Note: Descriptions are shown in the official language in which they were submitted.
~ 7~ 7
SEISMIC DE~ECTION ME~HOD AND APPARA~US
- The present invention relates to the acquisition of
seismic signals, and more particularly to a seismic detection
method and apparatus using a seismic pick-up composed of one or
more geophones.
Seismic interpretation, whether of surface data or of deep
data, is of capital importance in oil prospecting, and the
quality of the result depends ~or the most part on the quality
of the seismic signal as picked up together with its initial
processing.
Seiæmic waves are very often picked up using pick-ups made
of one or more electromagnetic type geophones. These geophones
essentially comprise a coil and a magnet, one of which i8 fixed
relative to the Earth, while the other is suspended by a spring
from a fixed support. Any relative displacement between the
coil and the magnet generates an electromotive force (e.m.f.)
across the terminals of the coil proportional to the speed of
the movement.
In conventional manner, the e.m.f. is fed through a
damping resistance connected directly in parallel with the
geophone so as to produce damping which is typically between
60% and 70%. Seen from the geophone, the damping resistance is
in series with the internal resistance of the geophone, and
consequently acts as a voltage divider, thus reducing the
signal presented to the voltage preamplifier. The input
impedance of the preamplifier iB high enough not to modify the
damping.
The voltage divider circuit has the drawback of reducing
the geophone's sensitivity. This first drawback can be
3 remedied either by connecting a plurality of geophone~ in
series, or by increasing the number of turns in the coil.
Each o~ these solutions has its own drawback: namely
increasing the volume of the pick-up per se and/or increasing
its internal resistance.
When seismic signals are being picked up at depth, the
pick-ups are situated inside an elongate sonde of outside
diameter which is less than 100 mm, and which is suitable for
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being suspended at the end of an electric cable inside a
_ borehole. Until very recently, single axis pick-ups comprising
four geophones in series have been used. However, progress in
seismic interpretation has lead very rapidly to consideration
of acquiring seismic signals along three orthogonal axes.
~urther, in deviation boreholes it is desirable for one of the
three axes to be vertical, which naturally leads to the use of
suspended pick-ups being considered. The use of three
orthogonal suspended pick-ups, with each pick-up being made up
of four geophones in series, requires a complex and voluminous
mechanical structure.
Recent studies have shown that the weight of a seismic
sonde should be as light as possible so as to obtain better
acoustic coupling between the pick-up and the wall of the
borehole. This weight condition is in direct opposition to the
above-mentioned line of development.
~ urther, increasing the number of turns in the coil, and
hence the internal resistance of the pick-up leads to
increasing input impedance for the amplifier and hence of its
negative feedback resistance, thus very rapidly reaching values
that are high and not recommended for connection to an
amplifier. ~he constraints of this upper limit are made even
more onerous by the fact that the amplifier circuit is
subjected to temperatures as high as 200C.
One aim of the invention is thus to guarantee optimal
transfer of the energy generated by a geophone to the input of
a preamplifier.
Another aim of the invention is to minimize the effect of
noise at the input to the preamplifier.
Seismic deteotion apparatus in accordance with the
invention comprises a seismic pick-up made of one or more
geophones, a damping resistance, and an amplifier, wherein said
damping resistance is essentially constituted by a resistance
connected in series between the said pick-up and said
amplifier.
In a preferred embodimentl the damping resistance is made
of two equal resistances, one of which is connected in series
0~
with the positive input of the amplifier while the other one is
~ connected in series with the negative input o~ the amplifier.
A seismic detection method in accordanee with the
invention in which the electrical signals generated by a pick-
up made of one or more geophones connected in series are dampedand the damped electrical signals are amplified, consists in
making all of the current delivered by the pick-up participate
in the amplification.
The following description will facilitate understanding
0 the invention.
Figure 1 is a circuit diagram of a prior art preamplifier
circuit;
Figure 2 is a circuit diagram of a preamplifier circuit in
which the dampïng resistance is connected in series between the
pick-up and the amplifier; and
~ igure 3 is a circuit diagram of a preferred embodiment of
a preamplifier circuit.
A conventional preamplifier circuit is briefly de~cribed
to begin with. The electromagnetic or speed-sensitive seismic
pick-up is symbolized in ~igure 1 by a voltage source 1
delivering an electromotive force (e.m.f.) el and having a
corresponding internal resistance 2 of value R2. A damping
resistance 3 of value R3 is connected in parallel with the
pick-up, and one of its terminals is grounded.
~his conventional circuit constitutes a voltage divider,
with the voltage across the terminals of the damping resistance
being equal to:
V(R3) _ el.R3/(R2 ~ R~)
3~
~he volt~ge V(R3) is applied via an input resi~tance 4
of value R4 to the negative input of an amplifier 9. ~he
amplifier is provided with a negative feedback loop constituted
by a resi~tance 5 of value Rs which is much greater than
R3, such that the gain g1 Of the amplifier 9 is equal to:
gl = - R5/R4
1~3t7~0~
A voltage source 6 placed at the negative input to the
_ amplifier 9 symbolizes a voltage Vn1 equivalent to the the
electrical noise of the circuit preceding the amplifier 9.
