Marine seismic system

A method for eliminating ghosts, or reflections from the air/water interface of a body of water, from seismic signals detected at a predetermined depth is provided. The method employs both a pressure sensor and a motion sensor. A seismic signal and its corresponding ghost signal detected by a pressure sensor is filtered as a function of depth of the sensor to provide a preselected amplitude and zero phase shifted band-limited spike signal at an arrival time corresponding to the arrival time midway between the seismic and ghost arrival times. Similarly, a seismic and ghost signal from a motion sensor is filtered as a function of depth of the sensor to provide a band-limited spike having an arrival time midway between the seismic and ghost arrival times. The filtered motion and pressure spikes are added together in proportion to their respective signal-to-noise ratios. The proportionally added signal is a ghost free seismic signal having a maximum signal-to-noise ratio for each frequency component and may be employed in further seismic processing steps.

BACKGROUND OF THE INVENTION 
This invention relates generally to seismic exploration of substrata 
beneath bodies of water and, more particularly, to a marine seismic system 
for sensing reflected seismic waves from such substrata. 
Marine seismic exploration is usually conducted by towing a seismic 
streamer at a given depth through the ocean or other body of water. The 
streamer is provided with a plurality of pressure sensors, such as 
hydrophones, disposed at appropriate intervals along the length thereof. 
Acoustic wave energy is provided in the vicinity of the cable by an air 
gun or other suitable means; this wavelet travels downwardly through the 
earth with a portion of it being reflected upwardly at levels where there 
is a contrast in the acoustic impedance characteristics of the strata. The 
plurality of reflections of the source wavelet generates a sequence of 
upwardly traveling reflection wavelets that are distributed in time. The 
pressure sensors detect these primary pressure waves produced in the water 
by the upwardly traveling reflection wavelets and provide electric signals 
indicative thereof to suitable processing and recording equipment located 
on the seismic vessel that is towing the streamer. The pressure sensors 
also receive secondary pressure waves reflected from the surface of the 
water as a result of the mismatch in acoustic impedance at the air-water 
interface; these secondary waves may adversely affect the seismic signals. 
Nearly total cancellation of certain frequencies of the seismic signal may 
result, since the pressure wave undergoes a 180.degree. phase shift when 
reflected at the air-water interface. The prior art, such as U.S. Pat. No. 
3,290,645, has attempted to overcome this problem by employing both a 
pressure sensor and a particle velocity sensor. The output signals of the 
pressure sensor in response to the primary and secondary pressure waves 
have opposite polarity; whereas, the output signals of the particle 
velocity sensor have the same polarity for the primary and secondary 
waves. The prior art combines the pressure wave signals with the particle 
velocity signals to cancel the surface reflected wave front or ghost; 
however, it has been found that the mere combination of a pressure wave 
signal with a particle velocity signal may severely degrade the 
signal-to-noise ratio of the lower frequencies in the seismic band so that 
the signal-to-noise ratio of the combined signal may be less than the 
signal-to-noise ratio of the pressure wave sensor alone. This high noise 
level in the lower frequencies of the output of the particle velocity 
sensor is a function of the mounting of the particle velocity sensor and 
the geometry and materials of the cable. Particle velocity sensors such as 
those disclosed in U.S. Pat. No. 3,281,768, which consist of either a 
particle displacement sensor in conjunction with a differentiating circuit 
or a particle acceleration sensor in conjunction with an integrating 
circuit may also be subject to the high noise levels. 
Therefore, it is an object of the present invention to provide a marine 
seismic system that eliminates the adverse effects of the reflected, 
secondary pressure wave on the seismic signal and provides a good 
signal-to-noise ratio over the seismic band. 
SUMMARY OF THE INVENTION 
The present invention supplies acoustic energy to the body of water above 
the substrata to be seismically explored. The primary pressure wave 
reflected from the substrata beneath the body of water and the secondary 
pressure wave caused by a secondary reflection of the primary pressure 
wave from the air-water interface are sensed at a predetermined depth, and 
a first signal indicative thereof is generated. This first signal 
comprises a first plurality of frequency components having a plurality of 
amplitudes and phase shifts. The particle motion of the water accompanying 
the primary pressure wave and the particle motion of the water 
accompanying the secondary pressure wave are also sensed at the 
predetermined depth, and a second signal indicative thereof is generated. 
