Piezoelectric seismic vibration device and method

A vibrator and method for transmitting seismic signals into the earth to obtain data on various subsurface characteristics. The vibrator is driven with piezoelectric transducers which can operate below 500 Hz, and can produce better signal resolution at higher frequencies than is possible with hydraulic vibrators. The piezoelectric vibrators can be used in various embodiments at the earth's surface, within a borehole, or while permanently buried in the earth.

BACKGROUND 
1. Field of the Invention 
This invention relates to a device and method for transmitting seismic 
signals into the earth from a surface or subsurface location, in which the 
signal is produced by the vibration of a piezoelectric transducer. In one 
embodiment, the device can be operated with a borehole. In another 
embodiment, the device can be permanently buried at a subsurface location. 
2. Discussion of the Art 
Information about subsurface geological characteristics is routinely 
obtained by oil and gas exploration companies using seismic technology. A 
seismic energy source generates sound waves which are reflected and 
refracted as they pass through the earth. The signal is recorded at 
another location and analyzed. 
Conventional methods for generating a seismic signal include explosives and 
vibrators. For example, detonation of dynamite in a hole can produce a 
single shock vibration. Truck-mounted heavy vibrators such as VIBROSEIS 
(trademark of Conoco) units are capable of multiple vibrations at 
controlled frequencies. For offshore exploration, gas exploders or other 
devices can be towed behind a boat. 
Additional methods are available but are not in widespread commercial use. 
For example, U.S. Pat. No. 3,28,456 to Picou discloses a seismic wave 
generator in which the driving force is provided by a magnetic coil. 
In addition to devices used on the earth's surface, downhole periodic 
seismic sources, such as a pneumatic oscillator discussed by H. C. Hardee 
("Downhole Periodic Seismic Sources," Geophysical prospecting, 31: 57-71, 
1983), have been proposed. Advantages include higher signal efficiency, 
improved resolution, and the capabilty for repetitive study of deep 
structures. Downhole periodic sources can be used with downhole geophones 
for improved vertical seismic profiling, or they can be used with surface 
receivers. 
Existing seismic signal generators have not been completely satisfactory. 
Inefficiencies in hydraulic or pneumatic vibrators result in wasted energy 
and diminished signal resolution as signal frequency increases. There is a 
need for a signal generator which is easily controlled and which avoids 
the shortcomings of existing vibrators. 
Piezoelectric transducers are not new to seismic prospecting; they are 
sometimes used in receivers for seismic signals known as accelerometers. 
Surface accelerometers are typically laid out in a spaced pattern some 
distance from the seismic source and have a short spike which is pushed 
into the soil to provide a firm coupling with the earth's surface. 
Vibration of the accelerometer causes vibration in a piezoelectric 
cyrstal, which generates a voltage in relation to the received signal. 
Hydrophones, sensors adapted for receiving seismic signals through water, 
often incorporate piezoelectric transducers. Other examples of 
piezoelectric crystals converting mechanical energy to electrical energy 
are phonograph cartridges and microphones. 
Piezoelectric crystals can also convert electrical energy to mechanical 
energy. Examples of such applications include sonar, fluid flow 
measurement, and ultrasonic jewelry cleaning baths. They have been used 
for dynamic positioning of observatory telescope mirrors and other large 
items. 
The patent literature discloses other uses for piezoelectric crystals. U.S. 
Pat. No. 4,491,759 to Kunz et al, describes a piezoelectric vibration 
exciter for generating mechanical vibrations over a 20 kHz to 100 kHz 
frequency range for the purpose of non-destructive material testing. The 
piezoceramic disk stacks are mechanically connected to a test specimen, 
and the disks vibrate at the frequency of an applied AC voltage. 
Ultrasonic piezoelectric transducers, designed to generate acoustical wave 
trains having frequencies of 50 kilocycles or more, and preferably greater 
than 100 kilocycles, are disclosed as useful in well logging in U.S. Pat. 
