Aug- 139 1953
w. A. ALEXANDER
2,649,579
DETECTOR FOR SEISMIC EXPLORATiON
Filed Feb. 1, 1950
3 Sheets~$heet l
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Aug. 18, 1953
w. A‘ ALEXANDER
2,649,579
DETECTOR FOR SEISMIC EXPLORATION
Filed Feb. 1, 1950
3 Sheets-Sheet 2
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Aug. 18, 1953
w. A. ALEXANDER
2,6495 79
DETECTOR FOR SEISMIC EXPLORATION
Filed Feb. 1, 1950
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Patented Aug. 18, 1953
2,649,579
UNITED ST
> orrc
2,649,579
DETECTOR FOR SEISMIC EXPLORATION
Warren A. Alexander, Tulsa, Okla., assignor to
Standard Oil Development Company, a corpo
ration of Delaware
Application February 1, 1950, Serial No. 141,660
4 Claims. (Cl. 340-—17)
The present invention relates to an improved
2
type of portable seismometer, or geophone, for
seismic exploration. More particularly, it relates
earth’s subsurface, based on the principle that
part of the energy of the arti?cial seismic shock
will travel downwardly and be re?ected back to
to a portable seismometer of the condenser or
ward the surface by various more or less well
capacitor type wherein seismic vibrations cause
de?ned substrata, and that this re?ected energy
minute changes in the spacing of elongated ?ex
will be detected by the geophones and be recorded
ible conductors comprising the plates of the ca
on the seismograph record. Hence, the desirabil
pacitor, the changes being detected by a suitable
ity of placing on one record the traces of as many
circuit as will be more fully described herein
geophone locations as is practical, since a re?ec
after.
10 tion from a well-de?ned substratum will appear
One well-known technique of exploration for
on the record as a Wave form of increased ampli
oil or other mineral deposits comprises seismic
tude on all of the traces in some de?nite time
prospecting or re?ection seismography wherein
relation, permitting the re?ection to be “lined
a hole is drilled into the earth and an explosive
up” on the record.
charge or other means of producing an arti?cial 15
In order to increase the e?ectiveness of the
shock is placed in the hole. The shock is initiated
records it has become frequent practice to employ
and the seismic waves traveling through the earth
a plurality of geophones at each station, all tied
are detected at various points on the surface of
to one trace on the record. This practice has
the earth by means of sensitive pick-ups or geo
many advantages, including the cancelling out
phones which translate the detected waves into 20 of near-surface anomalies. For example, if only
electrical impulses which after suitable ampli?
a single geophone is used and it happens to be
cation can be recorded on a seismograph.
placed near a buried stump or boulder, an anom
Usually the geophones are placed in line with
the shot point and are more or less evenly spaced
alous travel time may be obtained for seismic
waves reaching that geophone, Whereas if 30 or
25 50 geophones are laid out and all tied to the same
from each other and from the shot point. Geo
phone stations will in general be from about 100
trace, anomalous signals will be cancelled out
feet to 500 feet apart in a straight line. Each
and the re?ected wave will be picked up by the
geophone is connected through a suitable cable
majority of the geophones. Also, if a plurality of
to a recording station, usually a ?eld truck, pro
geophones are laid out in the direction of the line
vided with seimograph instruments.
30 of spread of the geophone stations and several
Conventionally, a seismograph record is ob
geophones are tied to the same trace the group
‘ tained by means of a number of moving coil
of geophones will act as a directional receiver
galvanometers each one of which has a mirror at
since signals that are not lined up will tend to
tached thereto, the galvanometers being arranged
cancel out, whereas nearly plane wave fronts,
in a battery in connection with a source of light 35 representing re?ections from substrata, will ar
in such relation to a moving strip of sensitized pa
rive at all of the geophones at substantially the
per or ?lm that there will be recorded on the pa
same time. Furthermore a plurality of geophones
per or ?lm a plurality of wave forms or traces
tied together in this manner will aid in discrim
representative of the seismic waves that have been
inating against disturbing background noise,
picked up by the individual geophones, suitably 40 erally referred to as “wind noise” and “ground
ampli?ed, and fed to the galvanometers. Many
unrest”; thus resulting in improved presentation
seismograph instruments are capable of recording
of the re?ected signal. A further result will be
as many as 24 or 36 individual traces simul
that weaker re?ected signals will be discernable,
taneously. The strip of paper or ?lm is moved
or conversely, smaller initiating explosive charges
longitudinally at a substantially constant speed 45 may be employed to give equal efficiency in de
and is provided with suitable timing marks so
tecting re?ecting layers.
that when the seismograph record or seismogram
It is evident that although the use of a plu
is later examined it is possible to determine the
rality of geophones at each geophone station pro
length of time required for the arrival of seismic
duces many advantages, the practice does add to
waves at any particular point on the earth’s sur 50 the time and labor involved for the making of
face either directly from the source or by re?ec
each record, as well as to the investment cost.
