U.S. patent application number 12/925658 was filed with the patent office on 2011-04-28 for acoustic imaging of objects in optically opaque fluids.
Invention is credited to Curtis F. Osterhoudt, Cristian Pantea, Dipen N. Sinha.
Application Number | 20110096622 12/925658 |
Document ID | / |
Family ID | 43898335 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110096622 |
Kind Code |
A1 |
Sinha; Dipen N. ; et
al. |
April 28, 2011 |
Acoustic imaging of objects in optically opaque fluids
Abstract
The present invention is a method and an apparatus that can
image objects immersed in optically opaque fluids using ultrasound
in a confined space and in a harsh environment. If the fluid is not
highly attenuating at frequencies above 1 MHz, where commercial
ultrasound scanners are available, such scanners can be adapted for
imaging in these fluids. In the case of highly attenuating fluids,
such as drilling mud, then a low frequency collimated sound source
is used.
Inventors: |
Sinha; Dipen N.; (Los
Alamos, NM) ; Pantea; Cristian; (Los Alamos, NM)
; Osterhoudt; Curtis F.; (Los Alamos, NM) |
Family ID: |
43898335 |
Appl. No.: |
12/925658 |
Filed: |
October 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61255014 |
Oct 26, 2009 |
|
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Current U.S.
Class: |
367/7 |
Current CPC
Class: |
G01S 15/8934 20130101;
G01S 15/8913 20130101; G03B 42/06 20130101; G01S 15/895
20130101 |
Class at
Publication: |
367/7 |
International
Class: |
G03B 42/06 20060101
G03B042/06 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0002] This invention was made with government support under
Contract No. DE-AC52-06NA25396, awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. An apparatus for imaging objects in opaque fluids comprising: at
least one transducer for generating a directed, ultrasonic sound
beam having a chosen frequency and also for receiving reflected or
scattered sound from the target object; a scanning head; a
protective casing placed around the scanning head; coupling gel
placed between the scanner head and the protective casing; an
acoustic imaging device; and an electronic circuit to receive image
data from the imaging device and display it on a screen.
2. The apparatus of claim 1, wherein the protective casing is a
metal pipe.
3. The apparatus of claim 1, wherein the scanner is the wobbler
type.
4. The apparatus of claim 1, wherein said electronic circuit
comprises a computer, for example including a processor, memory,
clock, display driver, and other components:
5. The apparatus of claim 1, wherein the imaged object includes
tools.
6. The apparatus of claim 1, wherein the depth of field of the
image is approximately 1 foot.
7. The apparatus of claim 1, wherein the resolution of the image is
approximately 2 mm.
8. The apparatus of claim 1, wherein the at least one transducer is
an array of transducers that can be electronically scanned in a
phased array manner.
9. An apparatus for imaging objects in opaque fluids comprising; a
collimated source; an acoustic imaging device; and an electronic
circuit to receive image data from the imaging device and display
it on a screen.
10. The apparatus of claim 9 wherein the collimated source is
broadband and has a frequency range of 50 kHz-1 MHZ.
11. The apparatus of claim 9, wherein the collimated source is made
using Fluorinate 43 or any other liquid in the same family.
12. The apparatus of claim 9, further comprising: a ring transducer
around the collimated source.
13. A method for imaging of an object within opaque fluids
comprising the steps of: generating a directed and collimated
ultrasonic sound wave having a chosen frequency, wherein a low
frequency ultrasonic sound wave is created by a parametric array by
combining two high frequency sound waves in an acoustic nonlinear
fluid; symmetrically placing a receiver transducer close to a
source transducer; directing the low frequency wave is directed
toward the object to be imaged; and detecting the reflected and
scattered signal from the object; and electronically analyzing the
reflected and scattered signal to produce an image of the
object.
14. The method of claim 13, where the acoustic nonlinear fluid is
Fluorinate 43.
15. The method of claim 13, further comprising obtaining the travel
time of the signal from the target object back to the receiver
transducer next to the source transducer.
16. The method of claim 15, wherein the image is constructed from
the travel time information.
17. The method of claim 13, further comprising the step of using a
collinear measurement system to provide an image of the object.
18. The method of claim 13, wherein the image may be produced by
scanning the object in 2D arcs.
19. The method of claim 13, wherein the image may be produced by
linear x-y scanning.
20. The method of claim 13, wherein the opaque fluid includes one
of the following: bubbling liquids, motor oil, water and thick mud.