No. 3,949,352 to Vogel. A beam of ultrasonic energy is transmitted from 
the borehole to produce shear waves in the adjacent formation, and the 
return signal is monitored from another location in the same borehole. 
U.S. Pat. No. 4,245,172 to Shirley describes an improved transducer using a 
plurality of piezoelectric elements in a bimorph condiguration for 
generating and detecting shear waves for the analysis of solid and 
semi-solid materials. The resulting displacement from this configuration 
is relatively high but the force is weak compared to other piezoelectric 
configurations. 
It is an object of this invention to provide a more efficient seismic 
energy source by converting electrical energy directly into acoustic 
energy and to avoid energy disadvantages associated with hydraulic and 
pneumatic systems, such as heat loss through friction, viscosity, and 
turbulence, 
It is another object of this invention to conserve energy compared to 
seismic methods involving dropped or vibrating weights. Most of the energy 
expended in this invention by raising the weight is stored and reclaimed 
in the subsequent downward cycle, while in conventional hydraulic 
VIBROSEIS technology much energy is wasted as heat. 
It is a further object of this invention to provide an efficient vibratory 
source signal that does not degrade at highe frequencies (e.g., above 100 
Hz). 
It is an additional object of this invention to produce a vibratory seismic 
device with dimensions and capabilities that would make it suitable for 
temporary or permanent placement in a borehole. 
SUMMARY OF THE INVENTION 
The invention relates to a vibrator device and method for transmitting 
seismic signals into the earth. The device comprises at least one 
piezoelectric transducer attached to a rigid plate, a means for applying 
pressure against the earth with the transducer and plate, and a means for 
supplying an electrical signal to the piezoelectric transducer. 
In a preferred embodiment useful for generating seismic signals from the 
earth's surface, a transducer is mounted between a heavy mass and a base 
plate. In an alternate embodiment useful for generating seismic signals 
within a borehole, one or more piezoelectric transducers are attached to 
curve plates which are held in tension against the walls of the borehole. 
In another embodiment, a device can be buried permanently to provide an 
inexpensive seismic energy source from a fixed location on a periodic 
basis.

DETAILED DESCRIPTION 
All embodiments of this invention involve one or more piezoelectric 
transducers for producing a vibration, together with a means for 
electrically energizing the transducer. The configuration of each device 
will vary depending upon whether it is designed for use on the earth's 
surface, within a borehole, or while permanently buried in the earth. The 
principal differences are found in the means for applying pressure against 
the earth. 
Referring to FIG. 1, one side of piezoelectric transducer 15 is placed 
adjacent to plate 5 which is in contact with the earth. On the opposite 
side of piezoelectric transducer 15 is part 10, which is a heavy reaction 
mass or other means for applying pressure against transducer 15 and plate 
5 to resist movement. Such means can include mechanical and other devices 
to hold transducer 15 and plate 5 firmly against the earth, such as 
jacking and bracing devices to hold transducer 15 and plate 5 against the 
lateral walls of a borehole. 
Signal controller 28, power amplifier 26, and power source 30 taken in 
combination are a typical means for supplying an electrical signal to 
transducer 15. In operation, a sine wave or other desired signal is 
generated by signal controller 28 and amplified by power amplifier 26 
connected to power source 30 (e.g., a generator). The piezoelectric 
transducer 15 is wired to the amplifier 26 and expands when the energizing 
signal is applied. The transducer is preferably driven at a selected 
frequency to cause the heavy mass 10 to vibrate. The vibrating mass 10 
results in a reactionary oscillating force transmitted to the earth 
through the base plate 5. The signal controller can be used to alter the 
frequency of the signal and, thereby, the frequency of the oscillating 
mass 10. A frequency sweeping signal, as known in the art, is preferred. 
Transducers 
The number, size, and type of piezoelectric transducers chosen will depend 
upon the needs of each application. The most appropriate transducer for 
this invention demonstrates vibration along a single axis when an 
electrical signal is applied, since the desired motion is in only one 
direction such as, for example, up and down. 
Some piezoelectric crystals are designed to expand and vibrate in several 
directions simultaneously, such as discs which vibrate in a radial mode. 
Bimorph elements, for example, bend out of a plane because of their 
orientation, typically two or more piezoelectric elements joined 
face-to-face which produces differential expansion and bending. 
Transducers which exhibit significant expansion along more than one axis 
should be avoided in practicing this invention. 
Although each single piezoelectric crystal has limitations on the 
displacement and force, these limitations can be partially overcome by 
increasing the number of crystals used, and by stacking them. For example, 
100 thin (0.04 inch each) piezoelectric discs stacked in a cylindrical 
arrangement can produce a greater displacement than a single crystal of 
the sme size and shape when the power supply is voltagelimited. Operation 
with limited voltages is desirable for technical as well as safety 
reasons. 
Greater aggregate force can be obtained by adding separate crystals (or 
stacks of crystals) alongside so that the transducers work in unison. For 
applications needing considerable displacement and force, such as the 
retrofitted VIBROSEIS truck discussed below, an array of about 10 stacked 
transducers, each consisting of 100 thin (0.04 inch) piezoelecric discs, 
could be used. 
The transducers should be capable of vibrating across the higher frequency 
spectrum desired for seismic prospecting, typically 20 to 500 Hz, 
preferably 50-500 Hz. The crystal should preferably be capable of a 
displacement of as much as 1 millimeter at 20 Hz (or 0.1 mm at 50 Hz). In 
general it should be capable of simulating or superseding the vibratory 
characteristics of a VIBROSEIS, particularly at frequencies above 100 Hz. 
Superior signal resolution can be expected from the piezoelectric system 
at higher frequencies, compared to a hydraulic system, where efficiency 
degrades rapidly at frequencies above 100 Hz. 
Published literature, such as a booklet on "Piezoelectric Ceramics" 
available from EDO Corporation, Western Division, Salt Lake City, Utah, 
can be helpful in choosing the piezoelectric transducers. The EDO booklet 
discusses the performance characteristics of its products and includes 
relevant information on element stacking, electrode characteristics, 
selection of materials and shapes, etc. 
Signal Source 
The signal to drive the piezoelectric transducers can be from a sinusoidal 
sweep signal generator of the type that is commonly used to generate sweep 
signals on VIBROSEIS equipment. Preferred signal generators are capable of 
generating a sinusoidal signal with an amplitude of plus or minus 10 volts 
at frequencies from 1 Hz to 1000 Hz. 
The signal is typically fed to a power amplifier which in turn drives the 
transducers. The amplifier best suited for this application is a four 
quadrant bipolar amplifier, which can function as both a source and sink 
of the current produced. The amplifier can produce both negative and 
positive voltage, and negative and positive current, which may be either 
in-phase or out-of-phase with the driving voltage. Further information on 
selection or operation of such amplifiers can be found in the literature, 
including the Kepco Power Supplies "Applications Handbook & Catalog" 
(Kepco. Inc., 131-38 Sanford Ave., Flushing, N.Y. 11352 USA). 
The power needed for operating the vibrator in the field is conveniently 
obtained from a generator mounted on a truck, although other sources could 
be used. The power required to vibrate a 4000 pound reaction mass, used in 
the example below, is about two kilowatts. 
Plate 
The plate can be of any material of sufficient thickness and rigidity to 
prevent flexion, such as steel. It is typically planar, but can be curved 
when desirable for special applications such as for placement against the 
walls of a borehole. 
Accelerometers can be mounted on the reaction mass and plate to monitor the 
motion and hence the driving force delivered to the earth. 
Surface Vibrator 
A surface vibrator with multiple piezoelectric drivers can apply energy to 
the earth similarly to a conventional hydraulic VIBROSEIS unit, but with 
greater efficiency. This efficiency can result in decreased apparatus 
size, which is desirable for field use. 
A VIBROSEIS truck can also be retrofitted as shown in FIG. 2 to obtain the 
benefits of this invention at very little cost. The truck 51 is shown in 
broken lines, and its existing base plate 53 is shown in an extended 
position. A new base plate 55 is constructed which is preferably about the 
same dimensions as the truck base plate 43. A plurality of piezoelectric 
transducers 65 are sandwiched between new plate 55 and truck base plate 
53, or between new plate 55 and an optional top plate 57. The 
piezoelectric transducers 65 are wired to accept signals from a signal 
source, 62 (e.g., a power generator, amplifier, and control) that can 
optionally be mounted on the truck. For operation, the truck lowers its 
base plate to contact the earth's surface and apply static force in the 
conventional manner, but the piezoelectric transducers provide the driving 
force instead of the truck's hydraulic system. For convenience only, the 
trademark VIBROSEIS shall be read to include all types of surface vibrator 
trucks. 
Borehole Vibrator 
A device useful for use within a borehole can comprise one or more 
piezoelectric transducers attached to one or more cylindrical plates with 
a tensional arm such as a spring or hydraulic piston. A heavy mass such as 
used on the surface may be included but is not absolutely necessary in 
this embodiment because sufficient pressure can be applied by holding the 
cylindrical plates against opposing sides of the borehole. Alternatively, 
a single plate could be used which extends to contact the borehole wall 
and holds the entire device firmly in place. The electrical signal is 
provided by a cable from the surface. 
In operation, the device is lowered to the desired depth in the borehole, 
the plate are extended to contact the sides of the borehole, and current 
is applied to the piezoelectric transducer. Signal receivers can be placed 
at the surface or in one or more additional boreholes. The process can be 
repeated at other depths. 
FIG. 3 illustrates a device for transmitting seismic signals from within a 
borehole into the earth. The device can be roughly cylindrical in shape, 
about 5-10 feet long and 6 inches at its widest diameter, so that it can 
be lowered into a borehole of about 8 inches diameter. Its outer shell 101 
is tapered at the lower, leading edge and the opposite end 151 has 
suitable attachments for support cables and entry ports (not shown) for 
electrical and control lines 164 leading to the earth's surface and the 
signal source 162. 
Outside, and not directly attached to, shell 101 are two base plates 105 
and 107, which are capable of moving away from the axis of the device in 
order to contact the walls of the borehole. The base plates 105 and 107 
have a radius of curvature to conform to the curvature of the borehole. 
The arc of each base plate need not exceed, and is preferably less than, 
90 radians. 
In a preferred embodiment (not illustrated), the device is equipped with a 
second pair of base plates 106 and 108 (not shown) positioned at right 
angles to the axis and to the first pair of base plates. This embodiment 
allows the operator to sequentially or concurrently vibrate both pairs of 
plates to produce compressional waves, shear waves, or a combination. 
The base plates 105 and 107 are attached by breakaway shoes 120 to pairs of 
piezoelectric transducers, 115 and 117 respectively. The shoes can be any 
conventional design to assure that the tool body can be detached from the 
base plate in case the tool becomes stuck in the borehole. Such design 
could consist of a weak mechanical link or a slip-on device, with a 
calibrated break-away force. 
The base plate, shoe, and transducer move as a unit away from the axis of 
the device. The transducer, preferably cylindrically-shaped, moves through 
an opening defined by the shell 101, similarly to a piston. Sets of 
springs 121 attached to the exterior of shell 101 and the inteior of base 
plates 105 and 107 provide retracting the force to withdraw the base 
plates in case of power failure. 
The force to extend the base plates outwardly to contact the walls of a 
borehole is provided by a hydraulic system. Hydraulic fluid is contained 
in reservoir 130 at ambient pressure, and enters pump 132 via line 131. 
Fluid channel 133 communicates with the transducers 115 and 117, so that 
pressure can be applied to the transducers to force the base plates out. 
O-rings 123 between the shell 101 and the transducers 115 and 117 provide a 
seal against leakage of the hydraulic fluid. A valve 134 is provided for 
relief of hydraulic pressure in the event of power failure to the device, 
and allows return of fluid to reservoir 130 via line 135. 
Optional equipment that can be used with this device include pressure 
sensors 141 to monitor the dynamic force exerted on the borehole wall. 
Movement of the device itself can be measured by radial accelerometers 143 
and a tangential accelerometer 145. A magnetometer 147 may be included to 
determine the azimuthal orientation of the tool in the borehole. 
Space can be provided within the device for an electronics control module 
149, or the device can be controlled primarily from controls at the 
earth's surface. A torque motor 153 can also be added to aid in orienting 
the device in the borehole. 
Permanent Subsurface Vibrator 
In another embodiment represented in FIG. 4, a piezeoelectric vibrator can 
be permanently placed at a specific subsurface location where it can be 
periodically activated to generate seismic signals. This offers the 
capability of repeatable seismic signals interrupted by large intervals of 
time, (e.g. months and years) which is impossible or impractical with 
other equipment. For example, conventional VIBROSEIS equipment may 
encounter disturbed surface conditions when attempting to duplicate 
seismic data acquired at a certain location at an earlier date. It is also 
difficult to position a surface vibrator in the identical spot repeatedly, 
particularly if few landmarks exist. In some areas, such as arctic tundra 
where the soil is too soft much of the year to support conventional 
surface vibrators, a permanently buried vibrator would permit data to be 
obtained throughout the year. 
Periodic seismic data is useful in monitoring changes in hydrocarbon 
reservoirs, to detect changes in the reservoir over time. These data could 
be used, for example, to track the location of gas/oil or oil/water 
contact zones, and to permit more efficient production from a reservoir. 
Additionally, the progress of enhanced oil recovery procedures could be 
monitored. 
FIG. 4 is a side view of a device for permanent subsurface burial, shown in 
a cross-section of a borehole with walls 221. A borehole is made to the 
desired depth in the earth. A quantity of cement 219 is optionally placed 
at the bottom of the borehole to form a more solid foundation. This cement 
is preferably about 20 feet or more in depth. 
The device comprises a lower plate 205 and an upper plate 207 separated by 
a hollow cylinder 209. Within the cylinder are one or more piezoelectric 
elements, with two such elements 215 shown one on top of the other. The 
elements 215 are in firm contact with, and are preferably bolted or 
otherwise attached to, plates 207 and 205. 
The walls of the cylinder 209 extend from the top and bottom plates (207 
and 205) to completely enclose the elements 215. The walls are preferably 
pressure-tight to exclude subsurface fluids from the interior of the 
cylinder 209, and are sufficiently flexible to permit movement of the 
plates 207 and 205 when force is exerted by the piezoelectric transducers. 
The cylinder walls are preferably made of flexible steel, but can consist 
of polymers or other suitable materials, or combinations of them. 
Once the device is placed in the borehole, additional cement or earth (not 
shown) can be placed in the borehole on top of plate 207 to provide an 
overburden weight to couple the device to the earth. It is preferred, 
however, to first attach an additional weight 210 to the top of plate 207 
to assure that sufficient reaction mass is present when the device is in 
place, rather than rely solely on the overburden of materials added to the 
borehole. While cement layer 219 and weight 210 are optional, they are 
preferred to insure good coupling and consistent operation. 
The signal can be provided by means of a cable 214 leading from the 
transducers 215 to a signal source 212 at the surface. The device is 
operated in the same manner as the surface piezoelectric vibrator 
discussed above. 
The device is buried in a hole below the earth's surface. It can be located 
at about the location of a known hydrocarbon reservoir, or nearer the 
surface, depending upon the desired signal source location. The device is 
preferably placed below the weather layer or low-velocity layer, which is 
the shallow, near-surface layer characterized by considerably slower 
velocities of sound than deeper layers. As a general guideline, the 
devices could be placed at or below about 200 feet in depth. 
