Seismic Vibrator Controlled by Directly Detecting Base Plate Motion
-by
Zhouhong Wei
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a non-provisional of U.S. Provisional Appl. Ser. No. 61/174,786, filed
01-MAY-2009, which is incorporated herein by reference in its entirety and to which
priority is claimed.
BACKGROUND
[0002] In a geophysical survey, a seismic source can be carried by a truck and
positioned at a predetermined location in an area of exploration. The seismic source
can be a single axis vibratory source and can impart compressing P-waves into the
earth once coupled to the earth and operated.
[0003] A vibrator 10 according to the prior art is illustrated in Figs. 1A-1C and is
diagrammatically illustrated in Fig. 2. The vibrator 10 transmits force to the ground
using a base plate 20 and a reaction mass 50. As is typical, the vibrator 10 is
mounted on a carrier vehicle (not shown) that uses a mechanism and bars 12/14 to
lower the vibrator 10 to the ground. With the vibrator 10 lowered, the weight of the
vehicle holds the base plate 20 engaged with the ground so seismic source signals
can be transmitted into the earth.
[0004] The reaction mass 50 positions directly above base plate 20 and stilts 52
(Fig. 2) extend from the base plate 20 and through the mass 50 to stabilize it.
Internally, the reaction mass 50 has a cylinder 56 formed therein. A vertically
extending piston 60 extends through this cylinder 56, and a head 62 on the piston 60
divides the cylinder 56 into upper and lower chambers. The piston 60 connects at its
lower end to a hub in a lower cross piece 54L and extends upward through cylinder
56. The piston 60's upper end connects to a hub on an upper cross piece 54U, and
the cross pieces 54U-L connect to the stilts 52.
[0005] To isolate the base plate 20 from the bars 14, the bars 14 have feet 16 with
isolators 40 disposed between the feet 16 and the base plate 20. As shown, three
isolators 40 are disposed under each foot 16. In addition, the feet 16 have tension

members 42 interconnected between the edges of the feet 16 and the base plate 20.
The tension members 42 are used to hold the base plate 20 when the vibrator 10 is
raised and lowered to the ground. Finally, shock absorbers 44 are also mounted
between the bottom of the feet 16 and the base plate 20 to isolate vibrations
therebetween.
[0006] Figs. 3A-3C show the base plate 20 for the prior art vibrator 10 in plan, side,
and end-sectional views. The top of the plate 20 has stilt mounts 24 for the stilts (52;
Fig. 2), and a reinforcement pad 21 surrounds these mounts 24. Retaining ledges
26 are provided for the isolators (40). The long edges near the corners have forked
hangers 28 to which ends of the tension members (42) connect, and reinforcement
pads 27 are provided around the outside edges of the plate 20 for connecting the
shock absorbers (44) to the base plate 20.
[0007] Overall, the base plate 20 can have a height H1 of about 6.9-in., a width W1
of about 42-in., and a length L1 of about 96-in., and the plate 20 can weight
approximately 4020-lbs. As shown in the end section of Fig. 3C, the plate 20 has
four internal tubes or beams 30 that run longitudinally along the plate's length. The
beams 30 are hollow tubes with rectangular cross-sections and have a height of
about 6-in., a width of about 4-in., and a wall thickness of about 3/8-in.
Interconnecting spacers 32 position between the beams 30 and between the long
cap walls of the base plate 20.
[0008] During operation, a controller 80 as shown in Fig. 2 receives signals from a
first sensor 85 coupled to the upper cross piece 54U and receives signals from a
second sensor 87 coupled to the reaction mass 50. Based on feedback from these
sensors 85/87 and a desired sweep signal for operating the vibrator 10, the controller
80 generates a drive signal to control a servo valve assembly 82. Driven by the
drive signal, the servo valve assembly 82 alternatingly routes high pressure hydraulic
fluid between a hydraulic fluid supply 84 and upper and lower cylinder piston
chambers via ports in the mass 50. As hydraulic fluid alternatingly accumulates in
the piston's chambers located immediately above and below the piston head 62, the
reaction mass 50 reciprocally vibrates in a vertical direction on the piston 60. In turn,
the force generated by the vibrating mass 50 transfers to the base plate 20 via the

stilts 52 and the piston 60 so that the base plate 20 vibrates at a desired amplitude
and frequency or sweep to generate a seismic source signal into the ground.
[0009] As the moving reaction mass 50 acts upon the base plate 20 to impart a
seismic source signal into the earth, the signal travels through the earth, reflects at
discontinuities and formations, and then travels toward the earth's surface. At the
surface, an array of geophone receivers (not shown) coupled to the earth detects the
reflected signal, and a recording device records the signals from the geophone
receivers. The seismic recorder can use a correlation processor to correlate the
computed ground force supplied by the seismic source to the seismic signals
received by the geophone receivers. The seismic source has a hydraulic pump
subsystem with hydraulic lines that carrying hydraulic fluid to the servo valve
assembly 80, and a cooler may be present to cool the hydraulic subsystem.
[0010] When operating such a prior art vibrator 10, operators experience problems
in accurately determining the ground force that the vibrator 10 is applying to the
earth and in accurately correlating the vibrator's operation with the generated source
signal. Ideally, operators would like to know the actual ground force applied by the
base plate 20 to the ground when imparting the seismic energy. Unfortunately, the
base plate 20 experiences a great deal of vibration and flexure that distorts readings
that can be obtained from the base plate 20. Moreover, the isolators 40, shock
absorbers 44, and other components required to isolate the base plate 20 from the
supports 14 and feet 16 limit what free and unencumbered space is available on the
base plate 20 to obtain acceleration readings.
[0011] For these reasons, a local sensor (e.g., accelerometer or geophone) is
typically positioned on the upper cross piece 54U of the vibrator 50, which positions
above the reaction mass 50 as best shown in Fig. 1C. Affixed at a location 55 on the
upper cross piece 54U, the accelerometer (85; Fig.2) couples to the base plate 20
via the stilts 52. In this location on the upper cross piece 54U, the accelerometer
(85) can avoid the flexure, undesirable noise, distortion, and the like that occurs at
the base plate 20, while still measuring acceleration for the base plate 20. For this
reason, typical work in the prior art to improve performance of such a vibrator 10 has