Description
PRIORITY STATEMENT
[0001] This application claims priority under 35 U.S.C. .sctn.119
from U.S. Provisional Patent Application No. 61/255,014, filed on
Oct. 26, 2009, the disclosure of which is incorporated herein by
reference to the fullest extent consistent with the present
disclosure.
BACKGROUND OF INVENTION
[0003] This invention relates to ultrasonic imaging. More
particularly, the present invention relates to obtaining ultrasonic
images of items that are immersed in optically opaque and often, in
addition, acoustically attenuating fluid media.
[0004] There are situations where it is necessary to image objects
that are in an environment, such as motor oil or crude oil, or mud
or other optically opaque media. For example, there is a need to
inspect fuel tanks of ships or large boats that may hide
contrabands. Such inspection is typically carried out using a long
stick to feel through the fuel compartment to locate objects.
Another example is to address the need in the oil/gas industry to
visualize the down-hole drilling environment where it is necessary
to monitor drill pipe, casing, and drilling collars and also to
identify and then retrieve dropped testing tools, logging tools,
and tubing. Currently, there is not an efficient and reliable
method of accomplishing this.
[0005] The current state-of-the-art is to lower a lead impression
disc down the pipeline and to then try to get an imprint of the
dropped object on the lead disc. The impression produced by this
method is highly inadequate and does not provide much depth of
field information. This method does not result in a detailed image
of any kind. It is very difficult to tell what the dropped object
is from the crude impression obtained by this method.
[0006] Another method recently introduced is to lower a regular
optical camera down the pipeline but this requires replacing the
existing drilling fluid with clear water that takes several days in
a regular well-bore and lots of adjustments before a useful image
can be captured. Infrared imaging has also been explored and it has
the ability to image a short distance through water and some low
API crude oil but no imaging is possible through heavy oil or
drilling mud. The environment encountered in well-bores for oil/gas
exploration is rather harsh (corrosive fluids, high pressure and
temperature) and the space available is confined, typically the
size of the bore hole. It is also important to be able to obtain
some depth of field information to understand the nature of the
object of just getting a surface image.
[0007] The present invention provides the much-needed ability to
provide clear and identifiable images of objects in optically
opaque and acoustically attenuating fluids, such as drilling mud,
heavy crude oil, brine/oil mixture, and bubbly fluids.
SUMMARY OF INVENTION
[0008] An aspect of the present invention is a method and an
apparatus that can image objects immersed in optically opaque
fluids using ultrasound in a confined space and in a harsh
environment. If the fluid is not highly attenuating at frequencies
above 1 MHz, where commercial ultrasound scanners are available,
such scanners can be adapted for imaging in these fluids. In the
case of highly attenuating fluids, such as drilling mud, the
requirements for the imaging system are a lot more stringent as
this requires a low frequency collimated sound source.
[0009] In one aspect of the invention the apparatus for imaging an
object in opaque fluids hereof may include: at least one transducer
for generating a directed, ultrasonic sound beam having a chosen
frequency and also for receiving reflected or scattered sound from
the target object. The transducer is mechanically scanned to direct
the sound beam in a two-dimensional pattern using a mechanical
wobbler such that an image of the target object can be produced
from the reflected signal through conventional electronics.
[0010] In another aspect of this invention the apparatus for
imaging an object in opaque fluids hereof may include: an array of
transducers that can be electronically scanned in phased array
manner to direct an ultrasonic beam and electronically scan the
beam over of an area. The same array is used for receiving the
reflected and scattered signal from the object. This signal is
processed to create an image of the object.
[0011] In a further aspect of this invention the apparatus for
imaging objects in opaque fluids in harsh environment hereof may
include: at least one transducer for generating a directed,
ultrasonic sound wave having a chosen frequency and also for
receiving reflected or scattered sound from the target object. The
transducer is protected inside a metal tube so the imaging is
performed through a metal plate.
[0012] In yet another aspect of the present invention the method
for imaging of an object within opaque fluids hereof may include
the steps of: generating a directed and collimated ultrasonic sound
wave having a chosen frequency, wherein a low frequency ultrasonic
sound wave is created by a parametric array by combining two high
frequency sound waves in an acoustic nonlinear fluid, such as
Fluorinert 43, and this low frequency wave is directed toward the
object to be imaged; detecting the reflected and scattered signal
from the object via a symmetrically placed receiver transducer
close to the source; and electronically analyzing the signal to
produce an image of the object.
[0013] Benefits and advantages of the present invention include
providing a quick way image objects. Although the examples below
use the present invention in the oil industry, the ability to image
objects in opaque fluids may be applied to many industries,
particularly the medical arena.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a schematic of the instrument including the
scanner head inside a protective metal tube.
