Title of Invention

"A METHOD OF GEOPHYSICAL EXPLORATION"

Abstract A method of geophysical exploration is disclosed. The method comprises: generating a seismic signal in the body of water; detecting said seismic signal with a plurality of co- located particle motion sensor assemblies (3) and pressure gradient sensors (5) positioned within a seismic cable (30) deployed in said body of water; modifying the output signal of at least one of said particle motion sensor assemblies (3) or said pressure gradient sensors (5) to substantially equalize the amplitude and phase response of said particle motion sensor assemblies and said pressure gradient sensors within at least a selected frequency range; and combining the modified output signals from co-located particle motion sensor assemblies (3) and pressure gradient sensors (5) within at least said selected frequency range.
Full Text

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention provides a method of geophysical exploration. The
present invention is related in particular to marine geophysical exploration. More
specifically, the invention is related to sensors for detecting seismic signals and to
marine seismic data gathering.
The present application has been divided out of Indian Patent application
No. 384/KOL/2003. (hereinafter referred to as "parent application")
2. Description of Relevant Art
In seismic exploration, geophysical data are obtained by applying acoustic
energy to the earth at the surface and detecting seismic energy reflected from interfaces
between different layers in subsurface formations. The seismic wave is reflected when
there is a difference in impedance between the layer above the interface and the layer
below the interface.
In marine seismic exploration, a seismic shock generator, such as an airgun, for
example, is commonly used to generate an acoustic pulse. The resulting seismic wave
is reflected back from subsurface interfaces and detected by sensors deployed in the
water or on the water bottom.
In a typical marine seismic operation, a streamer cable is towed behind an
exploration vessel at a water depth between about six to about nine meters.
Hydrophones are included in the streamer cable for detecting seismic signals. A
hydrophone is a submersible pressure gradient sensor that converts pressure waves
into electrical signals that are typically recorded for signal processing, and evaluated to
estimate characteristics of the earth's subsurface.
After the reflected wave reaches the streamer cable, the wave continues to
propagate to the water/air interface at the water surface, from which the wave is
reflected downwardly, and is again detected by the hydrophones in the streamer cable.
The reflection coefficient at the surface is nearly unity in magnitude and negative in sign.
The seismic wave will be phase-shifted 180 degrees. The downwardly travelling wave is
commonly referred to as the "ghost" signal, and the presence of this ghost reflection
creates a spectral notch in the detected signal. Because of the spectral notch, some
frequencies in the detected signal are amplified and some frequencies are attenuated.

Because of the ghost reflection, the water surface acts like a filter, making it
difficult to record data outside a selected bandwidth without excessive attenuation or
notches in the recorded data.
Maximum attenuation will occur at frequencies for which the distance between
the detecting hydrophone and the water surface is equal to one-half wavelength.
Maximum amplification will occur at frequencies for which the distance between the
detecting hydrophone and the water surface is one-quarter wavelength. The
wavelength of the acoustic wave is equal to the velocity divided by the frequency, and
the velocity of an acoustic wave in water is about 1500 meters per second. Accordingly
the location in the frequency spectrum of the resulting spectral notch is readily
determinable. For example, for a streamer water depth of 7 meters, as illustrated by
curve 54 in Figure 1, maximum attenuation will occur at a frequency of about 107 Hz.
and maximum amplification will occur at a frequency of about 54 Hz.
It has not been practical to tow cables deeper than about 9 meters because the
location of the spectral notch in the frequency spectrum of the signal detected by a
hydrophone substantially diminishes the utility of the recorded data. It has also not been
practical to tow cables at a depth shallower than about 6 meters, because the ghost
signal reflected from the water surface substantially attenuates the signal detected by a
hydrophone within the frequency band of interest.
It is also common to perform marine seismic operations in which sensors are
deployed on the water bottom. Such operations are typically referred to as "ocean
bottom seismic" operations. In ocean bottom seismic operations, both hydrophones and
geophones are employed for recording the seismic data, with the geophone normally
being placed in direct contact with the ocean bottom. To improve the contact between
the geophone and the ocean floor, the geophone assembly is typically made to be quite
heavy, with a typical density of between 3 and 7 grams per cubic centimeter.
A geophone detects a particle velocity signal, whereas the hydrophone detects
a pressure gradient signal. The geophone has directional sensitivity, whereas the
hydrophone does not. Accordingly, the upgoing wavefield signal detected by the
geophone and the hydrophone will be in phase, but the downgoing wavefield signals
detected by the geophone and the hydrophone will be 180 degrees out of phase.
Various techniques have been proposed for using this phase difference to reduce the
spectral notch caused by the ghost reflection.

U.S. Patent No. 4,486,865 to Ruehle, for example, teaches a system said to
suppress ghost reflections by combining the outputs of pressure and velocity detectors.
The detectors are paired, one pressure detector and one velocity detector in each pair.
A filter is said to change the frequency content of at least one of the detectors so that
the ghost reflections cancel when the outputs are combined.
U.S. Patent No. 5,621,700 to Moldovenu also teaches using at least one sensor
pair comprising a pressure sensor and a velocity sensor in an ocean bottom cable in a
method for attenuating ghosts and water layer reverberations.
U.S. Patent No. 4,935,903 to Sanders et al. teaches a marine seismic reflection
prospecting system that detects seismic waves travelling in water by pressure sensor-
particle velocity sensor pairs (e.g., hydrophone-geophone pairs) or alternatively
vertically-spaced pressure sensors. Instead of filtering to eliminate ghost reflection
data, the system calls for enhancing primary reflection data for use in pre-stack
processing by adding the ghost data.
U.S. Patent No. 4,979,150 provides a method for marine seismic prospecting
said to attenuate coherent noise resulting from water column reverberation by applying
a scale factor to the output of a pressure transducer and a particle velocity transducer
positioned substantially adjacent one another in the water. In this method, the
transducers may be positioned either on the ocean bottom or at a location in the water
above the bottom, although the ocean bottom is said to be preferred.
Four component system have also been utilized on the sea floor. A four
component system utilizes a hydrophone for detecting a pressure signal, together with a
three-component geophone for detecting particle velocity signals in three orthogonal
directions : vertical, in-line and cross line. The vertical geophone output signal is used
in conjunction with the hydrophone signal to compensate for the surface reflection. The
three orthogonally positioned geophones are used for detecting shear waves, including
the propagation direction of the shear waves.
The utility of simultaneously recording pressure and vertical particle motion in
marine seismic operations has long been recognized. However, a geophone (or
accelerometer) for measuring vertical particle motion must be maintained in a proper
orientation in order to accurately detect the signal. Maintaining such orientation is non-
trivial in a marine streamer and significantly more problematic than maintaining such
orientation on the ocean bottom. Exploration streamers towed behind marine vessels
are typically over one mile in length. Modern marine seismic streamers may use more

than 10,000 transducers. To maintain a particle velocity sensor (a geophone or
accelerometer) in proper orientation to detect vertical motion, the prior art has proposed
various solutions. The use of gimbals has been proposed repeatedly. One example is
a "gimbal lock system for seismic sensors" described in U.S. Patent No. 6,061,302 to
Brink et al. Another example is a "dual gimbal geophone" described in U.S. Patent No.
5,475,652 to McNeel et al. Still another example is a "self-orienting directionally
sensitive geophone" described in U.S. Patent No. 4,618,949 to Lister. Nevertheless, no
streamers containing both hydrophone and geophones are in commercial use.
In addition to the problem of maintaining orientation, severe noise from streamer
cables has been considered prohibitive to use of particle velocity sensors in streamers.
Because the voltage output signal from particle velocity sensors is normally not as
strong as the output signal from hydrophones, the noise level in streamer cables has
been a detriment to the use of particle velocity sensors.
In ocean bottom cables, the sensors are located on the sea floor and therefore
are less exposed to noise generated by vibrations in the cable. Geophones are typically
gimbaled to ensure a correct direction and are made of heavy brass or similar material
to ensure good contact with the sea floor. The geophone housing is typically filled with
fluid to improve the coupling between the sensor and the seafloor. However, because
of the variation in properties of the seafloor from location to location, impedance
mismatch between the seafloor and the sensor and sensor housing can cause
problems. Such mismatch in impedance can cause various types of distortion in both
the hydrophone signal and the geophone signal. Also, the boundary effects for the
hydrophone and the geophone due to their closeness to the sea floor can change the
response for the hydrophone and the geophone, giving rise to a need to correct the
amplitude values in processing to be able to use the signal for elimination of the surface
"ghost" reflection.
Accordingly a need continues to exist for an improved system for gathering
marine seismic data.
SUMMARY OF THE INVENTION
The "parent application" discloses a particle motion sensor assembly for use
in a seismic streamer cable, comprising :
a housing ;

a particle motion sensor gimbal-mounted in said housing ;
a fluid within said housing substantially surrounding said particle motion sensor,
said fluid having a viscosity providing sufficient damping of sensor movement to reduce
noise while enabling sufficient movement of said sensor to maintain said sensor in a
desired orientation as said housing is rotated ;
wherein said particle motion sensor assembly is constructed from components
selected so that said particle motion sensor assembly has an acoustic impedance within
the range of about 750,000 Newton seconds per cubic meter to about 3,000,000
Newton seconds per cubic meter; and
wherein said particle motion sensor assembly has a configuration that enables
said particle motion sensor assembly to be mounted within an internal diameter of a
seismic streamer cable having an internal diameter no greater than about 66
millimeters.
The present invention provides a method of geophysical exploration
comprising:
generating a seismic signal in a body of water;
detecting said seismic signal with a plurality of co-located particle motion sensor
assemblies and pressure gradient sensors positioned within a seismic cable deployed in
said body of water;
modifying the output signal of at least one of said particle motion sensor
assemblies or said pressure gradient sensors to substantially equalize the output
signals from said particle motion sensor assemblies and said pressure gradient sensors
within at least a selected frequency range ; and
combining the modified output signals from co-located particle motion sensor
assemblies and pressure gradient sensors within at least said selected frequency range.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described in detail with reference to preferred
embodiments shown in the accompanying drawings, wherein -
Figure 1 shows the frequency spectrum of a seismic signal detected by a
hydrophone at a water depth of 7 meters ;
Figure 2 illustrates a typical implementation of the invention, in which a plurality
of streamer cables are towed behind a seismic survey vessel;

Figure 3 shows the geophone assembly with the parts exploded or separated
out for illustration ;
Figure 4 shows a cross section of a geophone assembly;
Figure 5 shows particle velocity sensors and pressure gradient sensors in a
seismic streamer cable;
Figures 6A and 6B show a typical phase and amplitude response for a particle
velocity sensor;
Figure 7 shows the simulated output responses for a hydrophone and a
geophone at a water depth of 26 meters;
Figure 8 provides actual hydrophone and geophone data from a field test with
the cable at about 26 meters ;
Figure 9 shows a summation of the hydrophone and geophone data shown in
Figure 8; and
Figure 10 shows a simulation of streamer data at a one-meter depth.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 2 illustrates a typical geophysical exploration configuration in which a
plurality of streamer cables 30 are towed behind vessel 32. One or more seismic
sources 34 are also normally towed behind the vessel. The seismic source, which
typically is an airgun, but may also be a water gun or other type of source known to
those of ordinary skill in the art, transmits seismic energy or waves into the earth and
the waves are reflected back by reflectors in the earth and recorded by sensors in the
streamers. Paravanes 35 are utilized to maintain the cables 30 in the desired lateral
position. The invention may also be implemented, however, in seismic cables that' are
maintained at a substantially stationary position in a body of water, either floating at a
selected depth or lying on the bottom of the body of water, in which case the source
may be towed behind a vessel to generate shock waves at varying locations, or the
source may also be maintained in a stationary position. Seismic sensors, in accordance
with embodiments of the present invention are deployed in streamer cables, such as
cables 30.
In a particular implementation, the present invention comprises a particle
velocity sensor in the form of a geophone assembly. Such a geophone assembly is
shown in Figures 3 and 4. Figure 3 shows the geophone assembly 3 with the parts
exploded or separated out for illustration. Figure 4 shows a cross section of the
geophone assembly 3 of Figure 3 with the various parts assembled (not exploded).

With reference to Figures 3 and 4, geophone 10 is mounted in a housing 20
comprising outer sleeve 12 and end caps 1 and 13. Geophone 10 is secured in
mounting ring (or cradle) 8. Shafts 9 extend from opposite sides of mounting ring 10
into bushings 2. Bearings 4, which are positioned between shafts 9 and bushings 2
enable rotational motion of shafts 9 with respect to bushings 2, thereby providing a
gimbaled mounting. End caps 1 and 13 are secured together by means of bolts 16 and
threaded inserts 18. Spring 6 provides electrical contact between shafts 9, which are
electrically conductive and are electrically connected to output terminals (not shown) of
the geophone, and bushings 2, which are also electrically conductive, and which are
electrically connected to the streamer cable wiring. Thrust washers 7 provide pre-load
for bearings 4 to eliminate undesired bearing slack. O-rings 15 provide a seal between
outer sleeve 12 and end caps 1 and 13, and O-rings 14 provide a seal between bushing
2 and end caps 1 and 13. Plug 17 is utilized for plugging the conduit through which fluid
is inserted into the interior of the geophone housing comprising the two end caps 1 and
13, and the outer sleeve 12. The configuration of the geophone assembly illustrated in
Figures 3 and 4 is a particular implementation of an embodiment of the invention and is
not intended to be limiting. The geophone assembly 3 is secured to a seismic cable
strain member for positive location.
The housing 20, comprising end caps 1 and 13 and outer sleeve 12, contains a
fluid, preferably an oil, which substantially surrounds the geophone. The fluid provides
coupling between the geophone and the geophone housing of the geophone assembly.
The fluid should preferably surround the geophone, but preferably will not entirely fill the
housing so as to allow room for fluid expansion and contraction with changes in
temperature and pressure. The fluid has a viscosity that provides sufficient damping of
geophone movement to reduce noise, while enabling sufficient movement of the
geophone 10 on the bearings to maintain the transducer in the desired orientation. That
is, the viscosity of the fluid should be high enough to restrain the geophone from
unwanted movements but low enough to prevent the geophone from following rotational
movement of the housing and a streamer in which the geophone assembly may be
mounted. A preferred viscosity for such fluid is in the range of about 500 to about 5000
centistokes.
A positioner, such as a weight 11, may be mounted on the lower side of the
geophone 10 to assist in maintaining the sensor 10 in the desired orientation. Positioner
11 may be formed substantially from lead, although other materials having a density

greater than the density of the geophone may be utilized. Alternatively, or additionally, a
positioner (not shown) having a density lower than the density of the fluid that
substantially surrounds the geophone 10, may be installed on the upper side of the
geophone 10 to assist in maintaining the geophone 10 in the desired orientation.
Locating the center or gravity of the geophone below the rotational axis of the gimbal on
which the sensor is mounted will also assist in maintaining the geophone in the desired
orientation.
The particle velocity sensor in accordance with this invention is sufficiently small
to fit in the interior of a cylindrical streamer cable. Typical internal diameters of such
cylindrical streamer cables are either 55 millimeters or 66 millimeters. The space within
the streamer surrounding the seismic sensors and other apparatus (not shown)
positioned within the streamer is typically filled with a liquid, such as an oil, which
provides substantially neutral buoyancy to the cable. The space may also be filled with
a gel or semi-solid material, and the streamer may also be a solid streamer.
In a preferred embodiment of the invention, the density of the overall geophone
assembly (including the fluid and all other elements thereof) is selected to improve
coupling between the geophone assembly and its surroundings. In general, optimum
coupling is obtained when the acoustic impedance of the geophone assembly is about
the same as the acoustic impedance of its surroundings, which may be achieved by
making the density of the geophone assembly about the same as the density of its
surroundings, and the acoustic velocity of the geophone assembly about the same as
the acoustic velocity of its surroundings.
When an acoustic wave travelling in one medium encounters the boundary of a
second medium, reflected and transmitted waves are generated. Further, when the
boundary area of the second medium is much smaller than the wavelength of the
acoustic wave, diffraction results rather than reflection. For plane waves the
characteristic acoustic impedance of a medium is equal to density times velocity, i.e.,

in which, z = acoustic impedance
p o= density, and
c = velocity.

Let the incident and reflected wave travel in a fluid of characteristic acoustic impedance,
n = P1C1, where p1 is equilibrium density of the fluid and C1 is the phase speed in the
fluid. Let the transmitted wave travel in a fluid of characteristic acoustic impedance r2 =
p2c2. If the complex pressure amplitude of the incident wave is P1 that of the reflected
wave PR, and that of the transmitted wave Pr, then the pressure reflection coefficient R
may be defined as :

and since 1 + R = T, the pressure transmission coefficient T can be written as :

It follows from the foregoing explanation that improved reception will be achieved if the
particle velocity sensor is made in such a way that the density and speed in the sensor
assembly, including its housing and other components, is similar to that of the
surrounding fluid. It they are equal, the reflection coefficient will be R = O and the
transmission coefficient will be T= 1.
By making the acoustic velocity in the particle velocity sensor substantially equal
to the acoustic velocity in the water in which the sensor is deployed, and by making the
density of the particle velocity sensor similar to the density of the water, a good
impedance match is generated between the water and the particle velocity sensor. The
velocity sensor will have a good impedance match with the surrounding media and no
distortion of amplitude or phase will occur due to reflection, diffraction or other
anomalies of the travelling wave passing through the sensor and its housing.
In a preferred embodiment, the density of the particle velocity sensor is less
than about twice the density of water (about 2 g/cm3), and more preferably about the
same as the density of water (about 1 g/cm3). Accordingly, the density of the particle
velocity sensor should typically be between about 0.5 g/cm3 and 2 g/cm3, and more
preferably, about 1.0 g/cm3. It is understood, however, that water density may vary with
salinity, and that it may be useful to vary the density of the particle velocity sensor,
depending on the particular body of water in which the particle velocity sensor is to be
employed. Because the density of particle velocity sensor in accordance with a
preferred embodiment of the invention is substantially less than the density of

geophone assemblies typically available for use in ocean bottom seismic operations,
different components are selected from which to assemble the geophone assembly.
For example, at least a portion of the housing may be formed from a moldable
elastomeric, such as isoplast or polypropylene, or a moldable composite material, such
as fiber reinforced epoxy.
Over and above the need for good acoustic coupling, a low-weight particle
velocity sensor is useful because, in a preferred embodiment of the invention, the
seismic cable in which the sensors are included needs to be neutrally buoyant. As
many as 10,000 particle velocity sensors may be utilized in a single cable. Accordingly,
a particle velocity sensor having a density of less than 2 grams per cubic centimeter
facilitates the mechanical construction of the seismic cable to achieve neutral buoyancy.
In a particular implementation of the invention, particle velocity sensors 3 and
pressure gradient sensors 5 are utilized together in a cylindrical seismic cable 30, as
shown in Figure 5. Use of both particle velocity sensors and pressure gradient sensors
enables signal degradation resulting from surface ghost reflections to be substantially
eliminated from the recorded seismic data. Such signal improvement is achieved by
combining the output signals from a particle velocity sensor (or an array of particle
velocity sensors) with the output signal from a pressure gradient sensor (or an array of
pressure gradient sensors) positioned at substantially the same location. Particle
velocity sensors and pressure gradient sensors positioned at substantially the same
location may be referred to hereinafter as "co-located" sensors.
The phase and amplitude response for a pressure gradient sensor are
substantially constant in the seismic frequency band of interest (from about 2 Hz. to
about 300 Hz.). For example, for the T-2BX hydrophone marketed by Teledyne
Instruments, Inc., of 5825 Chimney Rock Road, Houston, Texas 77081, the variation in
amplitude over a frequency range of 2 - 300 Hz. has been measured at less than 1 db,
and the variation in phase at less than 0.1 degree. Figures 6A and 6B show a typical
amplitude and phase response for a particle velocity sensor. In Figure 6A, curve 56
represents amplitude variation, and in Figure 6B, curve 58 represents the phase
variation. In contrast to the amplitude and phase response of the hydrophone, it is
evident that there are substantial variations in both the amplitude and the phase
response for a particle velocity sensor in the seismic frequency range of interest.

Further, in prior art systems, in which the impedance of the particle velocity
sensor was not substantially matched to the impedance of the substance (either the
water or the water bottom) from which the seismic wave is coupled to the particle
velocity sensor, additional variations in amplitude and phase occur in the seismic
frequency range because of the impedance mismatch.
In accordance with a particular embodiment of the present invention, where the
impedance match between the water and the particle velocity sensor is more nearly
equal, such additional variations in amplitude and phase are minimized, and,
accordingly, the particle velocity sensor output and the pressure gradient sensor output
can be matched by utilizing an appropriate filter, of a type known to those of ordinary
skill in the art, without requiring additional matching for variations caused by impedance
mismatch.
In one implementation of the invention, the pressure gradient sensor is a
hydrophone and the particle velocity sensor is a geophone. The ratio of acoustic
pressure in a medium to the associated particle velocity speed is the specific acoustic
impedance ( p oc = p / u). For a hydrophone, having a good impedance match to the
medium surrounding the hydrophone, and having (for example) a pressure sensitivity of
20 volts per bar, i.e.,

which relationship may be expressed as

and a geophone or a group of geophones, having a good impedance match to the
medium surrounding the geophone, and having (for example) a voltage sensitivity of:

the scale factor ( K), expressing the relationship between velocity output signal of the
geophone and the pressure output signal of the hydrophone will be :

which indicates that the geophone velocity output signal needs to be multiplied with a
scale factor of K = 15 before the pressure and the velocity can be compared. It will be
understood that for hydrophones and geophones having different sensitivities than in the
example discussed above, the scale factor (K) will be different. Further, because of

the variation in the amplitude (as shown in Figure 6A) and phase (as shown in Figure
6B) of the geophone output as a function of frequency, it is necessary to compensate
for the amplitude and phase response of the geophone before applying the scale factor.
The amplitude response ( E) and phase response (ϕ) for the geophone as a
function of frequency may be represented by the following relationships:

in which, G = geophone voltage sensitivity;
f = frequency;
fn= natural resonance frequency;
r = winding resistance ;
R = load resistance ; and
bt = total damping .
Typical values may be : f„= 10; r = 350 ohms ; R = »; bt= 0.6.
If the amplitude and phase of the geophone output signal is adjusted to
compensate for this variation in phase and amplitude with frequency, the geophone
output signal will have substantially the same phase and amplitude curve as the
hydrophone signal. Normally the adjustment may be made on the basis of calculations
based on Equations 8 and 9.

As stated above, in a preferred embodiment of the invention, particle velocity
sensors are constructed to have an acoustic impedance substantially similar to the
acoustic impedance of the water in the body of water in which the particle velocity
sensors are deployed. Accordingly, problems encountered in prior art system, in which
the impedance of the sensor was not matched to the acoustic impedance of the medium
from which a seismic wave was coupled to the sensor, are avoided. In prior art systems
variations in amplitude and phase as a function of frequency caused by impedance
mismatch compounded the difficulty of matching the particle velocity sensor output to
the pressure gradient sensor output. Because of the impedance match achieved in a
preferred embodiment of the present invention, only the variation in amplitude and
phase of the particle velocity sensor itself needs to be compensated for to enable the
particle velocity sensor output to be combined with the pressure sensor output to
attenuate the spectral notches caused by the ghost reflection.
In a preferred embodiment of the invention, the phase and amplitude variations
with frequency of the particle velocity sensor may be calculated based on known (or
determinable) characteristics of the particle velocity sensor, itself. The output signal of
the particle velocity sensor may be modified accordingly to correct for amplitude and
phase variation with frequency using filter techniques well known to those of ordinary
skill in the art. For co-located pressure gradient sensors and particle velocity sensors,
the signal output of the pressure gradient sensor and the filtered output of the pressure
gradient sensor may then be summed to attenuate the spectral notches resulting from
the ghost reflection. Although, in a preferred embodiment of the invention, the phase
and amplitude of the particle velocity sensor output is modified to substantially match
the pressure gradient sensor output, those of ordinary skill in the art would understand
that the phase and amplitude of the pressure gradient sensor output could be modified
to match the particle velocity sensor output signal.
Because the noise level is generally at shallower water depths, placing the
streamer at depths greater than about nine meters (the greatest depth at which
streamer cables are typically deployed) may reduce noise detected by the sensors, and
the signal to noise ratio of the signals detected by the seismic sensors is accordingly
improved. However, for such greater depths, notches in a hydrophone spectrum
resulting from the surface ghost reflection are at lower frequencies, and such a
hydrophone signal is normally regarded as undesirable because of the spectral notches
in the frequency range of interest in seismic exploration. In accordance with an

embodiment of the present invention, the output signal from the particle velocity sensor,
which will have notches in its frequency spectrum at different frequencies from the
notches in the frequency spectrum of the hydrophone, may be combined with the
hydrophone output signal to compensate for the notches and a substantially ghost free
signal can be obtained. Figure 7 shows simulated output responses for a hydrophone
(curve 42) and a geophone (curve 44) at a water depth of 26-meters. The graph
indicates that two signals may be combined to compensate for the spectral notches
resulting from the surface reflection. Figure 8 provides actual data from a field test with
the cable at about 26 meters, which confirms the results indicated in the simulation. In
Figure 8 the geophone output signal is designated by numeral 46 and the hydrophone
output signal is designated by numeral 48. Figure 9 shows a summation (curve 60) of
the hydrophone and geophone data shown in Figure 8, and illustrates the attenuation of
the spectral notches.
Because of the potential high noise level in geophone signals at low
frequencies, resulting from mechanical vibrations in the cable, in a particular
implementation of the invention, low frequency geophone signals are not combined with
the hydrophone signal. In one specific implementation of the invention, frequencies in
the geophone signal lower than about the frequency of the lowest frequency spectral
notch in the hydrophone spectrum are removed from the geophone signal before the
geophone signal is combined with the hydrophone signal. In another implementation of
the invention, geophone signals of less than about 30 Hz. are not combined with the
hydrophone signal.
Improved results are also afforded for operations at shallow depths by the use
of particle velocity sensors in seismic cables in addition to pressure gradient sensors,
over operations using solely pressure gradient sensors. At shallower depths, i.e., less
than about 6 meters, a hydrophone output signal will be attenuated by the surface ghost
in the seismic frequency range of interest. Because of the phase difference between
the upgoing pressure gradient wavefield and the downgoing pressure gradient wavefield
within the seismic frequency band of interest, the downgoing wavefield is subtractive
with respect to the upgoing wavefield, and the downgoing wavefield effectively
attenuates the upgoing wavefield. For a geophone signal, however, the result is the
opposite, and the surface ghost signal effectively increases the amplitude of the signal
detected by a geophone. The difference in phase between the upgoing wavefield and
the downgoing wavefield is such that, for shallow depths, the signal detected by the

geophone is additive. Accordingly, substantially improved results are achieved by use
of particle velocity sensors in addition to pressure gradient sensors at shallow depths
over what is achieved by use of pressure gradient sensors alone. In coastal regions
where the water depth is quite shallow, it may be particularly useful to be able to deploy
the sensors at such shallower depths.
Figure 10 shows a simulation of a hydrophone signal (curve 52) and a
geophone signal (curve 50) at one-meter depth. The attenuation of the hydrophone
signal is evident. Combining the geophone output signal with the hydrophone output
signal for data recorded at the one-meter water depth also compensates for the
influence from the surface reflection.
Generally, a hydrophone signal will have an amplitude that is 10 to 20 times
greater than the amplitude of a geophone signal. This relationship will vary depending
on the particular sensitivity of the particular sensors used. Typically a group of
hydrophones, distributed across a selected spatial distance, will be connected in parallel
for noise attenuation, and the hydrophone output signal that is recorded for use in
seismic data processing and analysis is the combined output from a plurality of
individual hydrophones connected in parallel. Because of the lower signal amplitude of
the geophone output signal, in one implementation of the invention, a group of
geophones, associated with a group of hydrophones (co-located geophones and
hydrophones), will be connected in series, to increase the amplitude of the output signal
as well as to attenuate noise, and the geophone output signal that is recorded for use in
seismic data processing and analysis will be the combined output from a plurality of
individual geophones connected in series. However, depending on the needs of a
particular survey, the geophone groups may be connected in parallel or series, or in a
parallel/series combination. Although, in general, the discussion herein refers to an
output signal from various sensors, the output signal is typically the output signal from a
plurality of discrete sensors interconnected into a sensor array. Further, although the
discussion herein generally refers to a geophone and hydrophone, particle velocity
sensors other than geophones and pressure gradient sensors other than hydrophones
are intended to be within the scope of the present invention.
In one embodiment, groups of about 8 pressure gradient sensors will be used in
association with groups of about 2 to about 16 particle velocity sensors (with lower
numbers rather than higher numbers of particle velocity sensors preferred), and each
combined group will be about 12.5 meters apart from another such combined group. In

this embodiment, combined groups of both pressure sensors and particle velocity
sensors will be treated as single sensors.
In one embodiment of the invention, three-component particle velocity sensors
are included in the seismic cable. By "three-component" is meant that, in addition to a
particle velocity sensor (typically a geophone) mounted to sense motion in the vertical
direction, two particle velocity sensors are mounted in orthogonal directions with respect
to each other (and to the vertically mounted geophone) to sense horizontal motion.
Accordingly, a three-component particle velocity sensor will sense motion in the vertical
direction, in an in-line direction and a cross line direction. Positioning these sensors in
these three directions enables the propagation direction of an incoming signal to be
detected, and also enables the detection of strumming or other mechanical behaviour to
the cable.
Accelerometers could be used as particle motion sensor as an alternative to use
of geophones, although the output signal will need to be integrated to obtain velocity.
An example commercial accelerometer suitable for use in the present invention is the
VECTOR-SEIS ™, available from Input Output, Inc. in Houston, Texas. This particular
accelerometer generates a DC output signal which is indicative of the variation in
orientation of the accelerometer from a selected orientation. Accordingly, if sets of 2
(for situations in which the in-line direction is known) or 3 (if the in-line direction is not
known) of these accelerometers are utilized, the sensor orientation may be computed
and it is not necessary to gimbal-mount the accelerometers. A single accelerometer
could also be used, but it would need to be gimbal-mounted. Since the sensor can
measure acceleration to DC, it is possible to determine the true gravity vector by
analyzing the magnitude of G (the gravity vector) each sensor is operable under. The
results of this analysis are stored with the trace data as direction cosines and describe
the tensor rotation required to recover the signals as if the sensor were deployed at true
vertical orientation.
The foregoing description of the invention is intended to be a description of
preferred embodiments. Various changes in the described apparatus and method can
be made without departing from the intended scope of this invention as defined by the
appended claims.

WE CLAIM:
1. A method of geophysical exploration comprising:
generating a seismic signal in the body of water;
detecting said seismic signal with a plurality of co-located particle motion sensor
assemblies (3) and pressure gradient sensors (5) positioned within a seismic cable (30)
deployed in said body of water;
modifying the output signal of at least one of said particle motion sensor assemblies
(3) or said pressure gradient sensors (5) to substantially equalize the amplitude and phase
response of said particle motion sensor assemblies and said pressure gradient sensors within
at least a selected frequency range; and
combining the modified output signals from co-located particle motion sensor
assemblies (3) and pressure gradient sensors (5) within at least said selected frequency range.
2. The method as claimed in claim 1, wherein the amplitude and phase of the output
signals from said particle motion sensor assemblies (3) and said pressure gradient sensors (5)
are substantially matched within said at least a selected frequency range.
3. The method as claimed in claim 1, wherein modifying the output signals from either
said particle motion sensor assemblies (3) or said pressure gradient sensors (5) is performed
independently of the acoustic impedance of material through which said seismic signal
travels.
4. The method as claimed in claim 1, wherein the output signals of said pressure
gradient sensors (5) and said particle motion sensor assemblies (3) are substantially
equalized during processing and combined.
5. The method as claimed in claim 1, wherein output signals from said particle motion
sensor assembly (3) and said pressure gradient sensors (5) are combined to reduce spectral
notches above frequencies of about 20 Hz.