focused on optimizing the location of the local sensor (85) on the upper cross piece
54U to avoid issues with noise, flexure, and other problems.
[0012] In operation, the controller 80 shown in Fig. 2 measures the signal imparted
into the earth using the local sensor 85 located on the upper cross piece 54U and
the sensor 87 located on the reaction mass 50. The measured signals are
transmitted to a correlation processor, which also receives the signals from
geophones or other sensors making up the seismic spread. The correlation
processor uses various algorithms to distinguish wave signal data from distortions
and other spurious signals. A problem with this method is that the original source
signal distortion may vary making correlation difficult. Thus, the cleaner the source
signal imparted into the earth the easier the correlation at the recording end of the
seismic acquisition process. Also, the more accurate the source signal is, the more
energy the source vibrator 10 can impart to the earth.
[0013] Because the vibrator 10 works on the surface of the earth, which can vary
dramatically from location to location due to the presence of sand, rock, vegetation,
etc., the base plate 20 is often not evenly supported when deployed against the
ground at a given location. In addition, the base plate 20 will flex and directly affect
the control system during operation. As a result, the radiated energy produced can
vary from location to location depending on where the vibrator 10 is deployed.
Therefore, the vibrator's source signature is not the same (or nearly the same) from
location to location and is not characteristically repeatable, which is desirable when
performing seismic analysis.
[0014] When calculated ground force signals at the vibrator 10 are cross-correlated
with far-field signals measured in the field, it has been recognized in the art that
locating an accelerometer on a base plate can cause errors in the arrival time of the
seismic waves. One theoretical approach proposed in the prior art for reducing the
time shifts caused by the phase lag in the oscillations of the base plate 20 relative to
the actuator force from the piston 60 suggests locating the accelerometer on the
base plate at a radius that is approximately 68% of the plate's total radius. See A.
Lebedev and I. Beresnev, "Radiation from flexural vibrations of the baseplate and
their effect on the accuracy of traveltime measurements," Geophysical Prospecting,

2005, 53, 543-555. Yet, it has also been recognized that it may be practically
difficult to find the exact location of the base plate for the accelerometer to improve
the time shifts so that a more practical solution would be to modify the resonance of
the base plate 20 so that problematic modes of this resonance would lie above an
upper frequency of a sweep signal used during seismic survey. Although this
approach may be affective, the subject matter of the present disclosure is directed to
overcoming, or at least reducing the effects of, one or more of the problems set forth
above.
SUMMARY
[0015] A seismic vibrator has a base plate having a top surface and a bottom
surface. A frame supports the base plate so that the bottom surface can couple to
the ground to impart vibrational energy for seismic surveying. An actuator moves a
mass movably disposed above the base plate to impart vibrational energy to the
base plate. This actuator can include a hydraulic actuator having a servo valve
assembly that controls hydraulic fluid to a piston on which the mass positions.
Alternatively, the actuator can include an electric motor, such as a linear motor.
Using the hydraulic actuator, for example, stilts affixed to the base plate extending
through the mass and support the mass on the base plate. A first cross piece
supports one end of the piston to the stilts, and a second cross piece supports
another end of the piston to the stilts.
[0016] The top surface of the base plate has areas unencumbered by components
coupling the frame and stilts to the base plate. In particular, the base plate has at
least four isolators isolating the frame from the base plate. Each of these isolators is
disposed on shelves at corner locations of the base plate. These shelves are
disposed offset from a footprint of the base plate so that the isolators do not
encumber the top surface of the base plate.
[0017] A first sensor such as an accelerometer is disposed directly on the base
plate and detects first signals indicative of acceleration imparted to the base plate.
Similarly, a second sensor is disposed on the mass and detects second signals
indicative of acceleration of the mass. The second signals from the second sensor

tend to be reliable and accurately reflect the acceleration of the mass due to the size,
mass value, and dimensions of the mass on which the second sensor is disposed.
[0018] The base plate, however, tends to experience a great deal of bending and
flexing when it is coupled to the ground and vibration is imparted to it. For this
reason, the base plate preferably has an increased stiffness. For example, the
stiffness of the base plate can be approximately 2.5 times greater than the stiffness
of a base plate of a comparably rated vibrator of the prior art. Yet, the mass of the
base plate can be approximately the same as the mass of a base plate used on a
comparably rated vibrator of the prior art. The increase in stiffness and comparable
mass is achieved by increasing the height of longitudinal beams in the base plate
and giving them a decreased wall thickness to reduce weight.
[0019] The increased stiffness of the base plate reduces the amount of flexing and
bending that it experiences during operation, thereby making readings of its
acceleration by the first sensor disposed directly thereon more reliable. However, to
further improve the readings, the first sensor is disposed on a particular location of
the base plate that experiences transition between longitudinal flexing during
vibration. This transition location tends to better represent the actual acceleration of
the base plate during vibration and avoids the overly increased and decreased
acceleration readings that would be obtained from other locations on the base plate
that experience flexing or bending during vibration.
[0020] A controller is communicatively coupled to the actuator and the first and
second sensors. The controller controls operation of the actuator based at least in
part on the first signals detected from the first sensor disposed directly on the base
plate. For example, the controller computes a weighted-sum ground force using
acceleration values of the first and second signals and using mass values of the
mass and the base plate. Because the acceleration values from the base plate
sensor are more accurate, the controller can avoid overestimating or
underestimating the computed ground force during operation. This allows the
controller to better control the vibrator and allows the vibrator to impart more energy
into the ground in a way that better reflects the preferred reference or pilot signal
configured to operate the vibrator for a seismic survey.