[0015] FIG. 2a is a picture of pacman shaped metal object and FIG.
2b. is a still image from a real-time video obtained from the
ultrasound scanner of a pacman shaped object immersed in motor
oil.
[0016] FIG. 3a is a picture of a metals spring and FIG. 3b. Still
image made from a real time video obtained from the ultrasound
scanner of the spring in motor oil.
[0017] FIG. 4 is a schematic representation of the parametric
source using FC43 as the nonlinear medium and the ring-shaped
receiver.
[0018] FIG. 5 is a schematic of the imaging system using the
parametric source.
[0019] FIG. 6 shows the time-of-flight measurement used to produce
the image of the object shown in the inset.
[0020] FIG. 7a shows a picture of the object imaged using the
parametric source and the ring transducer. FIG. 7b is an image of
the object shown in FIG. 7a reconstructed from the time-of-flight
measurement in water at 550 kHz.
[0021] FIG. 8a is an image of the object shown in FIG. 7a
reconstructed from the time-of-flight measurement in water-based 10
ppg mud at 550 kHz. FIG. 8b shows the image of the object shown in
FIG. 7a reconstructed from the time-of-flight measurement in
water-based 100 ppg mud at 175 kHz.
[0022] FIG. 9 shows a demonstration of depth of field of the
imaging system where a single line cross-section of multiple
objects at various depths is shown.
DETAILED DESCRIPTION
[0023] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing a
particular embodiment of the invention and are not intended to
limit the invention thereto.
[0024] The present invention includes an apparatus and method for
acoustic imaging in optically opaque fluids with a varied amount of
acoustic attenuation from moderately low (e.g., brine mixed with
oil or light crude) to very high (e.g., drilling mud). Imaging in
such media is very challenging because of the conflicting
requirements that need to be satisfied. For example, higher
attenuation means shorter penetration depth and the attenuation
usually varies as square of the frequency. Therefore, it requires
lower frequency to obtain deeper penetration into the fluid.
However, lower frequency means higher beam spread that affects the
spatial resolution in the image. Lower frequency also requires
larger size transducers and this is difficult to accommodate if the
measurement needs to be made in a bore hole or in any other
confined space. Moreover, for a higher quality image in terms of
higher signal-to-noise ratio a higher frequency bandwidth is
required. Unfortunately, a lower frequency transducer has a lower
bandwidth. Based on these competing requirements, an imaging system
that is compact but provides a low frequency narrow beam is
required. In moderately low attenuating fluid, sound can penetrate
the fluid several tens of centimeters even at frequencies as high
as 3 MHz. Commercially available ultrasound scanners operate in the
frequency range above 1 MHz. However, the scanning head (the sensor
part) is very fragile and cannot be used in harsh environments,
such as where there is very high pressure. Developing an imaging
system capable of withstanding such environment can be very
expensive. In one aspect of the present invention the existing
commercially available scanning head is encased in a sturdy metal
pipe for protection but still performs its imaging functions even
when the sound beam from the scanning head has to go through a
metal plate.
[0025] FIG. 1 schematically shows one embodiment of the present
invention. A scanner head is placed inside a metal protective
casing 2. The image of the intended object 3 is shown on display 4.
In one example, the scanning head of a commercial ultrasound
scanner (the wobbler type) operating at 3.5 MHz was placed inside a
steel pipe with a wall thickness of 2.5 mm and diameter 5 cm.
However, any other type of ultrasound scanner may be used,
including the phased array kind. In the example described above,
the space between the front of the scanner head and the faceplate
of the pipe through which the sound beam exits was filled with
coupling gel. One of ordinary skill in the art would likely
hypothesize that the arrangement of the present invention would
severely degrade the imaging capability of the scanning system for
a variety of reasons, such as multiple reflections within the plate
thickness, refraction of the beam through the plate, and impedance
mismatch between the metal plate and the liquid in which the object
is immersed. However, the results of experiments with the present
invention indicate that a useful image can be obtained and the
metal pipe provides protection to the scanning head.
[0026] A photo of a metal object shaped as a Pacman that is 10 cm
in diameter is shown in FIG. 2a. In one experiment, the Pacman was
immersed in 10W-40 motor oil inside a 15 cm diameter pipe and was
placed at the bottom of the pipe. One embodiment of the present
invention was then used to image the object. FIG. 2b is a still
image captured from a real-time video from the scanner output. As
FIG. 2b shows, the object can indeed be imaged using the present
invention.