This second signal comprises a second plurality of frequency components 
having a plurality of amplitudes and phase shifts. The amplitudes of the 
first plurality of frequency components is multiplied by a first set of 
factors which equalize the amplitudes of all of the first plurality of 
frequency components, and the phase shifts of the first plurality of 
frequency components are modified so that each of the phase shifts, other 
than the phase shifts caused by the time at which the primary pressure 
wave and the secondary pressure wave are sensed, is zero to generate a 
third signal. The amplitudes of the second plurality of frequency 
components is multiplied by a second set of factors which equalize the 
amplitudes of all of the second plurality of frequency components to the 
equalized amplitudes of the first plurality of frequency components, and 
the phase shifts of the second plurality of frequency components are 
modified so that each of the phase shifts, other than phase shifts caused 
by the time at which the particle motion of the water accompanying the 
primary pressure wave and the particle motion of the water accompanying 
the secondary pressure wave are sensed, is zero to generate a fourth 
signal. The amplitude of each frequency component of the third signal is 
modified as a function of a first predetermined signal-to-noise ratio 
related to the pressure wave sensing step and a second predetermined 
signal-to-noise ratio related to the particle motion sensing step to 
generate a fifth signal, and the amplitude of each frequency component of 
the fourth signal is modified as a function of a first predetermined 
signal-to-noise ratio related to the pressure wave sensing step and a 
second predetermined signal-to-noise ratio related to the particle motion 
sensing step to generate a sixth signal. The fifth and sixth signals are 
then combined to produce a signal indicative of the substrata beneath the 
body of water. 
The multiple reflections of the source wavelet from the various interfaces 
of the substrata results in a sequence of reflection wavelets that are 
distributed in time. The time interval between generation of the acoustic 
energy and the subsequent reception of the received wavelet is an 
important parameter in seismic exploration. It is well known in the art 
that the phase spectra of the received wavelet will have a linear phase 
shift versus frequency component whose slope is proportional to the time 
delay. The phase shift associated with this time delay is preserved in the 
signal processing of the present invention. The signal processing is 
applied to all of the reflection wavelets; however, for the sake of 
clarity, a single, base reflection wavelet occurring at an arbitrary time 
zero can be considered. The response to the plurality of reflection 
wavelets would be the superposition of the time delayed versions of the 
base wavelet response. The pressure wave and particle motion signals, 
which are produced in response to the base wavelet, are normalized or 
modified so that all of the frequency components within the signals have 
the same amplitude. The phase angles associated with the frequency 
components of the pressure wave and particle motion signals are also 
modified so that all of the frequency components have zero phase shift, 
except for the phase shifts caused by the nonzero time of arrival of the 
particular wavelet. The normalized, zero phase pressure wave and particle 
motion signals are then weighted as a function of frequency of the 
relative predetermined signal-to-noise ratios applicable to the pressure 
wave and particle motion sensors. The weighted signals are then combined 
and transmitted to appropriate electronic equipment for further signal 
processing and recording. In the preferred embodiment of the invention, 
the normalizing, zero phasing, weighting and combining are performed by a 
digital computer and the motion of the water particles accompanying the 
primary and secondary waves is sensed by an accelerometer; however, any 
sensor which senses particle displacement or any derivative thereof can be 
utilized to sense the motion of the water particles. 