[0021] The foregoing summary is not intended to summarize each potential
embodiment or every aspect of the present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figs. 1A-1C show a vibrator according to the prior art in perspective, front,
and top views.
[0023] Fig. 2 schematically illustrates the prior art vibrator of Figs. 1A-1C.
[0024] Figs. 3A-3C illustrate the base plate for the prior art vibrator in plan, side,
and end-section views.
[0025] Figs. 4A-4C show a vibrator according to certain teachings of the present
disclosure in perspective, front, and top views.
[0026] Fig. 5 schematically illustrates the vibrator of Figs. 4A-4C.
[0027] Figs. 6A-6C illustrate the base plate for the disclosed vibrator in plan, side,
and end-section views.
[0028] Figs. 7A-7C illustrate the base plate for the disclosed vibrator in exposed
views.
[0029] Fig. 8 diagrammatically illustrates bending experienced by the base plate
during operation.
[0030] Fig. 9 is a plan view of the disclosed base plate showing a preferred location
for an accelerometer to affix to the top surface.
[0031] Fig. 10 is a flow chart illustrating operation of the disclosed vibrator.
[0032] Fig. 11A is a plot showing a power spectrum for a weighted-sum ground
force calculated for a prior art vibrator.
[0033] Fig. 11B is a plot showing an actual power spectrum of the prior art vibrator
as measured from a depth of 1000-ft downhole.
[0034] Fig. 12A is a plot showing a power spectrum for weighted-sum ground force
calculated for a vibrator according to the present disclosure.
[0035] Fig. 12B is a plot showing an actual power spectrum of the disclosed
vibrator as measured from a depth of 1000-ft downhole.
[0036] Fig. 13A shows a plot of the magnitude ratio (dB) relative to frequency (Hz)
for a prior art vibrator compared, to the disclosed vibrator.

[0037] Fig. 13B shows a plot of the phase (degrees) relative to frequency (Hz) for a
prior art vibrator compared to the disclosed vibrator.
[0038] Fig. 14 is a plot showing a first frequency response of a prior art vibrator
compared to the disclosed vibrator.
DETAILED DESCRIPTION
A. Assembly/General Operation
[0039] Figs. 4A-4C shows perspective, front, and top views of a seismic vibrator
100 according to certain teachings of the present disclosure. The disclosed vibrator
100 is also shown schematically in Fig. 5. The vibrator 100 has a base plate 110, a
frame 130, and a moveable reaction mass 170. The mass 170 and base plate 110
can be constructed mainly from a metal, such as steel or iron.
[0040] Vibrator 100 transmits force to the ground using the base plate 110 and
reaction mass 170. As is typical, the vibrator 100 is mounted on a earner or vehicle
(not shown) that uses the frame 130 to lower the vibrator 100 to the ground. The
vehicle can use a hydraulic, mechanical, or electromechanical mechanism to lower
the vibrator 100 to the ground surface. With the vibrator 100 lowered, the weight of
the vehicle holds the base plate 110 engaged with the ground so seismic source
signals can be transmitted into the earth during operation. Other details of how the
vibrator 100 couples to the earth with a vehicle or other carrier are well known in the
art and not detailed herein.
[0041] The moving reaction mass 170 acts upon the base plate 110 to impart a
seismic source signal into the earth. The seismic signal travels through the earth,
reflects at discontinuities and formations, and travels toward the earth's surface.
Sensors coupled to the earth are arranged in an array spaced apart from the vibrator
100. These sensors detect the reflected source signal, and a recording station
typically housed in a truck record signals from the sensors. The recording station
includes a seismic recorder and can also include a correlation processor. Such a
correlation processor receives a signal from the vibrator 100 indicative of the actual
source signal imparted into the earth and correlates the received signal with the
recorded signals.

[0042] As shown in Figs. 4A-4C, the reaction mass 170 positions directly above the
base plate 110. A support 140 extends from the base plate 110 through the mass
170 and stabilizes the reaction mass 170. The support 140 is typically constructed
using stilts 142, which can be tubular pipes or rods made of steel or the like. These
stilts 142 have ends affixed to the base plate 110 and extend upward from the base
plate 110 and through the reaction mass 170. An upper cross piece 145U, which
may be constructed from steel or iron I-beam, couples to the top ends of the stilts
142 and provides stability to the support 140 as the mass 170 vibrates. Similarly, a
lower cross piece 145L also couples to the stilts 142 below the mass 170. Isolators
144 are provided on the base plate 110 below the reaction mass 170.
[0043] As best shown in Fig. 5, the reaction mass 170 has a cylinder 160 formed
internally therein that fits onto a vertically extending piston 150. The piston 150
connects at its lower end to a hub in the lower cross piece 145L and extends upward
through cylinder 160. The piston's upper end connects to a hub in the upper cross
piece 145U. A head 156 on the piston 150 divides the cylinder 160 into upper and
lower chambers 162/164.
[0044] During operation, a controller 200 receives signals from a first sensor 210
coupled to the base plate 110 and a second sensor 220 coupled to the reaction
mass 170. Additional sensors (not shown) may be coupled to the carrier vehicle (not
shown) arid to the ground. The local sensors 210/220 can include, but are not
limited to, single or multiple axis accelerometers, geophones, micro electro-
mechanical systems (MEMS) sensors, analog accelerometers with suitable A/D
conversion, or other suitable sensors. Based on feedback from these sensors
210/220 and a desired sweep signal used to operate the vibrator 100, the controller
200 generates a drive signal to control a servo valve assembly 180. The drive signal
has characteristics of the desired sweep signal and is transmitted from the controller
200 to the valve's servo motor. In turn, the valve's motor operates a pilot valve in the
servo valve assembly 180 coupled to a main stage valve in the assembly 180 that
pressurizes and depressurizes hydraulic passages 182/184.
[0045] Driven by the controller 200, the valve 180 alternatingly routes high pressure
hydraulic fluid between a hydraulic fluid supply 230 to the piston 150 via ports