[0027] FIG. 3a shows a 7.5 cm long metal spring as the target
object to be imaged. The purpose of using the spring is to
demonstrate the high spatial resolution possible with the method of
the present invention. The spring is immersed in motor oil similar
to the Pacman shown in FIGS. 2a and 2b. FIG. 3b is a still image
captured from a real-time video of the ultrasound scanner output
and it shows that the spring can be quite clearly imaged even
through the pipe wall 2 that the scanner head 1 is encased in. The
quality and resolution of the image is excellent.
[0028] Several experiments were conducted in which measurements
were made in water and also in drilling mud using the embodiment
described in FIG. 1. Imaging in water was quite straightforward and
the sound penetration in water was several feet. However, imaging
in drilling mud was very difficult and the penetration depth was
less than 2 cm. The object 3 needed to be in very close proximity
with the scanner head 1. Although this approach works well for low
attenuating fluid, it is not practical for imaging objects in
drilling mud or other highly attenuating fluid. This is because the
high frequency (3.5 MHz) of the scanner that does not penetrate
beyond a few cm in the highly attenuating mud. It is not possible
to simply substitute the high frequency transducer used in this
scanner with a much lower frequency one as the beam spread will be
very large, which will severely limit the spatial resolution.
Typically, the frequency needs to be lower than 600 kHz to be
practical in terms of providing high quality image with sufficient
depth of field (e.g., >15 cm). Such low frequency transducers
will be quite large (both diameter and thickness) and heavy and the
simple mechanism of a typical wobbler would not work. It is also
not possible to devise a phased array imaging system that is small
enough at such a low frequency because of the size of the
transducer elements needed for the array.
[0029] This invention overcomes all these difficulties by using a
parametric sound source that is highly compact but can provide a
highly collimated beam at frequencies from as low as 50 kHz to 1
MHz. The design of one embodiment of the sound parametric source is
shown schematically in FIG. 4. It consists of a piezoelectric disc
7. The disc 7 may be approximately 5 mm in diameter. The disc 7 is
air-backed and is enclosed in a hollow cylinder 6, which may be
filled with a liquid (Fluorinert 43) that has high acoustic
nonlinearity (beta .about.7.5). The cylinder 6 may be made of any
solid. Other liquids within the Fluorinert family can also be used.
Fluorinert liquid (FC 43) has a very low sound speed (.about.650
m/s), which is a fraction that in water. If two high frequency
(f.sub.1 and f.sub.2) sound waves 8 are allowed to propagate in
this liquid in a collinear manner, these two frequencies mix in
this nonlinear medium and generate both a difference frequency and
a sum frequency. The sum frequency gets attenuated very quickly as
the sound attenuation is proportional to square of the frequency.
The two primary sound waves of high frequency (typically 1-10 MHz)
are also rapidly attenuated in the liquid. This leaves a difference
frequency sound beam to travel while maintaining the collimation of
the primary waves. For example, if a sound wave of 9 MHz is mixed
with another wave at 8.5 MHz, the difference frequency sound wave
will have a 500 kHz frequency but it will also have the beam spread
of the 8.5 MHz primary wave. This method is called parametric
generation of low frequency collimated sound. The mixing length in
the liquid for Fluorinert family of liquids is limited by what is
frequently called the shock length that is related to the primary
frequency and the beta of the liquid. The higher the frequency
shorter is the shock length. The shock length represents a maximum
length; beyond this length if too much power is applied to the
transducer, the system will produce shock wave or higher harmonics.
The shock length therefore limits the power generated in the
difference frequency wave. Typically, the strength of the
difference frequency signal is more than 1000 times less than the
strength of the primary signal. With a collimated beam this
difference can be much greater.
[0030] Another factor that limits the power of the difference
frequency signal generated is the frequency step down factor. If
the ratio of the difference frequency to the primary frequency is
more than 20 then the signal strength of the difference frequency
goes down rapidly as a function of the step down ratio. Therefore,
it is important to select the proper operating factors. If the
difference frequency required is very low, the primary frequencies
need to be lower as well. For example, the shock length at 9 MHz in
FC43 is approximately 1 cm and thus the mixing length must also be
approximately 1 cm. As a result, a compact source is developed. For
example, the length of the FC43-filled cylindrical tube used in one
embodiment of this invention was 2 cm, and it is possible for it to
be been half that length. However, the slightly longer length used
was to accommodate lower frequencies. This geometry allowed the
generation of a difference frequency collimated signal with
frequency that could be varied from a low of 150 kHz to 3 MHz. This
is an extremely high frequency bandwidth and cannot be obtained by
any conventional means.
[0031] A complete imaging system is described schematically in FIG.
5. This is for illustration purposes only and may have many
variations. This design shown in FIG. 5 follows closely the design
of a traditional wobbler type of imaging system where the sound
source is wobbled with mechanical drive to allow it to scan an arc
shaped path. However, this design can be easily converted to a
linear x-y scanning system as well. The mechanical scanning system
allows the sensor head (the parametric source) to move in two
orthogonal directions covering a 2-dimensional area. The sound that
is scattered or reflected from a target is detected by a ring
transducer that is placed around the mouth of the sound source.
This makes the system collinear. In another embodiment, small
transducers may be placed around the source and connected together
in parallel to act as the receiver. The receiver is selected for
its ability to respond in the low frequency so that it can detect
the difference frequency signal. The two high frequency primary
signals in the form of tone bursts are generated by two separate
DDS (direct digital synthesizers) modules at the same time and then
combined before being amplified by a power amplifier and then fed
to the source transducer which in the pictured embodiment is an
air-backed piezoelectric disc. The amplitude modulating a carrier
signal can also be used instead of combining two separate signals.
For better depth determination, the broadband characteristic of the
source is utilized.
[0032] In one experiment, a fixed frequency tone-burst signal is
combined with a frequency chirp signal. For example, a fixed
frequency signal at 9 MHz and of 100 .mu.s duration is combined
with a chirp signal that may vary between 8.4 MHz and 8.8 MHz and
of 100 .mu.s duration. This generates a difference frequency chirp
signal between 200-600 kHz. The chirp method provides significantly
higher spatial resolution for depth determination. In experiments,
it was determined that a single piezoelectric disc works better
than the conventional broadband commercial transducers, however
either may be used. The received signal is also amplified before
being sent to a DSP (digital signal processor module). The DSP
carries out all the data analysis and is controlled by a
microcontroller. The processed data is displayed on a screen.
[0033] The data processing and image construction consists of
determining the time-of-flight signal at each physical point of
scan on a 2-D surface. This is shown in FIG. 6. The inset shows a
photo of the object being imaged. The location of the return
tone-burst from the various depths of the object provides the
information for image reconstruction. The beginning of the tone
burst is used to determine the depth information. However, for a
more accurate image reconstruction, a cross-correlation is
performed where the peak in the correlation identifies the
time-of-flight information. A frequency chirp approach allows
significantly better time-of-flight determination through the
cross-correlation approach as the signal-to-noise improvement is
proportional to the product of signal duration and the frequency
bandwidth. The DSP allows such cross-correlation calculations in
real-time.
[0034] FIG. 7a shows a picture of the sample object being imaged.
The object is an aluminum block with machined sections of different
depths to provide a target for imaging. The length is 7 cm, the
height is 6 cm, and the width is 4 cm. The reconstructed image from
the time-of-flight data is shown in FIG. 7b. The measurement shown
is made with the object immersed under water and at a difference
frequency of 550 kHz.
[0035] The imaging under water-based mud of 10 ppg (pounds per
gallon) consistency is presented in FIGS. 8a & 8b for
difference frequencies 550 kHz and 175 kHz, respectively. The image
of the object is faithfully reproduced even at such low
frequencies. This is possible because of the collimated beam
produced by the parametric array method of sound beam
generation.
[0036] Finally, the depth of field measurement capability of this
imaging system is demonstrated by stacking multiple objects on top
of each other (not shown) inside an 8 inch diameter filled with
drilling mud of 10 ppg consistency. In one experiment, the objects
were at the bottom and the source was placed 6.7 cm from the top
surface. Therefore, the total distance between the source and the
bottom surface on which the object is placed is 23 cms. Only a
single line scan along the object is shown for clarity in FIG. 9.
The y-axis of the plot is travel time and so the longest travel
time indicates the farthest point and in this case the bottom
surface of the target. Measurements were made in oil-based mud as
well with similar success. The highest density mud used was 15 ppg
for imaging.
[0037] This invention shows that it is indeed possible to image
objects immersed in highly attenuating and optically fluids, such
as drilling mud and good depth of field measurement is possible.
There can be many other applications of this invention as it can be
used anywhere where it is important to have a low frequency
collimated beam for greater penetration and imaging, such a
biomedical applications, contraband detection, imaging under opaque
fluids and harsh environment in general etc.
[0038] The invention is not restricted to the illustrative examples
described above. Examples are not intended as limitations on the
scope of the invention. Methods, apparatus, compositions, and the
like described herein are exemplary and not intended as limitations
on the scope of the invention. Changes therein and other uses will
occur to those skilled in the art.
* * * * *