SUBSTRATE PENETRATING ACOUSTIC SENSOR

[0001] The U.S. Government may have certain rights in this invention.

FIELD OF THE INVENTION

[0002] The present invention relates to a substrate penetrating, sub-surface

monitoring acoustic sensor.

BACKGROUND OF THE INVENTION

[0003] Subterranean or other in-substrate acoustic sensors are known. Such devices

have been used for metrology or the classification of the subterranean environment, for

example, the identification of oil pocket location. Some such acoustic sensors have

included piezoelectric instruments to detect subterranean sounds.

[0004] Conventional accelerometer sensors utilize a seismic mass that floats freely

except for the attachment at a piezoelectric element. This configuration ensures that the

device will be insensitive to vibrations, including those from sound waves, that strike

the device off the primary axis, i.e., the line extending between the centers of gravity of

the mass and the piezoelectric element. Accelerometers for metrology purposes are

marketed by promoting this insensitivity to off-axis vibration, for example, that the

device has a transverse (off-axis) sensitivity of 5% or less. It would be useful to have an

acoustic sensor that could capture longitudinal as well as transverse vibrations for the

acquisition of additional sound signal energy.

SUMMARY

[0005] The invention relates to an acoustic sensor suitable for insertion into and use

in a subterranean or other in-substrate environment and method of using such a sensor.

In an exemplary embodiment of the invention, the acoustic sensor includes a

compressed piezoelectric element and a mass coupled to the device at a transverse

energy coupler. This coupling allows the acoustic sensor to capture acoustic energy

impacting both longitudinally and transversally for sound acquisition in three

dimensions.

[0006] These other features of the invention can be better understood based on the

following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a cross-sectional view of an acoustic sensor constructed in

accordance with the invention.

[0008] FIG. 2 is a partial cross-sectional view and FIG. 3 is a cross-sectional view of

the leading and trailing ends of the sensor of FIG. 1, respectively, taken along the same

cross-sectional plane of FIG 1.

[0009] FIG. 4 is a perspective view of the sensor of FIG. 1.

[0010] FIGs. 5 and 6 are cross-sectional views of other acoustic sensors constructed

in accordance with the invention.

[0011] FIG. 7 is a schematic view showing the acoustic sensor of FIG. 1 in use within

a substrate.

[0012] FIG. 8 is a cross-sectional view of an acoustic sensor constructed in

accordance with the invention.

DETAILED DESCRIPTION

[0013] The invention relates to an acoustic sensor that can be forcibly embedded in a

substrate and detect sound waves traveling through the substrate. The acoustic sensor

can be used with a variety of substrate materials, for example, soil, rock, sand, man-

made objects, and structures. The acoustic sensor has an accelerometer configuration

having a piezoelectric sensor. It can detect sound waves in three dimensions by

acquiring vibrations longitudinally, radially, and off-axis, with respect to a primary

longitudinal axis running length-wise through the sensor. The acoustic sensor can be

tailored in materials and dimensions to be sensitive to a wide range of sound

wavelengths.

[0014] Preferred embodiments of the invention will now be explained with

reference to the drawings, wherein like reference numbers indicate like features. FIG. 1

shows an acoustic sensor 10 which can be any size and can be made of many different

materials. The sensor 10 is generally axially symmetrical. The cross-sectional plane of

FIG. 1 extends through the axis of symmetry.

[0015] In a preferred embodiment, the sensor 10 is about 5 inches long from the tip

of a conical head 12 to the end of a tail piece 14 and is well suited for sensing sound

waves in the audible frequency range of about 5 Hz to about 20 kHz. By changing the

dimensions and/or materials of the acoustic sensor 10, it can be tailored to sense sound

waves of any frequency, e.g., from infrasonic frequencies relating to tectonic events to

ultrasonic frequencies relating to insect infestation.

[0016] The acoustic sensor 10 shown in FIG. 1 includes a conical head 12, a tail piece

14, and a housing 16, which are configured to form an exterior body to support sensors

within and to allow the acoustic sensor 10 to be forcibly embedded in a substrate 60

(FIG. 7), such as soil. The materials for the conical head 12, tail piece 14, and housing 16

should be suitable for the environment in which the acoustic sensor 10 is to be used,

e.g., subterranean earth or other substrate, and should be relatively stiff and sturdy. So

long as these features are incorporated into the conical head 12, tail piece 14, and

housing 16, no specific materials are required and those known in the art, e.g., steel,

aluminum, and titanium, can be used.

[0017] The conical head 12 can be connected to the housing 16 via threads 28. The

tail piece 14 can be connected to the housing 16 via threads 44. Alternatively, other

joining techniques can be used, such as welding, use of adhesives and others. Within

the conical head 12, tail piece 14, and housing 16, the acoustic sensor 10 includes a

piezoelectric element 18 and a seismic mass 20, which, together with electrodes 22

associated with the piezoelectric element 18, form an accelerometer sensor which can

detect vibrations from sound waves impacting the acoustic sensor 10. A sleeve bearing

36 in the tail piece 14 couples the mass 20 with the housing 16. The sleeve bearing 36

allows for linear motion of the mass 20 (longitudinally), while also coupling transverse

motions to the end of the mass 20.