6. The method as claimed in claim 1, wherein said particle motion sensor assemblies (3)
and pressure gradient sensors (5) are positioned in the interior of a seismic cable (30) having
an inside diameter of about 55 millimeters.
7. The method as claimed in claim 1, wherein said particle motion sensor assemblies (3)
and pressure gradient sensors (5) are positioned in the interior of a seismic cable (30) having
an inside diameter of about 66 millimeters.
8. The method as claimed in claim 1, wherein said seismic cable (30) is deployed at a
depth of less than six meters.
9. The method as claimed in claim 1, wherein said seismic cable (30) is deployed at a
depth of greater than nine meters.
10. The method as claimed in claim 1, wherein said particle motion sensor assemblies (3)
have an acoustic impedance within the range of about 750,000 Newton seconds per cubic
meter to about 3,000,000 Newton seconds per cubic meter.
11. The method as claimed in claim 1, wherein said particle motion sensor assemblies (3)
have an acoustic impedance substantially equal to the acoustic impedance of the water in
said body of water in which said cable (30) is deployed.
12. The method as claimed in claim 1, wherein said seismic cable (30) is a liquid-filled
cable.
13. The method as claimed in claim 1, wherein said seismic cable (30) is a gel-filled
cable.
14. The method as claimed in claim 1, wherein said seismic cable (30) is a solid cable.

15. The method as claimed in claim 1, wherein said seismic cable (30) is towed through
said body of water.
16. The method as claimed in claim 1, wherein said seismic cable (30) is maintained at a
substantially stationary position.
17. The method as claimed in claim 1, wherein at least a portion of the particle motion
sensor assemblies (3) are electrically interconnected in groups to generate a particle velocity
output signals.
18. The method as claimed in claim 18, wherein at least a portion of the particle motion
sensor assemblies (3) are electrically interconnected in series in groups of at least three
sensors.
19. The method as claimed in claim 18, wherein at least a portion of the particle motion
sensor assemblies (3) are electrically interconnected in parallel.
20. The method as claimed in claim 1, wherein said particle motion sensor assemblies (3)
have sensors (10) mounted in said seismic cable (30) in an orientation to detect signals in the
vertical direction, the cross line direction and in-line direction.
ABSTRACT
A METHOD OF GEOPHYSICAL EXPLORATION
A method of geophysical exploration is disclosed. The method comprises: generating
a seismic signal in the body of water; detecting said seismic signal with a plurality of co-
located particle motion sensor assemblies (3) and pressure gradient sensors (5) positioned
within a seismic cable (30) deployed in said body of water; modifying the output signal of at
least one of said particle motion sensor assemblies (3) or said pressure gradient sensors (5) to
substantially equalize the amplitude and phase response of said particle motion sensor
assemblies and said pressure gradient sensors within at least a selected frequency range; and
combining the modified output signals from co-located particle motion sensor assemblies (3)
and pressure gradient sensors (5) within at least said selected frequency range.


Documents:

00650-kol-2006 abstract.pdf

00650-kol-2006 assignment.pdf

00650-kol-2006 claims.pdf

00650-kol-2006 correspondence others.pdf

00650-kol-2006 description (complete).pdf

00650-kol-2006 drawings.pdf

00650-kol-2006 form-1.pdf

00650-kol-2006 form-2.pdf

00650-kol-2006 form-3.pdf

00650-kol-2006 form-5.pdf

00650-kol-2006-correspondence-1.1.pdf

00650-kol-2006-form-3-1.1.pdf

00650-kol-2006-priority document.pdf

650-KOL-2006-(13-02-2012)-CORRESPONDENCE.pdf

650-KOL-2006-ABSTRACT 1.1.pdf

650-KOL-2006-CLAIMS.pdf

650-KOL-2006-CORRESPONDENCE 1.1.pdf

650-KOL-2006-CORRESPONDENCE.pdf

650-KOL-2006-DESCRIPTION (COMPLETE) 1.1.pdf

650-KOL-2006-DRAWINGS 1.1.pdf

650-KOL-2006-FORM 1 1.1.pdf

650-KOL-2006-FORM 2 1.1.pdf

650-KOL-2006-FORM 3 1.1.pdf

650-KOL-2006-OTHERS 1.1.pdf

650-KOL-2006-OTHERS.pdf

650-KOL-2006-PETITION UNDER RULE 137.pdf

650-KOL-2006-REPLY TO EXAMINATION REPORT.pdf

abstract-00650-kol-2006.jpg


Patent Number 252928
Indian Patent Application Number 650/KOL/2006
PG Journal Number 24/2012
Publication Date 15-Jun-2012
Grant Date 11-Jun-2012
Date of Filing 30-Jun-2006
Name of Patentee PGS AMERICAS, INC.
Applicant Address 16010 BARKER'S POINT LANE, SUITE 600, HOUSTON,TEXAS 77079
Inventors:
# Inventor's Name Inventor's Address
1 SODAL AUDUN HASSELBAKKVN,10,7053 RANHEIM,
2 TENGHAMN STIG RUNE LENNART 3303 SAGE TERRACE KATY, TEXAS 77450
3 STENZEL ANDRE NMI 11003 MYRTILE DRIVE,RICHMOND, TEXAS 77469
PCT International Classification Number G01V1/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10/233,266 2002-08-30 U.S.A.