182/184 in the mass 170. As hydraulic fluid alternatingly accumulates in the
chambers 162/164 Ideated immediately above and below the piston head 156, the
reaction mass 170 reciprocally vibrates in a vertical direction on the piston 150. In
turn, the force generated by the vibrating mass 170 transfers to the base plate 110
via the piston 150 and the support 140. Consequently, the base plate 110 vibrates
at a desired amplitude and frequency to generate a seismic source signal into the
ground.
[0046] To reduce acoustic noise once the reaction mass 170 and the base plate
110 are set in motion, the controller 200 can control the force and phase of the
vibration using feedback from the sensor 210 on the base plate 110 and the sensor
220 on the reaction mass 170. Based on the sensor signals, the controller 200 can
then estimate respective motions of the base plate 110 and mass 170 using a force
and phase control algorithm. Based on the motions and the desired output of the
vibrator 100, the controller 200 can then modify the control signal transmitted to the
servo valve assembly 180 to regulate flow of the hydraulic fluid against the reaction
mass 170 and, thereby, control the phase and frequency of the seismic signal
produced.
[0047] Obtaining the best motion signals from the base plate 110 can improve the
control of the vibrator 100 and the output of seismic energy into the ground. For this
reason and as discussed below, the base plate 110 is preferably stiff and has
increased mass. In addition, the sensor 210 is preferably affixed directly to the base
plate 100 in an ideal location to achieve such a signal.
B. Isolation/Base Plate Structure
[0048] As noted above, the carrier vehicle applies its static weight to the base plate
110 via the frame 130 to hold the base plate 110 against the ground. Yet, the
contribution of the frame 130 and vehicle to the resulting seismic force applied to the
ground is preferably kept to a minimum by isolating motion of the base plate 110
from the frame 130 and vehicle using isolators 120.
[0049] As shown in Figs. 4A-4C and 5, the frame 130 has vertical support bars 134
and a horizontal bar 132 connected to the tops of these vertical bars 134. At their
distal ends, the vertical bars 134 connect to feet 136. In turn, these feet 136 connect

to the base plate 110 using an arrangement of isolators 120, pivotable pistons 137,
and tension members 139. The arrangement of these components (120, 137, &
139) essentially isolates the frame 130 from the base plate 110 and the movable
mass 170 supported thereon. In addition, the arrangement allows the vibratory force
of the mass 170 to be applied to the ground via the base plate 110 while minimizing
the amount of force permitted to transmit through the frame 130 to the supporting
vehicle.
[0050] Each vertical bar 134 couples to one of the feet 136. One end of four
pistons 137 pivotably connect at each inner corner of these feet 136, while the other
end of the pistons 137 pivotably connect to the base plate 110. The tension
members 139 connect the outer edges of the feet 136 to the outer edge of the base
plate 110 and support the plate 110 to the feet 136 when the vibrator 100 is lifted off
the ground.
[0051] For their part, the isolators 120 can be air bags or other isolating elements
known and used in the art. The isolators 120 are situated outside of the main
footprint of the base plate 110. In particular, the outside corners of the feet 136
extend beyond the base plate's footprint. Similarly, shelves 118 on the base plate
110 extend from its edges to support the isolators 120 disposed between these
shelves 118 and the extended corners of the feet 136. Use of the shelves 118 and
other features between the frame 130 and base plate 110 creates a particularly
useful amount of free space on the plate's surface.
[0052] Figs. 6A-6C show a plan view, a side view, and an end view of the base
plate 110. As shown in Figs. 6B-6C, the base plate 110 has a top surface 112a and
a bottom surface 112b. As shown in Fig. 6A, stilt mounts 113 are exposed in the top
surface 112a and are surrounded by reinforcement pads 111. Isolator mounts 115 in
these pad? 111 hold the isolators (144) that fit below the reaction mass (170).
Corners of the top surface 112a extend out from the sides of the plate 110 and have
retaining ledges for the isolators (120). The top surface's extending corners are
supported by shelves 118 extending from the sidewalls of the base plate 110. The
shorter edges of the base plate 110 have forked hangers 119 to which ends of the

tension members (139) connect, and reinforcement pads 117 for connection of the
pistons (137) are provided around the outside edges of the plate 110.
[0053] With the isolators (120) positioned on the shelves 118 off the main footprint
of the base plate 110, the top surface 112a of the plate 110 has large expanses of
free area on either side of the reinforcement pads 111, stilt mounts 113, and isolator
mounts 115. These expanses remain exposed and unhindered by interconnecting or
isolating components for the vibrator 100. In fact, the regions of the plate's top
surface 112a free from coupling to support components and free from the footprint of
the mass can define an area that is about 1/2 to 2/3 of the area of the plate's
footprint on its bottom surface 112b.
[0054] As detailed below, such a free expanse on both sides of the vibrating mass
(170) permits a local sensor, such as an accelerometer, to be particularly situated on
the base plate 110 at an advantageous location to obtain the plate's motion during
operation for use in computing the weighted-sum ground force.
[0055] Overall, the base plate 110 can have a height H2 of about 10.87-in., a width
W2 of about 42-in., and a length L2 of about 96-in. Additionally, the base plate 110
can weigh approximately 4345-lbs. in one implementation. Thus, the base plate 110
can have a weight approximately 1.08 times greater than the weight of the
conventional prior art base plate so that the weight of the base plate 110 is relatively
comparable to the weight of a base plate for a comparably rated vibrator used in the
art. However, the base plate 110 can have a height that is approximately 4-in. (or 1-
2/3 times) greater than the height of the conventional prior art base plate so that it
has a much greater stiffness.
[0056] As implemented, the base plate 110 exhibits considerably more stiffness
than the prior art base plate noted in the background section of the present
disclosure. As shown in the exposed views of Figs. 7A-7B, the plate 110 has four
internal tubes or beams 114 that run longitudinally along the plate's length and has
interconnecting spacers 116 positioned between the tubes 114 and between the long
cap walls of the base plate 110. Gussets 118' and the isolator shelves 118 extend
from the plate's long sidewalls, and the stilt mounts 113 position between pairs of
beams 114. As best shown in the end section of Fig. 7C, the beams 114 are hollow