[0018] The acoustic sensor 10 makes use of both on and off-longitudinal-axis

vibrations made by sound waves by coupling them from the device conical head 12

and/or housing 16 into the piezoelectric element 18, which reacts against the inertia of

the mass 20. While the acoustic sensor 10 can have up to 100% longitudinal sound

wave sensitivity, it can also have about 30% or greater transverse sound wave

sensitivity, which enables sound detection in three dimensions.

[0019] The electrodes 22 associated with the piezoelectric element 18 are electrically

coupled to one or more wires 40, which can be a bifilar wire having two conductors,

which runs the length of the housing 16, along the mass 20, to the tail piece 14. The

wire 40 transmits signals from the piezoelectric element 18 to a transmission means 64,

such as a cable or wireless transmitter, and thereby to a receiving means 62, such as a

computer or amplifier (FIG. 7). A region 34 is provided between the housing 16 and the

mass 20. This region 34 can be void of material or can be provided with means to

couple the housing 16 with the mass 20 so as to further transmit off-axis, transverse

vibrations to the mass 20. The coupling mechanism can be many different devices, such

as a sleeve, a shearable material such as an elastomeric compound, or one or more o-

rings.

[0020] FIG. 2 shows an expanded view of the conical head 12 and the portion of the

housing 16 with which it engages in accordance with the exemplary embodiment

shown in FIG. 1. The conical head 12 is coupled to the mass 20 with a compression bolt

24, via threads 26 and 32. The mass 20 can be many different materials with high

density and that allow rapid acoustical transmission so as to allow the mass 20 to be

considered a lump acoustical element. Preferably, the mass 20 is a tungsten alloy,

which has a density of about 18.5 g/cm ³ .

[0021] The compression bolt 24 should be made of a sturdy material, such as steel,

and can include an install notch 30, or other means, by which the compression bolt 24

can be secured within the mass 20. The compression bolt 24 is inserted through the

ring-shaped piezoelectric element 18, which can be, for example, lead zirconate titanate

(Pb[Zr _x TiI x]Os ₇ where x = 0.52, also known as lead zirconium titanate), which is a

ceramic perovskite material that develops a voltage difference across two of its faces

when compressed. Other piezoelectric materials, such as, for example, quartz crystal,

bismuth titanate, lead nickel niobate, and others, can be used also. When lead zirconate

titanate is used as the piezoelectric element 18, it can be modified to have a higher

dielectric constant, which is advantageous if the sensor is to be very small. Electrodes

22 are positioned on both sides of the piezoelectric element 18 for capturing charge

generated upon varying the compression of the element 18. After insertion through the

piezoelectric element 18, the compression bolt 24 can be screwed into the mass 20 via

the threads 32.

[0022] When the conical head 12 is attached to the housing 16 at threads 28, it also

engages the compression bolt 24 at threads 26. As the conical head 12 is attached to the

housing 16, tension is added to the compression bolt 24 and the piezoelectric element 18

is compressed 52 between the mass 20 and the conical head 12. Generally, about 5 MPa

(mega pascal) to about 40 MPa is sufficient compression 52 without being excessive,

with about 10 MPa to about 25 MPa being preferred. This compression 52 protects and

sensitizes the piezoelectric element 18 for operation of the acoustic sensor 10. The

compression 52 is designed to keep the piezoelectric element 18 in intimate acoustical

contact with the mass 20 and the conical head 12 and prevent the piezoelectric element

18 from going into tension at any time. The compression 52 couples the conical head 12

to the sensing features of the acoustic sensor 10 and enables sound detection from three

dimensions, e.g., longitudinally and off-axis relative to the length of the sensor 10. Too

much compression 52 can cause de-poling of the piezoelectric element 18, which would

prevent the element 18 from functioning. The amount of acceptable compression

depends on the material properties of the piezoelectric element 18.

[0023] FIG. 3 shows an expanded view of the tail piece 14 and the portion of the

housing 16 with which the tail piece 14 engages in accordance with the exemplary

embodiment shown in FIG. 1. The tail piece 14 engages the housing 16 at threads 44.

An attachment means 38 can be provided, for example in the form of wrench flats, for

engaging a tool for embedding the acoustic sensor 10 in a substrate. Proximate the

threads 44, the tail piece 14 includes a sleeve bearing 36, which supports the mass 20.

[0024] The sleeve bearing 36 is a transverse energy coupler and can be made of a

variety of materials, but is preferably a polyimide-polyamide blend. Alternatively, this

transverse energy coupler can be a metal, for example, bronze or cast iron, or plastic,

such as nylon, delrin, and polyethylene. Different materials can transmit or attenuate

acoustical energy differently and it may be desired to use specific sleeve bearing 36

materials for receiving certain acoustical wavelengths. Generally, the sleeve bearing 36

can be any material that is stiff in compression and non-lossy acoustically.

