Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-11-19
2004-04-20
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C702S002000
Reexamination Certificate
active
06725164
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to hydrophones employed in seismic exploration. More particularly, the invention relates to an improved hydrophone circuit that provides the frequency response characteristics of an accelerometer.
Due to the increasing difficulty and cost of finding petroleum resources in the world today, exploration techniques are becoming more and more technologically sophisticated. For example, many have found crystal hydrophones to be useful in petroleum exploration. Basically, hydrophones are used to measure seismic waves created by a source such as an air gun or a dynamite charge, to obtain detailed information about various sub-surface strata of earth.
As shown in
FIG. 1A
, a typical crystal hydrophone
100
includes a diaphragm
102
, a crystal
104
, and a housing
106
that is typically filled with a gas
107
. The diaphragm
102
, which has front and rear sides
102
a
,
102
b
, is made from a material such as Kovar or a Beryllium Copper compound, and is electrically connected to the crystal by a conductive epoxy
108
. The crystal
104
is typically made from a material such as Lead Zirconium Titanate, and is silver-plated on its top
104
a
and bottom
104
b
to achieve better conductivity. The crystal
104
is initially polarized by applying a high-voltage electrical charge to the crystal
104
. When the polarized crystal
104
experiences pressure resulting from a physical input such as sound, fluid pressure, or another type of pressure, it produces a voltage representative of the pressure experienced. The crystal
104
is electrically connected to electrical output leads
110
,
112
. To protect the crystal
104
from contaminants, and to maintain the crystal
104
in atmospheric pressure, the crystal
104
and the rear side
102
b
of the diaphragm
102
are sealed within the gas-filled housing
106
. The housing
106
protects the crystal
104
and diaphragm
102
, and facilitates mounting of the hydrophone
100
.
The diaphragm
102
functions to vibrate in response to physical pressures it experiences. The physical deflection of the diaphragm
102
is transferred by the epoxy
108
to the crystal
104
, deforming the electron structure of the crystal
104
and causing an electrical potential to be provided across the leads
110
,
112
.
Another apparatus that is also useful in petroleum exploration is the accelerometer. Accelerometers are commonly used to measure the motion of the earth's surface in response to seismic waves created by a seismic source, to obtain detailed information about various sub-surface strata in the earth.
As mentioned above, hydrophones and accelerometers are often used in petroleum exploration in conjunction with seismic equipment. In one example of such an application (FIG.
1
B), a cable
150
including one or more hydrophones and one or more accelerometers is placed on the sea floor
154
. Such a cable may be made up of cylindrical units
152
, where each unit
152
includes a geophone and an accelerometer.
Seismic waves are produced by a seismic source
156
that is towed behind a ship
158
; the seismic source
156
may comprise an air gun, a dynamite charge, or the like. The seismic source
156
produces a large explosion, creating seismic waves
160
. The seismic waves
160
travel through water
162
and various layers of earth
164
, and are reflected back to the cable
150
as upgoing incident waves
161
. Each unit
152
detects and measures the incident waves
161
and creates a real-time record of the results. This record is typically stored in a recorder (not
20
shown) that is linked to or contained within the cable
150
. Records of this nature help geologists determine the makeup of the earth
164
.
One problem with this arrangement, however, is surface ghost signals
166
. Surface ghost signals
166
are produced by incident waves
161
that are reflected from the water's surface
168
. At the wavelengths typically used for seismic signals, the surface
168
provides an effective mirror to reflect incident waves
161
and create downgoing surface ghost signals
166
. Surface ghost signals
166
contain no additional information regarding the composition of the earth
164
or the possible petroleum deposits therein, and they interfere with the proper receipt and interpretation of the incident waves
161
. Accordingly, it is desirable to eliminate the errors introduced by the surface ghost signals
166
.
A hydrophone-accelerometer combination, in theory, is naturally suited to eliminate surface ghost signals. Generally, hydrophones detect pressure omnidirectionally, and accelerometers detect force or acceleration, which is directional. Due to the relative strengths of the incident waves
161
and the surface ghost signals
166
at different depths, a hydrophone's output and an accelerometer's output will both vary with depth. For a seismic wave
161
of a given magnitude and frequency, a hydrophone's output will vary with depth sinusoidally (curve
180
, FIG.
1
C). Likewise, for the given seismic wave
161
, an accelerometer's output will vary sinusoidally with depth (curve
182
, FIG.
1
C). The hydrophone and accelerometer outputs may be scaled by external circuitry or by a mathematical algorithm in a computer, so that their peak values have the same amplitude; for example, in
FIG. 1C
, the hydrophone and accelerometer outputs are scaled to a maximum peak amplitude of 1 and a minimum peak amplitude of −1. After such scaling, the sum of the hydrophone and accelerometer outputs will always be 1, irrespective of the depth at which the hydrophone and accelerometer are both located (curve
184
, FIG.
1
C). Therefore, in theory, a hydrophone output and an accelerometer output may be combined to effectively eliminate the influence of surface ghost signals
166
.
One problem in applying this theory is that the frequency responses of hydrophones and accelerometers differ. Therefore, the hydrophone and accelerometer outputs will only complement each other as shown in
FIG. 1C
when the seismic wave
160
has a certain frequency. As a result, if the frequency of the seismic wave
160
were to change, the combined hydrophone-accelerometer output
184
would no longer be constant.
The difference between frequency responses of hydrophones and accelerometers will now be explained with reference to
FIGS. 2-4B
. When an electronic amplifier
200
(
FIG. 2
) is utilized to amplify the output of a typical hydrophone
202
, the frequency response of the hydrophone
202
(
FIGS. 3A
,
3
B) resembles that of a single-pole high pass filter, since it exhibits a single pole and a 6 dB/octave slope at frequencies less than its natural frequency (f
n
). The amplifier
200
may comprise an operational amplifier. The hydrophone may be modeled as a voltage source
202
a
and a capacitor
202
b
and resistor
202
c
in series; the capacitor
202
b
and the resistors
202
c
and
204
provide the single pole, and hence the 6 dB/octave slope. The natural frequency of the hydrophone
202
depends upon the value of the internal resistance
204
(R
i
) of the amplifier
200
, the resistance (R
H
) of the resistor
202
c
, and the capacitance (C
H
) of the capacitor
202
b
; this relationship is shown in equation 1.0, below.
f
n
=
1
2
π
(
R
H
+
R
I
)
C
(
Hz
)
For typical hydrophones, the natural frequency ranges from about 2 to 3 Hz.
In contrast to the hydrophone
202
, as illustrated in
FIGS. 4A and 4B
, the frequency response of a typical force-balance accelerometer, such as that disclosed in U.S. Pat. No. 5,852,242, issued on Dec. 22, 1998, the disclosure of which is incorporated herein by reference, resembles an electrical circuit having a differentiating element in combination with a pair of simple lag elements. The resulting frequency response exhibits a 6 dB/octave slope at frequencies less than a first cut-off frequency (F
c1
), a substantially flat response between the first cut-off frequency (F
c1
) and a second cut-off frequency (F
2
Barlow John
Input / Output Inc.
Madan Mossman & Sriram P.C.
Taylor Victor J.
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