 25 and backing 30, and further is highly
attenuating to sound energy which passes into it. A suitable such loaded rubber for
absorber 35 may be, for example, any high temperature silicone elastomer.
[0019] Thus, as previously described, the backing 30 and absorber 35 combination provides
essentially no opportunity for acoustical energy coupled thereto from the transducer
25 to find its way back to the transducer. That is, since the impedances of the materials
are matched, the interfaces between and among the several materials are almost invisible
to the sound energy, so very little is reflected. What reflections do occur in the
Wood's Horn backing 30 cause the sound energy to travel deeper into the Horn and not
to return to the transducer 25. Energy which enters the absorber 35 is quickly damped.
Thus maximum attenuation for the transducer is provided, substantially shortening
the ring-down time characteristics thereof.
[0020] While a Wood's Horn configuration for the backing is preferred, it will be immediately
apparent that other variations on the invention will conceptually and physically provide
remarkably improved attenuation. For example, a wedge-shaped backing is much easier
and less expensive to fabricate than one shaped as a Wood's Horn. Compared to the
simple prior art block of elastomeric sound absorbing material, the attenuating characteristics
of a suitably shaped wedge are extremely favorable. Such a wedge would terminate in
most cases in a line defined by the intersection of the major planar surfaces on the
rear of the wedge-shaped backing. The important concepts here are the absence of a
rear surface area which might reflect acoustic energy directly back to the front face
of the backing, and a small reflection angle that maximizes the number of reflections
suffered by the acoustic beam within the wedge before it can return to the front face.
Thus, as used in the present specification and claims, the phrase "terminating substantially
in a line or point" is to be taken to mean that there is not a surface, as such, where
the backing terminates opposite the transducer. Such a line may be straight or curved,
and the term "point" would be taken to be the extreme situation of a line infinitely
short.
[0021] Fig. 4 is an example of such a wedge 40 that is easy to machine, yet very efficient
in reducing unwanted energy returned to the transducer. The representative ray or
beam path 45 experiences six reflections before being reversed at the seventh to return
back to the face 31 -- a total of thirteen reflections in all. In the limit in which
the ray path approximation is valid, the total energy returned, assuming even a large
reflection coefficient of 0.1 (i.e., a poor match between the wedge and the tungsten-rubber
absorber in which it is embedded), will be reduced by a factor of 10⁻¹³. In common
usage at normal frequencies (1 MHz - 200 kHz) beam spreading and beam front curvature
will cause a fraction of the energy to turn back before seven reflections. However,
in practice reflection coefficients lower than 0.1 can be achieved with the result
that the reflected signal returning to the transducer is reduced several orders of
magnitude.
[0022] Effectively, this means that with the present invention, after a few reflections
the backing contributes no signal. Therefore, the only limiting factors remaining
are the quality of the attachment of the transducer to the backing, and the electronic
matching and signal-to-noise ratio (which should be better than 60 dB).
[0023] A progression of suitable shapes, all conceptually embraced by the present invention,
is thus now suggested. For example, intermediate the wedge shape and the Wood's Horn
would be a cone shape. Thus, within the context of the present invention, a cone may
be considered to be a special case of a wedge, and the Wood's Horn a special case
of a cone, all terminating substantially in a line or point opposite the face of the
backing at the transducer.
[0024] As may be seen, therefore, the present invention provides numerous advantages. Principally,
it significantly shortens the ring-down time of a transducer utilized in borehole
logging applications while maintaining a very good signal-to-noise ratio for the transducer.
With the present invention, a short pulse length of only about 2 cycles can be achieved
without reduction in the transducer signal-to-noise ratio. The acoustically matched
backing and absorber, and the special shape of the backing, while of convenient finite
physical length, appear acoustically to be virtually infinitely deep, so that essentially
none of the acoustic energy coupled thereto from the transducer returns to the transducer.
[0025] A further advantage of the matched backing is the much lower Q, hence increased bandwidth
of a transducer so backed. That is, by exactly matching the backing impedance to the
transducer the length of the transducer is effectively extended to infinity. The transducer
can then be electronically tuned to operate efficiently over a broad band of frequencies.
In tests, for instance, a 1 MHz transducer mounted on such a wedge has been run between
1 MHz and about 200 kHz with no noticeable change in peak voltage of the detected
signal.
[0026] Due to this finite size, the invention can be readily and easily incorporated into
many different types of borehole logging tools, such as the borehole televiewer illustrated
generally in Fig. 1, in which acoustical transducers are utilized in a pulsed mode.
The invention is fully functional in borehole environments, is inexpensive, uncomplicated,
durable, versatile, relatively easy and inexpensive to manufacture and implement,
and thus readily suited to the widest possible utilization in such borehole logging
applications.
[0027] While the methods and forms of apparatus herein described constitute preferred embodiments
of this invention, it is to be understood that the invention is not limited to these
precise methods and forms of apparatus, and that changes may be made therein without
departing from the scope of the invention.
1. An acoustical transducer provided with a sound attenuating backing for decreasing
the ring-down time of the transducer, comprising:
- a tapered backing having a face portion and a wall portion, said face portion being
acoustically coupled to said transducer, said backing having an acoustical impedance
substantially the same as said transducer, and the taper of said backing terminating
substantially in a line or point on said backing substantially opposite said face;
and
- sound absorbing means acoustically coupled to said wall portion of said backing.
2. The transducer as claimed in claim 1 wherein said backing further comprises a backing
formed of substantially the same material as said transducer.
3. The transducer as claimed in claim 2 wherein said transducer is formed of poled
ceramic material and said backing is formed of unpoled ceramic material of the same
type as said transducer.
4. The transducer as claimed in claim 1, 2 or 3 wherein said backing is shaped in
the form of a wedge.
5. The transducer of claim 4 wherein said wedge has the shape of a cone.
6. The transducer of claim 5 wherein said cone is curved and tapered in the shape
of a Wood's Horn.
7. The transducer as claimed in claim 1 wherein said sound absorbing means further
comprises a tungsten loaded rubber.
8. The transducer as claimed in claim 1 or 7 wherein said sound absorbing means further
comprises means embedding at least said wall portion of said backing in said sound
absorbing means.
9. The transducer as claimed in any one of the preceding claims for use in borehole
logging applications.
10. A method for attenuating sound from the back of such a transducer for decreasing
the ring-down time of the transducer, comprising the steps of:
- absorbing substantially all the acoustic energy from the back of such an acoustical
transducer in a tapered backing having a face portion and a wall portion, the face
portion being acoustically coupled to the transducer, the backing having an acoustical
impedance substantially the same as the transducer, and the taper of the backing terminating
substantially in a line or point on the backing substantially opposite the face; and
- attenuating the sound in the backing with a sound absorber acoustically coupled
to the wall portion of the backing.
11. The method as claimed in claim 10 using the transducer as claimed in any one of
the claims 1 to 9.
12. Acoustical transducer provided with a sound attenuating backing for decreasing
the ring-down time of the transducer substantially as described in the description
with reference to the appended drawings.
13. Method for attenuating sound from the back of a transducer for decreasring the
ring-down time of the transducer substantially as described in the description with
reference to the appended drawings.