[0025] A hole 42 can be provided through the mass 20 for passage of the wire 42.

The wire 42 is collected in the tail piece 14 and is connected to solder points 46, or other

connection means, and thereby to a printed circuit board 48, or other processor means.

The circuit board 48 can include an amplifier for the signals produced by the

piezoelectric element 18. The signals can be output from the acoustic sensor 10 via a

transmission means 64, for example a cable or powered wireless transmitter, provided

within a void 50, to some user interface means 62, such as a computer or amplifier

(FIG. 7).

[0026] FIG. 4 shows an exterior view of the acoustic sensor 10. As shown, the

acoustic sensor 10 has a hard conical head 12, housing 16, and tail piece 14. An

attachment means 38 is shown on the tail piece 14. As shown, the bullet-shape of the

acoustic sensor 10 provides for insertion into a variety of substrates and the dimensions

of this shape can be tailored to fit the substrate, as well as the sound wave frequency

desired to be sensed.

[0027] FIG. 5 shows an alternative exemplary embodiment in accordance with the

invention. In this embodiment of the acoustic sensor 10a, the conical head 12 is not

directly coupled to the mass 20. The tail piece 14 is coupled to the mass 20 by a

compression bolt 24, like that shown in FIG. 1, which is provided within the void 54

shown in FIG. 5. As with the embodiment shown in FIG. 1, in this embodiment the

compression bolt 24 engages threads 58 in the mass 20. The tail piece 14 also has

threads 14, which engage the compression bolt 24 and apply tension thereto to

compress 52 the piezoelectric element 18, which can fit inside the mass 20. Electrodes

22 are provided in this embodiment as well.

[0028] FIG. 6 shows an alternative exemplary embodiment in accordance with the

invention. The acoustic sensor 10b includes a substrate engagement means 68, which

can be adapted for use with any other embodiment of the invention. As shown, the

substrate engagement means 68 is an auger shaped protrusion on the housing 16 of the

acoustic sensor 10b. However, the substrate engagement means 68 can be in a variety

of forms. For example, the substrate engagement means 68 could be a spiral thread, a

friction surface, fins, hooks, or any other device suitable for engaging or embedding the

acoustic sensor 10b with or in a substrate.

[0029] In another exemplary embodiment as shown in FIG. 8, the acoustic sensor 10c

has a piezoelectric element 18, which can have electrode surfaces 22a and 22b that can

be integral to the piezoelectric element 18 and can be-formed by screen-printing or

sputtering metal or another conductor. As in other embodiments (FIGs. 1 and 2), the

piezoelectric element 18 is compressed between a conical head 12 and a mass 20. In the

embodiment shown in FIG. 8, components are provided for electrical shielding of the

piezoelectric element 18 from electro-magnetic interference (EMI).

[0030] The shielding components are included in the region compressed by the

compression bolt 24, between the conical head 12 and the mass 20. The shielding

components can include a thin, stiff, electrically insulating washer 70a, which can be

made of mica or a ceramic, for example, and a conductive tubular shield 72 electrically

attached to a bottom conductive washer 74a and the electrode surface 22a. The

shielding components can further include an upper conductive washer 74b over the

electrode surface 22b, an insulating washer 70b over the upper conductive washer 74b,

a conductive washer 74c over the insulting washer 70b, an insulating washer 70c over

the conductive washer 74c, and the mass 20.

[0031] This shielding component arrangement surrounds the piezoelectric element

18 with a layer of conductive material, forming a faraday cage. EMI currents enter this

faraday cage, since the shield 72 and conductive washers 70a, 70b, and 70c and are

shorted to ground, rather than coupling into the signal output of the piezoelectric

element 18 at electrode surface 22b. EMI currents exit the faraday cage at a gap 76. The

bifilar wire 40 can include a signal wire 40a connected to a conductive washer 74b and

electrode surface 22b and a shield wire 40b connected to the shield 72.

[0032] FIG. 7 shows an exemplary embodiment of an acoustic sensor 10 in use in

accordance with the invention. The acoustic sensor 10 is embedded in a substrate 60.

This may be accomplished in a variety of ways, for example, by screwing or pushing it

into the substrate 60. The acoustic sensor 10 is connected to a user interface, such as the

computer 62 shown, via a cable 64 or other means, such as a wireless transmitter. The

acoustic sensor 10 can receive sound waves 66 traveling through the substrate 60 in

three dimensions and transmit signals of the sounds to the user interface. The types of

substrates 60, environments, and uses of the acoustic sensor 10 are not limited to any

specific types and the acoustic sensor 10 can be used in any circumstance where sound

waves 66 travel through a substrate 60.

[0033] Various embodiments of the invention have been described above. Although

this invention has been described with reference to these specific embodiments, the

descriptions are intended to be illustrative of the invention and are not intended to be

limiting. Various modifications and applications may occur to those skilled in the art

without departing from the true spirit and scope of the invention as defined in the

appended claims.

[0034] What is claimed as new and desired to be protected by Letters Patent of the

United States is: