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@c?. ‘i, 39%.
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I
o. s. PETTY
29499478
SEISMI-C APPARATUS AND METHOD
Original Filed March 14, 1940
E2.
I
-
3 Sheets-Sheet 1
W
4%
4'2
3mm
(04“; a? 9m?
57
Oct. 1, 1946.
if
'
o. s. PETTY
~
2,408,478
SEISMIC APPARATUS AND METHOD
‘v
Original Filed March 14, 1940
3 Sheets-Sheet 2
o. s. YPETTY
_
2,48,478
SEISMIC APPARATUS AND METHOD
Original Filed. March 14, 1940
3 Sheets-Sheet 3
Patented Oct. 1, 1946
2,408,478
UNITED‘ STATES PATENT OFFICE
’ 2,408,478
SEISMIC APPARATUS AND METHOD
Olive Scott Petty, San Antonio, Tex.
Continuation of application Serial No. 324,013,
March 14, 1940. This application November 17,
1943, Serial No. 510,685
1
19 Claims. (Cl. 1'77—352)
2
This invention relates to portable seismometers,
intended particularly for use in conducting geo
physical studies of the type wherein an arti?cial
shock is imparted to the earth and the seismic
waves emanating therefrom are received, after
refraction through certain strata 0r re?ection
The whole of the apparatus is enclosed in a
watertight housing through which the necessary
conductors are passed;
The two condenser plates which are spaced
closely to each other are mounted respectively
for movement with the housing and with a steady
from various strata interfaces, at one or more
mass hinged thereto;
seismometer stations which are customarily
connected for electrical communication with a
A novel means of applying and adjusting the
balancing orrestoring force is provided for set
'
recording station equipped with apparatus for 10 ting the plates at a predetermined repose spac
producing on a chart one or a, plurality of traces, .
each one representative of changes in the fre
quency and amplitude of the seismic wave vi
111%‘ I
'
Adjustments are provided for the natural pe
riod of the seismometer, the rate of damping, the
spacing of the plates and for other characteris
brations ‘received by its particular seismometer.
More particularly, the invention relates to a 15 tics;
portable seismometer of the condenser or capac
The repose setting can be adjusted electrical
itive type wherein seismic vibrations cause
ly from a remote position to permit adjustment
changes in the relative spacing of a pair of con
of the sensitivity and frequency responsiveness
denser, plates so arranged with other impedance
of the seismometer;
elements in an oscillating circuit that the fre 20
Novel means is employed for mounting the
quency of oscillation is dependent upon and
vacuum tube and other circuit components
changes with the spacing of these plates. This
whereby they are given the greatest possible pro
circuit should have such characteristics that
tection against damage or change in character
changes in frequency are substantially directly
re?ected in changes in the plate current of the 25
thermionic tube forming a portion of the oscil- ,
lating circuit.
These changes in plate current
istics;
Novel means are used for supporting the
framework of the mechanism within the hous
ing and conducting the currents to and from the
various electrical portions of the apparatus;
A novel circuit arrangement is provided which
are transmitted to the recording station where
they serve to actuate appropriate known mech
anism for forming the trace.
30 insures against phase displacement while at the
A number of problems present themselves and _ same time producing such changes in' the plate _
each must be solved satisfactorily for the produc
current as can be readily transmitted to the re
tion of a seismometer capable of operation in
cording station without being subject to extrane
?eld service in various parts of the ‘country where
ous ?elds and the like which might cause varia
temperatures have large ranges and change rap 35 tions in the characteristics of the trace.
idly; where the characters of the upper and sub
The apparatus is capable of use in multiple
strata are entirely different; and where various
with such spacing between seismometers that
other factors of great variability are encountered.
the effects of “ground roll’ can be substantially
The apparatus must be sui?ciently rugged to
eliminated.
'
withstand rough treatment in handling and 40 Other and further objects and lesser features
transportation; it must be watertight to permit
ofthe invention will be more apparent to those
of, use in swamps and beneath the surfaces of
skilled in the art upon a consideration of the
bodies of water; it must be extremely compact
accompanying drawings and following speci?ca
and of comparatively light weight to permit
tion wherein are disclosed several exemplary
portability. The device must be substantially 45 embodiments of the invention with the under
self-contained; it must be capable of rapid, ac
standing that‘ such changes and combinations
curate and permanent adjustment of the several
in and of the features thereof may be made‘ as
elements, and some of these adjustments must
fall within the scope of the appended claims
be made without opening the casing while others
should be capableof change from remote points,
without departing from the spirit of the inven
50 tion.
This application is a continuation of my
such, for instance, as the recording station.
co-pending application, Serial No. 324,013, ?led
In order togsolve the above problems, it is ‘ March 14, 1940, now abandoned.
In said drawings:
therefore among the objects and features of the
present invention to provide a portable seis
Figure 1 is a central vertical section through
mometer of the condenser type in which:
55 a seismometer constructed in accordance with
2468,1178
3
one form of the present invention, the section
being taken on a plane at right angles to the
axis of movement of the steady mass
Figure 2 is a plan view of the apparatus with
the cover removed;
scribed.
4
These plates are incorporated in the
oscillating circuit of a three element thermionic
tube l2, the ?lament or cathode l3 of which is
energized by current from A battery l3’ through
conductors M and I5. The plate It of the tube
is directly supplied with plate current by means
Figure 3 is a transverse section on line 3-3 of
of the conductor H which isconnected to the
Figure 1, the section plane passing between the
positive side of B battery Ill’. The oscillating
two condenser plates;
circuit includes the intermediate tapped induct
Figure 4 is a schematic wiring diagram of the
circuits of the apparatus contained within the 10 ance coil l8 having the portion !9 which may be
referred to as the grid winding and which is con’
housing;
7
nected through the biasing battery 2'3 to the grid
Figure 5 is a fragmentary view similar to Fig
21‘ of'th‘e tube. The coil also has the section 22
ure 3, but showing the thermionic tube mounted
or plate winding connected to the plate by means
on the steady mass;
’
of conductor 23' in which is interposed the block—
Figure 6 is a schematic showing of the ‘use of a
ing condenser‘l'lt serving to keep the direct plate
plurality of condenser seismometers in multiple
current out of this oscillating circuit. The inter
grouping to reduce or eliminate the major por
mediate tap of the coil is connected to the oath
tion of the direct waves; and
ode by means of the wire 25. The grid coil has
Figure '7 is a circuit diagram of the same.
its
outer, end also connected, through resistance
Inthe conduct of geophysical investigations for
26, to plate H of the variable condenser by
the determination of subsurface strata disposi
means of the wire 21 while the outer end of the
tion, depth, and formation in the search for oil,
plate coil is connected to the opposite plate H]
minerals and the like, it has heretofore been
of this condenser by means of wire 28 which is
customary to fire a charge of high explosive at
also connected at 23} to the B—- lead.
some distance below the ground surface and to
When ap ropriate potentials are applied to the
record from one or more seismometers traces
conductors i4, 55, H, and 29, the circuit willcoscil
representative of the seismic waves reflected
late at a radio frequency which‘ is dependent upon
from the strata interfaces and back to the sur
the characteristics of the various’ elements of the
face. Comparisons of the waves received by the
circuit. The inductances of the‘ two parts of
various seismometers at di?erent distances from
the; coil IS, 22 areappropriately selected as well
the‘ source of the earth shock permits a deter
as- the value of the resistance 26, condenser 24,
mination, by means forming no part of the
and other portions to provide aconvenient reso
present invention, of the desired characteristics
nance frequency for any‘ desired ?xed spacing
of the reflecting surfaces.
' The seismometers used have taken a great 35 of the plates H) and H. This spacing can be
?xed in repose‘, as will be later described, and
number of widely differing forms operating on
adjusted to give’ the desired frequency of oscilla
several principles, and that most favored at the
tion, so that thereafter. with all of the other ele
present time is of the so-called magnetic type
mentsyremaining constant, any changes in the
Where voltage changes are generated by move
ment of a permanent magnet relative to a coil. 40 spacing of the' plates [0‘ and H will cause corre
sponding changes in the oscillation frequencies.
There are a number of reasons, however, for
The usable output from the detector travels as a
favoring. the capacity type as exempli?ed by the
pulsating direct current over. the B conductors
present invention, particularly in certain 1oca~
to the recording station where it is convenient to
tions. It is not necessary to go into all of the 5
position the B batteriesv as Well as the recording
reasons for the superiority of this type, but one
apparatus‘, which for the sake of the present dis
or two important reasons may be pointed out.
cussion willbe considered to include a string type
With the construction in accordance with the
galvanometer' G. When the condenser plates are
present invention, the signal to be used at the
in repose the current feeding the plate is a ?xed
recording station is transmitted from the seis
mometer in the form of a high frequency, pul 50 high-frequency pulsating direct current from the
B battery and reservoir condenser 24' to the plate.
sating direct current, the changes in R. M. S.
This pulsating direct current has a frequency out
values of which actuate the recording mecha
side‘ of the range of vibration of the galvanome
nism. Such a current is less subject to influence
ter string to which it is preferably coupled'by
by extraneous sources, such as emanations from
means of a suitable transformer T. and hence
power lines, static discharges, earth currents,
has no’ effect in moving this string. However, as
changes in the earth’s magnetic ?eld, and the
later described, substantially all of .the radio fre
The condenser type seismometer of proper
quency alternating and pulsating direct currents
characteristics records magnitude of displace
are" con?ned‘within the casing of the seismometer.
ments rather than velocities or accelerations and
With the plates at rest, and the ‘circuit oscillat
hence produces higher ?delity results with less
opportunity for error. Furthermore, it might
me, there is present in the grid coil- a high fre
be said that this type of construction is more
quency alternating current of ,?xed amplitude.
readily capable of use and adjustment for sensi
The changes in frequency of oscillation brought
tiveness, frequency response, damping, rejection 11
about by relative movements of the plates‘ I0 and
of unwanted waves, and forother factors, while 65 H are caused to change the quantity of current
capable of being constructed with great rugged
?owing to the plate of the tube as follows. The
ness. The instrument of the present invention
grid bias is preferably set for operation in a non
is primarily intended to respond to the vertical
linear portion of the EFL, curve of the vacuum
components of the seismic waves.
tube for instance near cut-01f by appropriate ad
Referring now‘ to the drawings, and ?rst to
justment ofv the bias battery 28 so that subse
Figure 4, for an understanding of the operation
quently applied changes in grid potential of equal
of the device of the present invention, there is
value but opposite sign will change the plate cur
shown in that ?gure at H) and l l a pair of closely
rent by unequal amounts. Changes in condenser
spaced relatively movable condenser plates, the . ‘
plate spacing change the capacity of the con
mechanical mounting of which will be later de
9,408,478
5
6
denser and hence the frequency of the oscillating
5a which is secured to the backbone by. means of
circuit, according to the formula
one of the three spacing rods 59 which‘are ver~
tically disposed. This one has a reduced‘ threaded
end passing through a hole in the plate" 58 and
received in a threaded aperture 60 in the ‘casting
55. The others which are attachedt'higher up
on the casting 55 aredivided at the plate 58 and
screwed together to clamp to the plate. The lower
~
7
~
_
where f is the frequency in cycles per second, L
is the inductance in henries, and C is the capacity
in'farads. Therefore, when the plates move closer
together and the capacity of the condenser in-'
ends of these rods are attached by screws 6| to
creases, it reduces the frequency of oscillation of 10 an aluminum base plate 62 adapted to bear
against the bottom of the bore of the housing
the circuit. With this reduction in frequency, the
which is counter-bored ‘as shown at 63. The
mutual inductance between the two portions of
space between the bottom plate and the shield 58
the coil i9, 22 will be lessened, therefore decreas
ing the oscillating voltage applied to the grid
is convenient for the mounting of various of the
by the coil IS. The smaller grid swing will pro 15 elements of the electric circuit. For instance, the
thermionic tube I2 is shown as horizontally posi
duce a correspondingly smaller amount of recti
?ed plate current. The recti?ed high frequency
portion of the plate current is impeded ‘and
tioned and held in place in an aperture in a mass
of sponge rubber 64, whereby it is protected from
vibrations and microphonics eliminated.
smoothed by the use of chokes such as the radio
In this mechanical illustration of the features
frequency chokes 30 shown in conductors l1 and 20
of the invention the electric wiring has been
29 so that the average variation of amplitude, rep
omitted for the sake of convenience and clear
resented by changes in B'current at seismic wave
ness. The tube is wrapped in lead as shown in
frequencies, may be converted to alternating cur
Figure 1 to reduce microphonics and has the
rent by the transformer T which couples the seis
mometer output (through an ampli?er, if neces 25 usual base prongs cut off and wires directly sol
dered to the remaining portions thereof. A ?ber
sary) to the galvanometer string. It may be con
separator 65 holds the sponge rubber in position
sidered that the usable output of the seismometer
and provides space above it and beneath the
is a current curve averaging the high frequencies.
shield plate 58 for a ?lter condenser 66, for the
One form’ of apparatus‘ for assembling in a
compact unit the components of Figure 2 and the 30 combined grid and plate coils I9, '22 and for the
?xed condenser 24 in the oscillating circuit. The
necessary mechanism for mounting and adjust
C battery 20 and the resistor 26 may also be ac
ing the condenser plates l0 and II is shown in
Figures 1 to 3 inclusive.
commodated in thisv space, but are not Visible in
The outer casing or
,Figure 1. These various parts may be conven
is conveniently a casting of aluminum or similar 35 iently clamped to the iron plate 58 or to an'in
sulating plate 61 beneath the same by means of
metal having a flat bottom 36 and projecting feet
housing 35 is preferably cylindrical in form and
appropriate fastening elements such as studs or
31. The side walls are relatively thin and uni
form in thickness except at the top where they
screws.
-
Figure 2 shows in plan the top of the backbone
are thickened as at 38 to provide increased
strength to accommodate the threads 39 by means 40 casting which is provided with four compart
ments 10, ‘H, 12, and 13, to receive the radio fre
of which the cap or cover is secured in position.
This cover may be a casting of the same material
quency chokes ‘I4, 15, 16, and 11, which are ‘shown
including the ?at portion 60, the handle 4! and
the depending ring 42 threaded to cooperate with
in the A and B battery leads in Figure 4. These
A suitable water-tight gasket may be interposed
between seismometers whose cables lead to the
chokes serve to keep the high or oscillating fre- ,
the'threads 39 for bringing together the shoulder ‘‘ 45 quency alternating currents out of the conduc
tor cables so that there will be no intercoupling
flange 43 and the upper end 44 of the casing.
here.
‘
The underside of the lid is channeled for stiff
ness and lightness and is recessed as at 45 to ac-\
commodate the annular insulation plate 46v se
cured in position by screws as shown. This disk
has inset in its lower face ?ve coaxial contact
rings 48 for cooperation with stationary spring
to
same recording station. These radio frequency
chokes are well shielded from each other by the
heavy metal of the casting and of the closely ?t
ting cover.
'.
,
-
The ?at annular top 5'! of the casting is notched
at diametrically opposite positions as are the par
allel vertical walls 18 and ‘I9 separating the radio
An appropriate 55 frequency choke compartments. Each pair of
notches accommodates an insulation strip, the
one at the upper side of Figure 2 being numbered
at one end of the handle in which conventional
80 and the one at the lower side 81. One of
contact plug receptacles are arranged for connec
these strips mounts three spring contacts 82 and
tion to the ?ve conductors leading from the rings
48 through the passages 5i and 52. The recepta» 60 the other mounts two of‘these contacts, each
spaced from the center a distance to engage with
cles are spaced to receive the prongs of-a multiple
the proper one of the contact rings 48 so that
plug ‘53 to which a five conductor cable is attached
when the cover is screwed in position the cir
for carrying the battery conductors shown in Fig
cuits are completed from these spring contacts to
ure 4 back to the recording station.
All of the apparatus within the housing is car? 65 their respective rings and the screw cover‘ does
not have to be stopped in any ‘special position to
ried by or supported from a backbone 55 in the
insure these connections being completed.
form of an irregularly shaped and compart
A thin insulation plate 84, shown in Figure l,
mented casting generally circular in outer/con
is perforated to pass the spring contacts 82 and
?guration to closely fit the bore of the casing 35
and having a substantially ?at bottom 56 and flat 70 is placed on top of the casting after it' and its
attached parts has been positioned in the hous
top 51. It is preferably formed of some light
ing and a locking ring 85 screwed down on top
metal such as aluminum and is kept as light as
of the same by means of a spanner wrench. This
possible by a cellular construction, as will be later
serves to-clamp the casting and all of its assem
described. The bottom of the casting 55 is com- I
bled parts tightly in the protecting housing and
pletely closed by a soft iron cover plate and shield
contacts to be later described.
?tting 50 of watertight construction is provided
2,408,478
and supporting the hinge springs for the steady
prevents any relative movement under the sever
est vibrations.
-
mass which carries the lower condenser plate
H. There are two sets of these springs, one set
.
A portion 86 of the main casting in the form of
a transverse septum provides .the ?oor for the
compartments housing the radio frequency chokes
98, the vertical, being more widely spaced apart,
and are secured against the vertical face 91, each
by a pair of screws as shown clamping overlying
metal plates 99 tightly thereon. The other set
of hinge springs I00 are between but spread al
and for the channel 81 de?ned bythe walls 18 and
‘I9 and extending at right angles to the contact
strips 80 and 8|. Closely adjacent the edge of
most as widely as the vertical set. They are
this septum and on its under-face is'attached the
insulating disk 88 which supports the upper con 10 horizontal as shown and are secured beneath and
against the ?at face 95 by fastening elements
denser plate H] which is thereby made rigid with
entering the backbone.
the housing and frame of the seismometer. This
The steady mass I02 is a heavy casting prefer
plate I0, which is preferably made of some non
corrosive material, such as “Monel metal” is ec- > ably of brass or other non-magnetic material.
It includes the horizontal arm portion I03 ad
centrically attached to a post‘89 extending from
jacent the two sets of binge springs just de
the rear face thereof, as shown, and passing
scribed and having a vertical dimension consid
loosely through an aperture in the insulating
erably less than the vertical dimension of the
disk. A helical spring 98 surrounding the por
weight portion I04. Its top surface is ?at so
tion of this post above the insulating disk bears
that the horizontal springs I00 can be secured
with its lower end on the disk and with its other
end on a nut 9! screwed on the post.
thereto by suitable fastenings passing through
It serves
to bias the plate upwardly and to permit its ad
justment for relative height and for paralleling
it to the movable plate. These adjustments are
eifected by three screws having round ends bear 25
them and the cover plates I05 while the right
hand edge has a vertical surface for attachment
of the vertical springs 98 by similar fastening
means and overlying cover plates [06. This type
ing on the back of the plate, one of which is
of spring hinge arrangement is entirely free from
shown at 92, passing loosely through the insulat
ing disk and engaged with the nut 93 below the
disk and nut 94 above the same. By appropriate
adjustment of these several screws, and their
eventual locking by means of the nuts, the upper
plate I0 can be positioned in height and inclina»
tion in respect to the portion 85 of the backbone
and made parallel to plate II in its repose posi:
any looseness whatsoever in any direction, and
within the small range of movement can be made
tion.
.
Proper initial spacing of the condenser plates
is best obtained by laying the seismometer on its
side with the center line thereof absolutely level
and the axis of the hinge of the steady-mass
pendulum horizontal. The plates are now ad
justed by operation of the adjusting screws 92
so that the distance between them is the correct
and desired working distance for use when the
seismometer is set on end in its normal position.
When the seismometer is placed horizontal the
pendulum is permitted to swing free without be
ing pulled by its supporting spring. The proper
setting of the plates with the seismometer in this
to be substantially frictionless and have a mini
mum of resisting and restoring torque. Thetwo
pairs of springs spaced Widely apart at opposite
sides of the rigid arm insure against any move
ment of the steady mass in a lateral direction
and con?ne all movement to that about an axis
35 passing through the intersection of the planes of
the two sets of springs. The hinge axis is ar
ranged in horizontal alignment with the center
of mass of the steady mass, ?nal adjustment be
ing made by the method discussed above.
The major or weight portion of the steady mass
40
more remote from the hinges is, as previously
stated, much greater in vertical height than the
arm. In fact it substantially ?lls the space be
tween the upper condenser plate l0 and ‘the iron
45 plate 58, leaving only room for the very limited
Vertical movement of the housing in respect to
the steady mass. It is the intention to concen
trate as much weight as possible within the lim
position assures that when the seismometer is
ited space and as remotely as possible from the
set on end the center of gravity of the steady 50 hinge, which contributes to the inertia of the
mass will then be in the same horizontal :plane
steady mass. Care is exercised to maintain the
as the hinge support. This insures that. the seis~
natural period of the steady mass, as sprung,
mometer will not pick up horizontal components
well outside of the range of seismic waves. If
of earth movements.
'
the fundamental frequency of the steady mass,
For re?ection surveys the frequency of the
55 as sprung, is so low that it approaches zero the
moving system is usually adjusted either to match
device very nearly records pure displacement.
The top surface of the steady mass has secured
thereto a plate H0 of insulation, which has rig
the re?ected wave frequency or to a somewhat
higher frequency in order .to eliminate as much
as possible low frequency “ground roll.” For re
idly attached to its upper surface the lower con
fraction work it is desirable that the frequency 60 denser plate ll. Beneath the insulation plate
of the moving system be much lower than the
H0, the casting of the steady mass is bored out
frequency of the waves to be recorded so as to
as at III for its full vertical height to accommo
get good amplitude of movement at the begin
ning of the initial impulse.
At the right hand side of Figure 1 and simi
larly positioned in Figure 3 is shown the rela
tively thin depending portion ‘55' of the back
bone 55 relieved on its inner left corner as at 95
date an electromagnet assembly H2, including
the soft iron sleeve H3 closely ?tting the bore
65 and having threaded into it the headed core I I4,
as shown.
Between the .core and sleeve is the
magnet wire winding I IS, the lower end of which
is closed over by an insulating washer H6. The
to provide a substantially horizontal overhang
core and tube de?ne a shell type electromagnet
ing surface 96. The inner face of the recess is 70 whose central and annular poles are spaced from
provided with a vertical surface 97. Both of
the iron plate 58 by the small air gap H1, as
these surfaces 96 and 91 extend for almost the
clearly seen in Figure .1. This air gap may be
full diameter of the casing in a direction at right
initially adjusted by a vertical movement of the
angles to the plane of Figure 1, as clearly, seen
whole ‘magnet assembly within the steady mass in
in Figure 3, for the purpose of Widely ‘spacing 75 any desired ‘manner. It is ?nally locked int-po
2,408,478
9
I10
sition to give the desired air gap for a purpose
‘adjusted by the elevating screw I2‘l’,v determines,
to be later described.
at least partially, the damping of the steady
The weight of the whole steady mass is sup
mass, because of viscosity of air between plates
ported by a cable or wire H8, attached by means
I0 and-I I ,’ which'damping, however, is a condi
of a clip H9 as nearly as possible to the center
tion controlled by several other factors and more
of percussion of the steady mass. By means of
particularly by the electromagnet previously de
this wire M8 the weight of the steady mass is
scribed.v The damping which it affords is, how
applied to the outer end of a cantilever spring
ever, not the primary purpose of the magnet.
I26 arranged in the transverse groove 8'! in the
The winding of this magnet, as shown in Fig
top of the casting, as previously described. The 10 ure 4 is connected between the A—-— battery source
wire passes through a suitable aperture I2I in
is and a conductor I42 which returns to the re
the wall 86 of the casting. This cantilever spring
cording station, passes through a suitable rheo
is so mounted as to be subject to a plurality of
stat, and is connected to the A+ terminal. ‘In
adjustments for ?xing the repose position, rate
serted in the circuit may be an appropriately
of oscillation, size of condenser air gap, damp [5 graduated 'ammeter to roughly adjust the
ing, and the like, of the steady mass.