The output volta~e sl from the amplifier can thus be
expressed as a function o~ e1 and Vn1:
s1 = g1~e1-R3/(R2 + R3) + g1.Vn1
The ratio R3/(R2 + R3) i8 obviously less than 1, and
is typically about 60~, such that the contribution of the
e.m.f. e1 to the signal s1 is considerably reduced to the
detriment of the electrical noise Vn1. In other words the
following expression may be written:
The Gain of e1 R~
_______________ _ <
The Gain of Vn1 R2 + R3
It is thus essential, in the above-described arrangement,
to keep the electrical noise as low as possible, ie. to use
relatively low values of resistance R2 and R3 in the range
of about 300 ohms to 1000 ohms.
lhis condition obviously limit~ the sensitivity of the
pick-up, since it determines the maximum number turns in the
coil.
With reference to Figure 2, the damping resistance 13 of
value R13 is connected in series with the pick-up which is
represented by a voltage source 11 delivering a voltage e2
and having an internal resi~tanoe 12 of value R12. ~he
re~iatances R12 and R13 thus con~titute the input
resi~tance to the amplifier 19 whose gain is marked g2.
The voltage amplifier 19 may, for example, be an HA-5130
type low-noise operational amplifier manufactured by Harris Co.
The pick-up thus delivers current into a damping
re~istance R13, to obtain damping of 60% to 70%, but the same
current participates in the amplification.
~3 -~30~
As in the preceding circuit, the electrical noise i~
represented by a voltage source 16 delivering a voltage Vn2
which is added to the negative input of the amplifier 19. A
resistance 15 of value R15 provides the amplifier 19 with
5 negative ~eedbac~.
The gain g2 f the amplifier is given, in practice, by
the ratio of the feedback resistance 15 divided by the sum of
the internal resistance 12 of the pick-up plus the damping
resistance 13. ~he output voltage s2 can thus be expressed
as follows:
S2 = - (e2 ~ Vn2).Rls/(R12 + R13)
~he output voltage s2 is thus not degraded by the
electrical noise to the same extent as in the conventional
circuit, which may be expressed as follows:
~he Gain of e2
____________ = 1 .
~he Gain of Vn2
It is thus quite possible, by means of such a circuit, to
increase the sensitivity of the pick-up, by adding turns to the
coil, for example, without thereby increasing the contribution
o~ the electrical noise to the signal s2-
~igure 3 shows a more elaborate embodiment. As befere,
the pick-up is represented by voltage source 21 delivering a
voltage e3 and having an internal resietance 22 of value
R22. In thi~ cirouitl the damplng re~i~tance i~ co~po~ed of
two reeistance~ 231 and 232 equal to half the value o~ the
damping resistance R13, with each o~ them being connected in
series with a respective one of the output~ ~rom the pick-up.
~he output~ from the two resistances 231 and 232 are connected
to respective input~ of the amplifier 29.
~urther, the connection between the pick-up and the
amplifier is fitted uith a guard ring 30 to improve electrical
noise immunity with temperature. ~he voltage applied to the
__ _.. _.. ~ . , . ,__, __ .___ __ __,_ . .~, .. .. . ., ,.. , .. _ .. . _ __ .
07
guard ring is equal to s3/2 and is taken from a voltage
divider formed by two equ~l resistances 27 and 28.
Negative feedback for the amplifier 29 is provided at its
negative input by a resistance 251 connected in parPllel with a
capacitance 261. Common mode rejection is assured by a
differential circuit comprising a resistance 252 connected in
parallel with a capacitance 262 and connected to the positive
input. ~he resistances 251 and 252 are equal and determine the
gain of the amplifier 29. The capacitances 261 and 262 are
equal and determine its upper cut-off frequency.
~ his differential circuit provides improved immunity to
common modes.
~ he gain g3 of such a circuit may be expressed as
follows:
e3 . R251
3 --____
R231 + R22/2
The gain g3 is chosen in such a manner as to ensure that
the signal S3 has a sensitivity of 2500 V/m/s. Such a
circuit makes it possible to use an electromagnetic pick-up
having an internal impedance of about 3.5 Kohms ar.d whose
sensitivity is about 80 V/m/s. A damping resistance of 12
Kohms, ie. 6 Kohms for each of the resistances 231 and 232,
combined with a feedback resistance of 220 Kohms provides a
gain of about 30.
~he use o~ such a det0ction device m~kes it po~ible to
reduce appreciably the number of geophones that need to be
connected in series; it providee an eleeant solution to the
problems of bulk and electrical noise de~cribed above.