tubes with rectangular cross-sections. To provide increased stiffness over prior art
base plates, the beams 114 in the disclosed base plate 114 have a greater height,
making the beams 114 about 10-in. in height. In other words, the height (about 10-
in.) of each beam 114 is greater than 10% (and more particularly approximately
11 %) of a length (about 92-in.) along which the beam 114 can bend during vibration.
[0057] The beams 114 also have a width of about 4-in. To maintain weight, the
wall thickness of these beams 114 is preferably about 5/16-in. so that the base plate
110 will have a weight approximately the same as that used on a comparably rated
vibrator in the art.
C. Location of Local Sensor on Base Plate
[0058] For best results when processing seismic source signals, operators
preferably know what force the vibrator 100 has applied to the ground to generate
the seismic source signals so that a correlation process can correlate the generated
ground force with the data received by geophone receivers in the field. In reality, the
true ground force that the vibrator 100 applies to the ground during operation may
not be precisely known, and the ground force must instead be computed based at
least in part on the masses and accelerations of the base plate 110 and reaction
mass 170. Correlating the computed ground force with the received seismic signals
uses known convolution techniques not described here.
[0059] The vibrator 100 of the present disclosure has several features that help to
more accurately compute what force it applies to the ground during operation. As
noted previously, the base plate 110 preferably has an increased thickness that
increases the plate's stiffness. In one implementation, the base plate 110 has a
thickness of approximately 10-in. This thickness produces a stiffness of
approximately 2.5 times greater than the conventional stiffness used for comparable
vibrators of the prior art that typically have a base plate thickness of about 6-in. The
plate's increased stiffness reduces potential flexing of the base plate 110 that could
produce undesirable readings by the base plate accelerometer210 during operation.
As is known, the base plate 110 can be operated on all kinds of ground surfaces,
and the base plate 110 can undergo all kinds of unknown deflections. The stiffness

reduces the potential deflection, making the deflection of the stiff base plate 110
more characterizable.
[0060] The plate's increased thickness likewise increases the plate's mass to some
extent. Along the same lines, the mass value for the reaction mass 170 is also
increased so that its mass is about 3 times the mass of the base plate 110, thereby
increasing the mass ratio of the vibrator mass 170 to the base plate 110 from the
conventional 2 times currently used on prior art vibrators. More precisely, the
conventional mass ratio of a reaction mass to a base plate is about 2.02. In one
implementation of the present vibrator 100, the preferred mass ratio between the
base plate 110 and the reaction mass 170 is increased to 2.8 (i.e., about 3 times).
[0061] Finally, as noted above, the isolators 120 used to isolate the base plate 110
from the frame 130 are situated off the footprint of the base plate 110, freeing
additional surface area on the base plate 110 for locating the accelerometer 210 and
reducing interference from isolating components.
[0062] Locating the accelerometer 210 directly on the base plate 110 is done to
obtain a more accurate or "true" reading for computing the ground force and to better
control the vibrator 100's operation. Having more precise accelerometer readings of
the base plate's motion, for example, the controller 200 can put higher frequency
energy in to the ground, giving the seismic signals more bandwidth into the ground
for seismic imaging. In other words, because the controller 200 uses standard
weighted sum algorithms to control the vibration of the mass 170, knowing more
precise motion of the base plate 110 allows the controller 200 to achieve greater
energy efficiency.
[0063] A preferred or ideal location of the accelerometer 210 on the base plate 110
is determined using finite element analysis. In this analysis, a "sweet spot" or"
preferred location on the stiffened base plate 110 is determined where the base plate
110 would experience less undesirable flexure and noise during operation of the
vibrator 100. The best locations for the accelerometer 210 would typically be
situated where isolators are conventionally located on the prior art vibrator.
Therefore, having the isolators 120 moved from conventional locations to comer

locations off the plate's footprint as in the disclosed vibrator 100 makes more of the
base plate's surface accessible for best locating the accelerometer 210.
[0064] Fig. 8 diagrammatically illustrates bending movement experienced by the
stiffened base plate 110 during operation. The base plate 110 is represented two-
dimensionally along the plate's longitudinal axis. Because the plate 110 is three-
dimensional and is vibrated against a surface such as the ground, the actual bending
movement is more complex than illustrated, as one skilled in the art will appreciate.
Finite Element analysis can be used to model the bending movement of the base
plate 110. Being stiffened, the base plate 110, however, can be better modeled for
finite element analysis due to more predictable motion. For all practical purposes,
only longitudinal bending is of concern here, and lateral bending and twisting can be
ignored.
[0065] During upward vibration, a center region A of the base plate 110 bends or
flexes upwards when the entire plate 110 is translated upward, while edge regions B
bend or flex downwards while also being translated upwards in the vibration. As a
result, the center region A would experience increased acceleration over the
acceleration induced by the base plate's translation, whereas the edge regions B
would experience less acceleration. In downward vibration, the center region A'
bends or flexes downwards when the entire plate 110 is translated downward and
would experience increased acceleration. The edge regions B' would bend or flex
upwards while also being translated downwards in the vibration and would
experience less acceleration.
[0066] Regions C-C on the plate 110, however, would be subject to less of the
increased and decreased acceleration because these regions C-C would represent
areas on the plate 110 where the bending or flexure transitions during vibration.
Therefore, regions C-C would best reflect the base plate's acceleration when
translated upward and downward because these regions C-C avoid at least some of
the acceleration changes caused by bending.
[0067] To locate the preferred location on the base plate 110, finite element
analysis was performed on a model of the stiff base plate 110 to develop select
locations for the accelerometer 210. Then, actual testing was performed by

positioning an accelerometer 210 at a selected location on the base plate's top
surface 112a. The plate 110 was then vibrated, and the ground force was measured
with a load cell or other sensor for directly measuring the actual force. The reading
from the positioned accelerometer 210 was used to calculate the weighted-sum
ground force, which was then correlated with the actual force measured by the load
ceil. This was repeated with other locations for the accelerometer on the base plate
110. In the end, the testing verified the ideal or preferred spot on the base plate 110
for locating the accelerometer 210 that achieves the most accurate or "true" measure
of the base plate's acceleration best correlated to the actual ground force measured
during experimentation.
[0068] In the plan view of Fig. 9, an ideal or preferred location 212 for the
accelerometer 210 on the base plate 110 of the present disclosure is illustrated.
Again, this base plate 110 has the dimensions of 10.87" (H) x 42" (W) x 96" (L) and a
weight of 4345-lbs. Measuring from the center of the base plate 110, the preferred
accelerometer location 212 is a longitudinal distance X of approximately 20-in. (±
0.5-in.) from the center C along the length or longitudinal axis of the base plate 110.
In addition, the preferred accelerometer location 212 is a lateral distance Y of
approximately 0-in. (± 0.5-in.) from the center along the width or lateral axis of the
base plate 110. Thus, the preferred accelerometer location 212 lies along the
plate's longitudinal axis, although the distance Y may actually be any value along the
width of the base plate 110 as long as bending or twisting along the plate's lateral
axis is negligible. In any event, the longitudinal distance X along the plate's
longitudinal axis is approximately 41 to 43% of the plate's longitudinal distance from
its center to a lateral edge (i.e., 48" which is half of the plate's total length of 96"). In
other words, the longitudinal distance X of the location 212 is about 20% to 21 % of
the overall length of the base plate 110.
[0069] Of course, a reverse location on the other side of the base plate 110's
center would also be preferred. Thus, one or more accelerometers may be
positioned on the base plate 110 in one or both of these identified locations 212. A
preferred accelerometer for affixing to the base plate 110 at this location is the M5TC
model accelerometer from Pelton Land Energy System. This accelerometer has the

dimensions of 4.00" (w) x 4.25" (1) x 2.25" (h) and would be coupled to predrilled
holes in the plate's top surface 112a.
D. Operation using Local Sensor
[0070] Given the above description of the vibrator 100, base plate 110, and other
components, discussion now turns to operation of the disclosed vibrator 100. Fig. 10
is a flow chart illustrating operation 300 of the disclosed vibrator 100 of Figs. 4A-4C
and 5. During operation 300, the seismic source 100 is coupled to the earth by using
a lift mechanism on a truck to lower the source 100 with the frame 130 so that the
base plate 110 engages the ground (Block 302).
[0071] With the base plate 110 properly positioned, operators then initiate seismic
operation (Block 304). Here, the controller 200 configures the control signal for the
servo valve assembly 180 having the desired bandwidth and sweep duration and
controls the supply of hydraulic fluid to the piston 160 to produce the desired
vibration of the mass 170 applied to the base plate 110 and ground. The seismic
vibrator source signal is typically a sweep signal with a sinusoidal vibration ranging
from 2-Hz to 100-Hz or 200-Hz and having a duration on the order of 2 to 20
seconds depending on the terrain, the subsurface lithology, etc.
[0072] During operation, the vibrator 100 produces the seismic source energy that
is transmitted into the ground, reflected by subsurface interfaces, and detected by
geophone receivers at the surface. Concurrently, the controller 200 obtains readings
from the local base plate sensor 210 and the mass sensor 220 (Block 306). In
addition, the controller 200 can obtain readings from additional sensors, such
pressure sensors, etc.
[0073] The reading of the mass 170 is generally accurate because it does not
undergo deflection or the like due to its size, mass, and dimensions. As noted
previously, the local sensor 210 for the base plate 110 can be an accelerometer or
the like that has been particularly located on the base plate 110 to obtain
advantageous acceleration readings of the plate 110 as the mass 170 vibrates and
transfers the vibration to the base plate 110. As discussed previously, these
readings from the local sensor 210 in the preferred location 212 are more accurate
than can be obtained from other locations because the location is unencumbered by

other components (i.e., isolators, tension members, etc.) for the base plate 110.
Furthermore, these readings from the local sensor 210 in the preferred location are
more accurate because the preferred location experiences transition between
longitudinal flexing along a length of the base plate during vibration and thereby
better reflects the actual motion of the base plate 110 without added motion from the
flexing and bending of the base plate 110.
[0074] The controller 200 processes the readings obtained from the sensors 210
and 220 (Block 308) and configures operating parameters for the vibrator 100 based
on those readings (Block 310). In controlling the vibrator 100, the controller 200 may
use a particular type of phase lock that locks the phase of the controller's control
signal to the phase of the base plate 110, the reaction mass 170, the ground force,
or the like.
[0075] For example, the controller 200 uses a reference or pilot signal to operate
the vibrator 100 in a preferred manner having a particular duration, sweep
frequencies, ground force, and the like. Based on that pilot signal, the controller 200
configures a drive signal to operate the vibrator 100 by controlling the hydraulic
actuator. Phase locking the vibrator 100 to the ground force uses the signal from
sensor 220 indicative of the acceleration of the reaction mass 170 and the signal
from sensor 210 indicative of the acceleration of the base plate 110. Using these
signals, the controller 200 computes a weighted-sum ground force exerted by the
vibrator 100 on the ground. This weighted-sum ground force is calculated using a
standard equation in the art that sums the momentum of the base plate 110 (mass
value of base plate 110 multiplied by its acceleration) with the momentum of the
reaction mass 170 (mass value of the reaction mass 170 multiplied by its
acceleration).
[0076] Once configured, the controller 200 then controls the vibrator 100 with the
newly configured parameters (Block 312). Then, using ground force phase lock, the
controller 200 correlates the computed ground force's phase with the phase of the
pilot signal to adjust the drive signal supplied to the vibrator 100. This correlation
removes or reduces discrepancies between the ground force and pilot signal so that
they are in phase or in sync with one another. This improves the operation of the