.It is accordingly one object of the present inven
tion .to provide a single geophone which will re
manner above referred to is of value in that it.
place such a plurality of geophones at each sta
gives information regarding the nature of the 65 tion. 5 Another object is to provide a single'in
tion from underlying strata.
The making of seismograph records in the
2,649,579
5
tially rather than radially with respect to the
shot hole.
One suitable circuit for detecting changes in
capacitance in the seismometer capacitor unit is
shown in the circuit diagram of Fig. 4. A vacuum
tube oscillator 20 is provided having output ter
6
.ance with the receipt of a seismic signal, the
bridge becomes unbalanced, causing a variation
in potential, produced at terminals 31 and 38 of
the bridge circuit. This variation in potential
is impressed on the control point of a radio fre
quency ampli?er 4| and the ampli?ed signal thus
minals 2| and 22. One plate of the capacitor of
obtained is fed into diode demodulator 42 and
the seismometer unit, represented in Fig. 4 by
then through cable [8 to the recording ampli?er
numeral 24, is connected to terminal 22 and the
and recording instruments in the recording truck.
other plate of the capacitor is connected through 10 A related circuit is described in Electronics, April
?xed resistor 23 to output terminal 2|. The two
1949, page 126.
plates of the capacitor 24 are also connected
It is to be understood that the speci?c em
through a ?lter section 25 to terminals 29 and
bodiments of the invention depicted in the fore
30 which can be connected into cable 18 of Fig. 3,
going description are by way of example only and
leading to the recording instruments on the truck. a are not intended to limit the scope of the inven
Oscillator 20 may be set up to produce an alter
tion, the same being limited only by the follow
nating current of 100 kilocycles, for example, and
ing claims.
?lter unit 25 will be designed to eliminate the
What is claimed is:
100 kilocycle frequency and to pass frequencies
1. Improved portable seismometer of the capac
in the normal seismic range, i. e. about 10 to 100 20 itor type comprising an elongated essentially ?at
cycles. Filter 25 comprises a recti?er unit 26, an
?exible tube, an elongate thin flat ?exible con
inductance unit 27, and a capacitor 28.
ductor carried by the bottom wall of said ?exible
As long as the capacity of capacitor 24 does not
tube, a second elongate thin ?at ?exible con
change, no voltage of seismic frequency will be
ductor carried by the top wall of said ?exible
produced across terminals 29 and 30. However,
tube out of electrical contact with said ?rst con
when the capacity of capacitor 24 is changed as,
ductor, an elastic non-conducting non-liquid me
for example, when being disturbed by the receipt
dium disposed between said conductors, and a
of a seismic impulse, a seismic frequency signal
plurality of flat mass-providing plates carried by
will be set up across terminals 29 and 30 which
the top wall of said ?exible tube.
can be ampli?ed and recorded by the conven 30
2. Improved seismometer according to claim 1
tional seismograph equipment. When utilizing
in which said non-liquid elastic medium com
the circuit of Fig. 4, oscillator 20, resistor 23 and
prises a layer of sponge rubber.
?lter 25 will be contained within unit 16 of Fig. 3.
3. Improved seismometer according to claim 1
If greater sensitivity is desired, a circuit such
in which said non-liquid elastic medium com
as that depicted diagrammatically in Fig. 5 may 35 prises a gas under pressure.
be used. A modulated oscillator 3|, which, for
4. Improved seismometer according to claim 3
example, produces an alternating current of 500
including a pressure valve in the wall of the tube
kilocycles with a modulation of 500 cycles per
to control the pressure of the gas.
second may be employed. The current from this
WARREN A.
oscillator is applied across terminals 32 and 33 of 40
a balanced A. C. bridge circuit 34 comprising in
References Cited in the ?le of this patent
ductances 35 and 36 and capacitors 39 and 40,
UNITED STATES PATENTS
the latter being a variable capacitor whose ca
Number
Name
Date
pacitance may be adjusted to bring the bridge
into balance, and 39 representing the capacitor 45
comprising the two plates of the seismometer
device depicted in Fig. 1. The bridge is balanced
after the seismometer spread has been set out
so that as the capacitance of 39 varies in accord
1,584,613
Comstock et al ____ __ May 11, 1926
2,257,187
2,272,984
2,305,717
2,408,478
Owen ____________ __ Sept. 30,
Ritzmann ________ __ Feb. 10,
La Bell __________ __ Dec. 22,
Petty _____________ __ Oct. 1,
1941
1942
1942
1946