Other objectives, advantages and applications of the present invention will 
be made apparent by the following detailed description of the preferred 
embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a seismic exploration vessel 10 is shown towing a 
marine streamer cable 12 through a body of water located above the 
substrata that is to be seismically explored. Cable 12 can be quite 
lengthy, for example, about 2 miles, and is normally composed of a number 
of individual sections 14 connected end to end. The forward section of 
cable 12 is connected to vessel 10 by a typical lead-in section 16. Each 
section 14 contains a number of hydrophones (not shown) and motion sensors 
(not shown) that are positioned in each of sections 14 so that they are 
interspersed as is known in the art. Acoustic wave energy is provided in 
the vicinity of cable 12 by an air gun 18 or other suitable means. This 
wavelet travels downwardly through the earth with a portion of it being 
reflected upwardly at levels where there is a contrast in the acoustic 
impedance between layers of the strata, for example, at point 20. The 
plurality of reflections of the source wavelet generates a sequence of 
upwardly traveling reflection wavelets that are distributed in time or 
phase shifted. The hydrophones sense the acoustic pressure waves produced 
in the water by the upwardly traveling reflection wavelets. The 
propagating wave fronts also cause movement of the water particles as they 
pass, which is sensed by the motion sensors. 
FIG. 2 illustrates a schematic block diagram of the seismic data 
acquisition system of the present invention. The primary pressure waves 
reflected from the substrata and the secondary pressure waves reflected 
from the air-water interface are detected by hydrophone 22. A sensor 24 
for detecting the motion of the water particles accompanying the primary 
and secondary pressure waves can comprise an accelerometer 26, or 
alternatively, sensor 24 can comprise a particle displacement sensor, a 
particle velocity sensor, or any sensor that senses any derivative of 
particle displacement. Accelerometer 26 must measure the vertical 
component of the water particle motion or acceleration for all 
orientations. Accelerometers that are axially symmetric, that is, their 
response is invariant only for rotations about their axis and are 
sensitive only to particle movements that are parallel to their axis are 
unsuitable, because the particle motion is predominantly vertical and the 
cable rotation causes the accelerometer axis to be nonvertical. One 
accelerometer that has been found suitable for use is the WH-1 
accelerometer manufactured by Litton Resources Systems of Houston, Tex. 
Preferably, accelerometer 26 is mounted in a low-noise mount as disclosed 
in my copending U.S. patent application, Ser. No. 300,430, which was 
filed Sept. 8, 1981, and is assigned to a common assignee. 
The outputs of hydrophone 22 and accelerometer 26 are provided to signal 
processor 30 which can be located on seismic vessel 10 of FIG. 1. Signal 
processor 30 may be a digital computer or other suitable electronic 
processing equipment which performs the signal processing illustrated in 
the flow diagram of FIG. 5. As discussed above, the multiple reflections 
of the source wavelet from the various interfaces of the substrata results 
in a sequence of reflection wavelets that are distributed in time as shown 
in FIG. 3 by the wavelets for two reflection events. The time interval 
between generation of the acoustic energy and the subsequent reception of 
the received wavelet is an important parameter in seismic exploration. It 
is well known in the art that the phase spectra of the received wavelet 
will have a linear phase shift versus frequency component whose slope is 
proportional to the time delay. The signal processing of the present 
invention is applied to all of the reflection wavelets; however, for the 
sake of clarity, a single, base reflection wavelet occurring at an 
arbitrary time zero can be considered. The response to the plurality of 
reflection wavelets would be the superposition of the time delayed 
versions of the base wavelet response. 
Referring to FIGS. 3 and 5, the output signal from hydrophone 22, which is 
shown in FIG. 3(a), is provided to filter 32, and the output signal from 
accelerometer 26, which is shown in FIG. 3(b), is provided to filter 34. 
FIG. 3(c) illustrates a sample output of the particle velocity embodiment 
of motion sensor 24 which could be provided to filter 34 in place of the 
output from accelerometer 26. At filters 32 and 34 the amplitude of each 
frequency component contained in the particle acceleration and pressure 
wave signals is modified or normalized. In addition, filters 32 and 34 
remove the phase angles associated with the signals to make all of the 
frequency components zero phase so that the signals can be added properly 
after the amplitude terms are weighted by the signal-to-noise filters, as 
discussed hereinbelow. 