vibrator 100, the imparted ground force, and the resulting seismic data in the various
ways discussed herein.
E. Plots
[0077] As discussed previously, determining whether a weighted-sum ground force
of a vibrator truly represents the vibrator's actual ground force remains a persistent
issue in vibroseis technology. Freeing available space on the base plate 110 and
particularly locating the accelerometer 210 at the preferred location 212 in this free
space improves the operation of the disclosed vibrator 100 and the results it
achieves, as noted previously. What follows is a discussion of these results.
[0078] For comparison, a plot 400 in Fig. 11A shows the power spectrum for a
weighted-sum ground force calculated for a prior art vibrator, while a plot 420 in Fig.
12A shows the power spectrum for a weighted-sum ground force calculated for a
vibrator 100 according to the present disclosure. This plot 420 is based on the
disclosed vibrator 100 having the base plate 110, preferred accelerometer location,
and other features disclosed herein.
[0079] When comparing the power spectrums for the weighted-sum ground forces
in the plots 400/420, it would appear that the prior art vibrator (Fig. 11 A) performs
better at high frequencies than the disclosed vibrator (Fig. 12A). However, when the
downhole signals at 1000 feet deep are examined, it is clear that the signal for the
disclosed vibrator 100 more closely resembles what is actually being input into the
ground. In particular, an actual power spectrum 410 for the prior art vibrator is
shown in Fig. 11B as measured from a depth of 1000-ft downhole. In contrast, Fig.
12B shows a plot of an actual power spectrum 430 for the disclosed vibrator as
measured from a depth of 1000-ft downhole. As seen in these two plots, the
disclosed vibrator's spectrum 430 has a higher power level as measured downhole
than the prior art vibrator's spectrum 410.
[0080] During operation, the prior art vibrator apparently operates under the notion
that it is doing a sufficient job of inputting a flat spectrum all the way up to 201-Hz as
seen in Fig. 11A. In contrast, the disclosed vibrator 100 apparently recognizes that a
limit has been reached in the vibrator's physical capability to input frequencies above
about 150-Hz efficiently into the ground as evidenced by is power spectrum in the

plot 420 of Fig. 12A dropping off somewhat. However, this strongly indicates that the
disclosed vibrator's weighted-sum is more in agreement with the signal actually
being input into the ground than is achieved by the prior art vibrator when the
spectrums 410/430 are compared.
[0081] The better agreement between the disclosed vibrator's weighted-sum
ground force and the actual ground force is best illustrated in Figs. 13A-13B. As
shown in a plot 500 of Fig. 13A, a first curve 510 shows the magnitude ratio (dB)
relative to frequency (Hz) for a prior art vibrator, and a second curve 520 shows the
magnitude ratio (dB) relative to frequency (Hz) for the disclosed vibrator 100. These
magnitude ratios compare the weighted-sum ground force computed for the vibrator
to an actual force measured by a load cell during test operations. Comparison of
these two curves 510/520 indicates that the disclosed vibrator produces a more
steady magnitude ratio over the frequency range, showing that throughout the
frequency range the disclosed vibrator's weighted-sum ground force computed
during operation closely matches the actual ground force measured by the load cell.
The prior art vibrator, however, apparently experiences increasing discrepancy in its
computed weighted-sum ground force and the actual ground force at higher
frequencies.
[0082] The same is true for the phase ratio of the disclosed vibrator. As shown in a
plot 550 of Fig. 13B, a first curve 560 shows the phase ratio (degrees) relative to
frequency (Hz) for a prior art vibrator, and a second curve 520 shows the phase ratio
(degrees) relative to frequency (Hz) for the disclosed vibrator. Comparison of these
two curves 560/570 indicates that the disclosed vibrator's phase during operation
more closely matches the actual measured phase over the frequency range than is
experienced by the prior art vibrator. Again, this confirms that using the disclosed
vibrator 100 can offer a better agreement between the weighted-sum ground force
and the actual ground force.
[0083] As discussed above, the base plate 110 with increase stiffness, the
preferred location 212 of the accelerometer 210 on the base plate 110, and the other
features disclosed herein help the disclosed vibrator 100 to produce a greater
ground force and more bandwidth for seismic imaging over prior art vibrators. The

stiffened base plate 110 and other features help in achieving this improved
operation. Yet, even on its own, proper placement of the accelerometer 210 on the
base plate 110 can improve operation.
[0084] In Fig. 14, for example, a plot 600 shows a first power spectrum 610 for the
disclosed vibrator 100 having the base plate 110 and other features disclosed herein
but having an accelerometer disposed on the upper cross piece (145U; Figs. 1A-1C)
as done in the prior art. This first spectrum 610 is shown in comparison to a second
power spectrum 650 for the disclosed vibrator 100 having the accelerometer 210 on
the base plate 100 located in the preferred location 212 according to the present
disclosure. The two vibrators for both spectrums 610/650 are operated using a
sweep signal from 1-201-Hz over 20 seconds.
[0085] The first spectrum 610 shows a persistent decline in power (dB) with
increased frequency. By contrast, the second spectrum 650 shows a more
sustained response in the mid and higher frequencies. For example, the second
spectrum 650 at 652 shows 3 dB at 50 Hz and declines less rapidly with increased
frequency in comparison to the first spectrum 610. Additionally, the second
spectrum 650 at 654 shows 14 dB at 150 Hz, which is significantly greater than the
first spectrum 610 at this frequency. This indicates that locating the accelerometer
210 at the preferred location 212 on the base plate 110 improves the disclosed
vibrator's power spectrum over positioning the accelerometer on the upper cross
piece as conventionally done.
[0086] Although the disclosed vibrator 100 has been described with respect to a
hydraulically actuated reaction mass, those skilled in the art will appreciate that the
teachings of the present disclosure can be applied to other types of actuators for
reciprocating a reaction mass. In general, therefore, the disclosed vibrator 100 can
reciprocate a reaction mass using a linear induction motor, a linear synchronous
motor, a controlled hydraulic actuator, or any other actuator used in the art. In
addition to vibrating vertically, the disclosed vibrator 100 can also produce seismic
shear waves ("S-Waves"). The present disclosure has focused on a single axis
seismic source for brevity and without limiting the scope of the disclosure. Those
skilled in the art would recognize that a multi-axis vibratory source capable of

imparting both P and S waves into the earth can be configured according to the
present disclosure. For example, details related to coupling the disclosed vibrator 10
to the earth and details related to other actuators for the disclosed vibrator 100 can
be found in U.S. Pat. Pub. Nos. 2007/0250269, 2007/0240930, and 2009/0073807,
which are incorporated herein by reference.
[0087] The foregoing description of preferred and other embodiments is not
intended to limit or restrict the scope or applicability of the inventive concepts
conceived of by the Applicants. In exchange for disclosing the inventive concepts
contained herein, the Applicants desire all patent rights afforded by the appended
claims. Therefore, it is intended that the appended claims include all modifications
and alterations to the full extent that they come within the scope of the following
claims or the equivalents thereof.