strength of the magnet, although this is not re‘
The construction of the spring mounting is
quired since the primary purpose of the magnet
clearly shown in Figures 1 and 2. The spring is
is to‘ ?x' the spacing of the condenser plates Ill
a straight ?at metal‘ plate whose free end sup
and H just prior to the time of taking a read
ports the adjustable screw terminal I22 to which
ing, The rheostat permitsa remote adjustment
the wire i is is attached. The opposite end of the
here, since the magnet in attracting the iron
spring‘ is secured by a pair of screws I23 to the
plate beneath it acts in opposition to the canti
upstanding lug I24 on a cradle I25 which is
lever spring and provides a means for not only
hinged, just below the attachment of the spring
increasing the pull on the spring but for remote
I20, to the backbone 55 of the whole assembly by ‘ ‘ly adjusting the gap between the condenser
means of a stiff spring I26 secured to both the
plates. The size of this gap can be very accu
backbone and the rear end of the cradle by means
rately read at the control station by the use of a
of suitable screws. This cradle carries at its ‘out
milliammeter MA in the plate circuit of the tube.
ermost end and beyond the free end of the spring
It will be recalled that this plate current bears
an elevating screw I21 whose head is exactly at ' 30 a de?nite and known relation to the frequency
the center of the housing so that it is beneath an
of oscillation of the tube and,~once having'cali
opening I28 in the cover plate 40 and a corre
brated the system, the gap between the con‘
sponding hole !29 in‘ the handle. This permits
denser plates can be accurately determined,. for
the use of a screw driver for adjusting the posi
instance, in fractions of a millimeter by "a read
tion of the cradle. which in turn changes the po- 1" ing of the milliammeter at the'recording'station,
sition of the steady mass and hence the repose
v‘if'such factors as A and B voltages are held reas
spacing of the condenser plates. The opening
onably constant or are compensated for. Remote
£255 is closed by a suitable plug and gasket I30 to
adjustment of the condenser plate gap is import
maintain the housing watertight. The elevating
ant to permit compensation for temperature
screw I2‘! is engaged in a threaded opening in the
outer end of the cradle and by means of suitable
changes and other transient variables.
'
vWhere it is desired to have the seismometer op
erate about a certain fundamental frequencyof
friction mechanism I32 is prevented from chang
ing its adjustment. Its lower rounded end‘bears
the output current while yet permitting changes
in plate spacing to adjust the damping in accord
ance’with external conditions, the frequency can
upon a hardened plate I33 secured to the upper
face of the partition plate 86 of the casting. A
hole I 2i’ in the cradle floor permits passage of
be adjusted by varying the current in they tube
the-wire H8.
?lament.
The cradle carries a second adjusting screw
I34 substantially at right angles to the elevating
screw and parallel to the spring I 20. This screw
has its ends reduced as at I35 and I36 to provide
trunnions for the same bearing in holes in the
lug I3‘! near the free end of the cradle and in
the lug I24 at the hinge end.
5O
'
'
In Figure 5 is shown a further embodiment of
the seismometer in which the thermionic tube
' !2A has been moved from the position shown in
.Figure 1 in the'sponge rubber mass to a mount
ing on the steady'mass in the space designated
in Figure'l by the reference character ‘I50 and
Threadedly
lying just above the arm portion of the steady
mounted on the screw for movement along the
same as the screw is rotated is the adjustable ful
crum block I38. The lower face of this block
bears closely against the flat surface I39 of the
cradle which is parallel to the axis of the adjust
in? screw and svf?cient friction is available here
mass. ' As shown, the tube is secured to the steady
mass arm by a pair ‘of metal bands I5l passing
around the same’and compressing sponge rubber
cushion stripsbetween the bands and the sur
face of the tube.
'
-
‘
‘
By placing ‘the tube as just described, several
advantages are gained' In the ?rst place, the
60 ..
to insure against looseness. The upper end of the
block is inclined to provide the sharp fulcrum
tube is so mounted that his not subjected to the
edge I 40 upon which the spring I20 bears, Ad'
seismic shocks since'the steady 'mass is intended
justment of the block I38 by rotation of the screw
'to' remain stationary while the remainder of the
I34 determines the active length of the cantilever
seismometer moves about'it. In the second place,
spring and hence the period of the steady mass
‘several inches in vertical height can be cut off
and its rate of movement in respect to the hous
of the detector,‘ reducing both the volume and
ing and thus ?xes the range of response of the
weight of the same; ..Where these devices are
seismograph in accordance with the known rate
‘used in' dif?cult terrain and far from base, they
‘of vibration of the seismic waves with which it 70 must be transported by trucks, by pack animals
is to operate. In most regions,’ it is found that
-or by human‘ carriers, so that any saving in?size
‘these waves have a frequency of from 20 to '70
‘or weight is extremely valuable. 'Thirdly, the
cycles per second.