Because of the shallow depths, it is convenient to use conventional water 
drilling equipment to make the holes for the permanently buried device. 
For this reason, the subsurface vibrators are preferably cylindrical with 
a diameter of about 8 inches or less. However, other shapes could be used. 
Multiple vibrators can be buried to provide multiple single source 
locations. A geophone can also be placed on or in the buried vibrator, so 
that recording can be accomplished at one or more subsurface locations 
while a signal is generated at another subsurface location. This 
embodiment also offers the benefit of reduced noise from surface 
activities. 
EXAMPLE 
A piezoelectric transducer was assembled from commercially available 
components. The piezoelectric transducer, Model P-243-3 was manufactured 
by Physik Instrumente (PI) GmbH & Co. of Waldbronn, West Germany, and was 
obtained from Polytec Optronics, Inc., of Merrick, N.Y. The transducer was 
about 100 mm in length and 50 mm in diameter overall in a cylindrical 
shape. Each element in the transducer was about 1mm in thickness. The 
maximum displacement was approximately 60 microns over the length of 100 
mm. The transducer is capable of delivering a maximum force of 2000 kg 
over this displacement range. In this experiment the device required about 
1.0 kilowatts of RMS power (at up to 1500 volts at one amp, depending on 
frequency). Efficiency was estimated to be at least 90%. 
A small, hydraulic, automotive jack about 6 inches tall was obtained from a 
Sears retail store. The base of the jack was firmly bolted to a 
12.times.12.times.3/4 inch steel plate. the piezoelectric transducer was 
then bolted to the top of the jack and a second, smaller steel plate was 
bolted to the top of the transducer so that the transducer was sandwiched 
between the second steel plate and the top of the jack. 
This apparatus was placed upright on a large concrete slab under an 
automobile weighting about 2000 lbs. The hydraulic jack was extended to 
lift the front left corner of the automobile, thus providing about 500 
lbs. of load to the top plate to serve as a resistance force. 
The piezoelectric transducer was driven with a Hewlett-Packard Model HP 
3312 function generator capable of providing a 50-500 Hz signal, and was 
coupled to a Bruel and Kjaer Model 2713 ower amplifier capable of 
delivering 100 Volt RMS at 1 Amp RMS. 
A first geophone was placed on the concrete slab about 19 feet from the 
jack and transducer. This geophone was connected to a lock-in amplifier, 
Princeton Applied Research Model 5301, and an oscillascope. Ambient noise 
at this location was measured at about 1 microvolt. 
A fixed frequency signal was amplified and fed to the transducer, and the 
resulting vibration of the transducer and mass produced a noticeable 
vibration of the concrete slab above a frequency of 200 Hz. 
A second geophone was placed above the transducer on top of the second 
steel plate. This second geophone was also moved to a rigid part of the 
automobile to obtain data from which the dynamic force due to the 
accelerating mass of the vehicle could be inferred. 
Several observations and conclusions were made from a series of tests at 
different frequencies. The effectiveness of the piezoelectric transducers 
(i.e., the measured velocity at the surface of the transducer when it is 
loaded by the heavy vehicle) did not decresae when the load was applied. 
The voltage measured from the second geophone placed on top of the second 
steel plate (near the piezoelectric drive surface) was essentially the 
same as that measured at various rigid locations on the automobile. This 
observation is consistent with the manufacturer's claim that the 
transducers can put out a minimum force of 4400 lb. 
The amplitude of the geophone signals ranged from about 0.2 to 2 mV p-p at 
50-300 Hz at a distance of about 10 feet. The signal to noise ratio at 280 
Hz was in excess of 1600 in a 1 Hz measurement bandwidth. The signal 
strength may be increased considerably by increasing the input electrical 
power, or by using multiple transducers. This can be compared to a typical 
monitoring geophone signal of 75 mV observed near a VIBROSEIS unit 
operating in the field.