CLAIMS:
1. A seismic vibrator, comprising:
a base plate having a longitudinal axis;
a mass movably disposed relative to the base plate for imparting vibrational
energy thereto;
an actuator coupled to the mass for moving the mass relative to the base
plate;
a first sensor disposed on the base plate and detecting first signals indicative
of acceleration imparted to the base plate, the first sensor disposed at
a location on the base plate experiencing transition between flexing
along the longitudinal axis during vibration of the base plate; and
a controller communicatively coupled to the actuator and the first sensor, the
controller controlling the vibrator based at least in part on the first
signals from the first sensor.
2. The vibrator of claim 1, wherein the first sensor is selected from the group
consisting of a single axis accelerometer, a multiple axis accelerometer, a geophone,
a micro electro-mechanical systems (MEMS) sensor, a digital accelerometer, and an
analog accelerometer with an analog-to-digital converter.
3. The vibrator of claim 1, further comprising a second sensor disposed on the
mass and detecting second signals indicative of acceleration of the mass.
4. The vibrator of claim 3, wherein the controller is communicatively coupled to
the second sensor and computes a weighted-sum ground force based on
acceleration values from the first and second signals and based on mass values for
the mass and the base plate.
5. The vibrator of claim 1, wherein the actuator comprises:
a servo valve controlled by the controller, and
a piston disposed within the mass and hydraulically coupled to the servo
valve.
6. The vibrator of claim 5, wherein the piston couples to a support disposed on
the base plate, the support supporting the piston and the mass above the base plate.

7. The vibrator of claim 6, wherein the support comprises:
a plurality of stilts affixed to the base plate and extending through the mass;
a first cross piece supporting one end of the piston to the stilts; and
a second cross piece supporting another end of the piston to the stilts.
8. The vibrator of claim 1, wherein the base plate comprises a plurality of beams
disposed parallel to one another along the longitudinal axis, each of the beams
having a height that is at least equal to or greater than 10% of a longitudinal length
along which the beam bends during vibration of the base plate.
9. The vibrator of claim 1, further comprising a frame supporting the base plate
relative to the ground and having at least four isolators isolating the frame from the
base plate.
10. The vibrator of claim 9, wherein each of the at least four isolators is disposed
at a corner location of the base plate and disposed offset from a footprint of the base
plate.
11. The vibrator of claim 10, wherein the base plate comprises a plurality of
shelves supporting the isolators, the shelves disposed at the comer locations and
offset from the footprint.
12. The vibrator of claim 9, wherein the frame comprises first and second feet
each disposed on one side of the mass and each isolated from the base plate by at
least two of the isolators.
13. The vibrator of claim 9, wherein the frame comprises a plurality of tension
members coupled between outside lateral edges of the base plate and outside
edges of the first and second feet.
14. The vibrator of claim 9, wherein the frame comprises a plurality of shock
absorbers coupled between a top surface of the base plate and bottom surfaces of
the first and second feet, the shock absorbers disposed along edges of the base
plate.
15. The vibrator of claim 1, wherein the base plate has a bottom surface defining
a first footprint with a first area, and wherein the base plate has a top surface having
regions being free from coupling to support components and being free from a

second footprint of the mass, the regions defining a second area that is from about
1/2 to 2/3 of the first area.
16. The vibrator of claim 1, wherein the base plate has a longitudinal length, and
wherein the first sensor is disposed at a longitudinal distance from a center of the
longitudinal length, the longitudinal distance being about 41 to 43% of half of the
longitudinal length of the base plate.
17. The vibrator of claim. 16, wherein the base plate comprises a plurality of
beams disposed parallel to one another along the longitudinal axis, each of the
beams having a height that is at least equal to or greater than 10% of the longitudinal
length of the base plate.
18. The vibrator of claim 16, wherein the first sensor is disposed on the
longitudinal axis passing through the center of the base plate.
19. The vibrator of claim 1, wherein a first mass value for the mass is
approximately three times greater than a second mass value for the base plate.
20. A seismic vibrator comprising:
a base plate having a top surface and a bottom surface and having a
longitudinal axis, the bottom surface coupleable to the ground;
a frame supporting the base plate relative to the ground and having at least
four isolators isolating the frame from the base plate, each of the at
least four isolators disposed at a corner location of the base plate and
disposed offset from a first footprint of the bottom surface of the base
plate;
a mass movably disposed above the base plate and imparting vibrational
energy thereto;
an actuator coupled to the mass and moving the mass relative to the base
plate;
a first sensor disposed on the base plate and detecting first signals indicative
of acceleration imparted to the base plate, the first sensor disposed at
a location of the base plate experiencing transition between flexing
along the longitudinal axis during vibration of the base plate; and

A seismic vibrator has a base plate
with at least four isolators isolating a frame from the
base plate. Each of these isolators is offset from the
plate's footprint on shelves to free up area on the
plate's top surface. An accelerometer disposed directly
on the base plate detects the acceleration imparted
to the plate. To reduce flexing and bending, the plate
has an increased stiffness and approximately the same
mass of a plate for a comparably rated vibrator. The
accelerometer disposes at a particular location of the
plate that experiences transition between longitudinal
flexing along the plate's length. This transition location
better represents the actual acceleration of the
plate during vibration and avoids overly increased
and decreased acceleration readings that would be obtained
from other locations on the plate.