weight of the'steady mass is increased without
The size of the gap between the condenser
any‘ attendant totalv weight increase in the seis
plates I 0 and II, when in repose, which may be 75 .mometer.' -: g
'
'11,
Condenser type seismometers constructed ac
rding to the present invention lend themselves
,rticularly well to gauging or multiple use for
e purpose of reducing or entirely eliminating
e pick-up of ground waves. As is well-known
Seismology, the waves of largest amplitude
.‘liCh reach the seismometers as the result of
L explosion are in most cases those travelling in
e surface or weathered layer of the earth, or
ry near the same, and since ‘they do travel so
:se to the surface they follow nearly the
.ortest path, and are often the ?rst to reach the
,rious detectors. In any event such ‘waves have
nplitudes much greater than those of the
12
may include only the mechanical parts as dis
closed in the several previous ?gures together
with the variable condenser plates. Thus in Fig
ure '7 seismometers 284, 204a, and 2041) are repre
sented merely ‘by condenser plates although of
course they include the adequate mechanical
mounting for the same so that these plates will
respond exactly as the ones in the seismometer
described in detail above. The condensers of
each of the four seismometers are set to substan
tially the same plate spacing and are connected
in parallel as shown. Then, by appropriate ad
justment of the proper elements the tube circuit
is'made to oscillate at the desired frequency but
anted re?ected waves. If the apparatus is made 15 under the control of all four of the sets of con
denser plates. It wil be seen that if two of these
nsitive enough to record the desired waves with
are widely opened and two others are brought
proper degre of amplitude on the chart, then
.e unwanted waves are recorded with such over
closely together the total change in capacity of
the set will be practically zero so that the total
tensity as to be detrimental to the equipment
id to extend off of the recorded chart. The 20 e?’e'et of “ground roll” will hardly be noticeable.
When, however, re?ected waves are received, the
me of arrival of the ?rst of these waves may
condenser plates 'of the several seismometers
e of importance but the remainder of them are
,move substantially in unison and thereby exert
no consequence and merely serve to confuse
an augmented control on the frequency of oscil
re recording.
‘In accordance with the present invention the .25 lation of the circuit, resulting in greater changes
in the output.
)ove difficulties may be partially or entirely
For purposes of convenience it might be de
iminated by ganging a number of condenser
sirable to have the auxiliary seismometers 2M,
:ismometers in place of the usual one at each
‘1304a, and 2041) also equipped with the magnetic
ation and spacing them apart in the line of
aproach of the ground Waves at approximately ,30 coil for determining their repose setting and
hence their damping as well as control of the
ie-half wave length intervals so that their out
oscillating circuit.
its substantially cancel each other. The re
With the arrangement just described no ad
ected waves, however, having more nearly a
ditional conductors need be returned to the re
zrtical component reach the units of this gang
:‘seismometers almost simultaneously and there 35 cording station above the number required when
a single seismometer is used. It will be appre
7 augment each ‘other, thus increasing the size
ciated that a plurality of seismometers arranged
i the wanted record and decreasing the size'
in a row and connected to control a single oscil
? the unwanted record, making them more nearly
lating circuit give an output averaging the
:‘ the same order. ‘Since the ?rst seismometer
1 line is affected without corresponding can 40 “ground waves.” "If they are placed suf?ciently
close together, they become the equivalent of a
allations from the others when surface waves
continuous ?exible condenser and it is within
rst.arrive the time of arrival will be nicely shown
the import of this invention to include such a
:1 the chart.
”
continuous condenser which should have a length
Reference to Figure 6 will make this arrange
not much greater than one-half wave length.
lent clear for there is shown at 200 the weathered
It might be constructed in the nature of a coaxial
tyer of the ground with the shot point 20| ar
cable with suitably resilient spacers or it could
inged just below this layer for better ‘trans
take other ‘forms, and in each case be connected
lission .of the arti?cially propagated seismic
to an oscillating circuit of the type described to
'aves andat 202 is shown an assumed re?ecting
‘give an average result. It would of course give
irface whose depth is not in proportion-to other
a summation for reflected waves.
oacings on the chart because of lack of space.
While the invention has been illustrated as
rhe surface .of the ground is shown to be formed
applied 'to a seismometer of the capactive type,
ito exaggerated waves ‘203 representing the so
certain features of the invention pertaining espe
alled “ground roll,” “ground wave” or the like,
cially thereto, it will be apparent from the fore
nd four condenser type seismometers 204, 204a,
going description that certain other features,
014b, and 2040 are shown positioned on the sur
"for instance the provision for indicating and re
1.08 of the .ground in a line with the shot point
motely controlling the spacing of the relatively
nd with the spacing 205 between centers of any
movable elements of the seismometer, are ap
djacent pair preferably equivalent to substan
plicable to various other types of instrument,
ally one-half wave length of the “ground wave.”
such as the electromagnetic type, ‘the reluctance
‘he length of this wave, in most any type of
'eathered layer, is a matter of record or can be
scertained before-hand very simply. Great
xactitude in this matter is not essential. It will
e seen from the position of the waves that any
wo adjacent seismometers are either in phase
pposition or are at a substantially neutral point
nd at the same amplitude. Two might be suf
.cient but four are preferred, arranged in circuit
s shown in Figure 'I.
For the purpose of this arrangement seis
iometer 2040 is videntical with that previously
.escribed in this application andy'the circuit is
be same as that of Figure 4. The remaining
eismometers, however, can be much simpler and '
‘type, the piezo-electric type, and various types
‘less commonly employed in which accurate de
termination of the repose position of the movable
element is desirable.