The particle acceleration and pressure wave signals can be represented by 
the functions M.sub.a e.sup.j.phi. a and M.sub.p e.sup.j.phi. p, 
respectively, which are illustrated in FIGS. 4(a) and 4(b). For the sake 
of clarity, the spectra of a single, base reflection wavelet occurring at 
an arbitrary time zero has been considered in FIG. 4. The frequency 
spectra associated with both the hydrophone and accelerometer signals 
exhibit periodic notches caused by the secondary reflections from the 
surface of the water. The notches in the pressure wave signals occur at 
multiples of the frequency defined by the wave propagation velocity of the 
body of water, which is approximately 1500 meters per second, divided by 
two times the depth of the detector. The first notch frequency is equal to 
the reciprocal of the time interval T, which is the time for a wave to 
propagate from the detector to the surface and back to the detector. The 
notches in the particle acceleration signals occur at frequencies midway 
between the notches in the pressure wave spectra. Thus, peaks in the 
particle velocity response occur at pressure wave notch frequencies and 
vice versa. Filters 32 and 34 consist of the inverses of the particle 
acceleration and pressure wave functions, i.e., 
##EQU1## 
Filtering or multiplying in the frequency domain with the functions 
PF.sub.a and PF.sub.p accomplishes the normalization and zero phasing, 
except that the phase shifts caused by the nonzero time of arrival of the 
particular wavelet are preserved. The output from either filter 32 or 
filter 34, as shown in the exemplary waveform of FIG. 3(d), consists of 
primary and secondary pulses that have been collapsed to bandlimited 
spikes. It should be noted that the noise level is higher in the outputs 
of filters 32 and 34 than it is in the outputs of hydrophone 22 and 
accelerometer 26, because the notch frequencies are boosted by the 
filtering. The filter functions can be obtained by utilizing the 
theoretical equations for the particle acceleration and pressure signals 
in conjunction with laboratory measurements of the sensitivity of 
accelerometer 26 and hydrophone 22. Alternatively, the filter functions 
can be obtained by actual measurements with the cable in which the 
particle acceleration and pressure wave responses to seismic wave 
reflections from a known reflector are measured. In this case the effects 
of the source signature and the reflector must be removed prior to the 
calculation of the filter functions. 
The signal-to-noise ratios of the pressure wave signals and the particle 
acceleration signals, as a function of frequency, are determined by tests 
performed prior to operation of the data acquisition system. The noise 
level is ascertained by measuring the noise detected by hydrophone 22 and 
accelerometer 26 when the cable is being towed and there is no deliberate 
acoustic excitation, and the signal level is ascertained from some strong 
reflection event. These tests are repeated to ensure the statistical 
accuracy of the results. The signal-to-noise ratio weighting functions for 
the pressure wave signal and the particle acceleration signal consist of a 
weighting factor for each frequency contained in the respective signals. 
These factors do not vary with changes in either signal or noise level 
provided that these changes are common to both hydrophone 22 and 
accelerometer 26. The relative signal-to-noise ratio weighting function or 
zero phase filter that is applied to the Fourier transform or frequency 
domain components of the pressure wave signal by filter 36 is defined as 
the signal-to-noise ratio for the pressure wave signal divided by the 
quantity consisting of the signal-to-noise ratio for the pressure wave 
signal plus the signal-to-noise ratio for the particle acceleration 
signal. Similarly, the relative signal-to-noise ratio weighting function 
or zero phase filter that is applied to the Fourier transform or frequency 
domain components of the particle acceleration signal by filter 38 is 
defined as the signal-to-noise ratio for the particle acceleration signal 
divided by the quantity consisting of the signal-to-noise ratio for the 
pressure wave signal plus the signal-to-noise ratio for the particle 
acceleration signal. The weighted pressure wave signal from filter 36 and 
the weighted particle acceleration signal from filter 38 are combined at 
point 40 to provide the bandlimited spike shown in FIG. 3(e). It should be 
noted that the noise level has been reduced by filters 36 and 38. The 
signal at point 40 is transmitted to further signal processing and 
recording equipment. Accordingly, the present invention provides a signal 
that has the maximum signal-to-noise ratio for each frequency component 
contained in the base wavelet. 
It is to be understood that variations and modifications of the present 
invention can be made without departing from the scope of the invention. 
It is also to be understood that the scope of the invention is not to be 
interpreted as limited to the specific embodiments disclosed herein, but 
only in accordance with the appended claims when read in light of the 
foregoing disclosure.