‘Having thus described the invention, what is
claimed as new and desired ‘to be secured by
‘Letters Patent is:
1'. In a portable seismometer of the condenser
type, in combination, a ‘housing, a ‘condenser
plate rigidly supported for movement with the
housing, a second ‘condenser plate spaced closely
to the ?rst, a steady-mass mounting said second
plate and being hinged to the housing ‘for move
ment relative thereto in a direction at right
2,408,478‘
anglesto the planes of said plates, spring means
biasing said second plate toward the ?rst one,
and adjustable magnetic means acting between
the housing and steady-mass for adjusting the
plate spacing against said bias.
2. In a portable seismometer of the condenser
type, in combination, a housing, a condenser
plate rigidly supported for movement with the
a second condenser plate spaced closely to the
?rst, a steady-mass mounting said second plate
and being hinged to the housing for movement
relative thereto in a, direction at'iright angles to
the planes of said plates, spring ‘means carried
by the housing for supporting said steady-mass,
said spring means comprising av cantilever spring,
means suspending said steady-mass from the free
end of said spring, and an adjustable fulcrum for
housing, a second condenser plate spaced closely
to the ?rst, a steady-mass mounting said second 10 said spring.
" '
plate and being hinged to the housing for move
8. In a portable seismometer of the condenser
ment relative thereto in 'a direction at right‘
type, in combination, a housing, a condenser plate
angles to the planes of said plates, spring bias
rigidly supported for movement with the housing,
ing means supporting the steady-mass and acting
a second condenser plate spaced closely to the
to move the same about its hinge in one direc
?rst, a steady-mass mounting said second plate
tion only, and a magnet carried by said steady
and being hinged to the housing for movement
mass and arranged to ,eXert a force to always
relative thereto in a direction'at right angles to
oppose the e?ect of said means.
the planes of said plates, spring means for sup-'
3. In a portable seismometer of the condenser
porting said steady-mass from the housing, said
type, in combination, a housing, a condenser 20 spring means comprising a cantilever‘ spring,
plate rigidly supported for movement with the
means suspending said steady~mass from the free
housing, a second condenser plate spaced closely
end of said spring, a cradle for said spring sup
to the ?rst, a steady-mass mounting said second
ported on said housing and having the opposite
plate and being hinged to the housing for move
end of said spring secured thereto, a knife edge
ment relative thereto in a direction at right 25 fulcrum for said spring slidable in said cradle,
angles to the planes of said plates, means acting
and means to adjust said fulcrum longitudinally
to move said steady-mass about its hinge,’ mag
of the cradle.
netic means for opposing the e?ect of said means,
9. In a, portable seismometer of the condenser
and means to adjust the strength of said mag
type, in combination, a housing, a condenser plate
netic means to vary the initial spacing of said 30 rigidly supported for movement with the hous
plates.
4. In a portable seismometer of the condenser
type for use with other elements to form an oscil
ing, a second condenser plate spaced closely to
the ?rst, a steady-mass mounting said second
plate and being hinged to the housing for move
lating circuit and including relatively movable
ment relative ‘thereto in a, direction at right an
condenser plates the spacing of which determines 35 gles to the planes of said plates, spring means
the frequency of oscillation of said circuit, means
for supporting said steady-mass from the hous
mounting said plates for relative position change
ing', said spring means comprising a cantilever
in response to seismic waves and including bias
ing means tending to ?x the normal plate spac
ing, and means adjusted remotely from the seis
spring, means suspending said steady-mass from
the free end of said spring, a cradle for said spring
40 supported on said housing and having the oppo
mometer and adapted to oppose said biasing
site end of said spring secured thereto, a knife
means to thereby adjust the initial setting of
edge fulcrum for said spring slidable in said cra
the plates and thus the oscillation frequency.
dle, means pivoting the spring-attached end of
5. In a portable seismometer of the condenser
said cradle to said housing, and means'to move
type, in combination, relatively movable con 45 said cradle about its pivot to change the repose
denser plates, means mounting said plates for
relative position change in response to seismic
waves and including biasing means tending to
spacing of said condenser plates.
10. A portable condenser type seismometer in
cluding, in combination, a closed bottom tubular
?x the normal plate spacing, and electromagnetic
metal casing, a metal closure cap ‘for the same,
means having an adjustable pull in opposition to 50 a backbone casting immovably mounted in said
said biasing means whereby adjustment of the
casing and having a recess therein, a steady
repose spacing of the plates may be e?ected re
mass and attached lever in said recess, means
hinging said lever to said casting, means suspend
6. In a portable condenser type seismometer
ing and providing a restoring force for said
for operating a remote recorder, in combination, 55 steady-mass, a pair of closely positioned con
a closed housing adapted for movement in re
denser plates, one supported by said casting and
sponse to seismic waves, a steady-mass hinged to
the other by said steady-mass, a thermionic tube
said housing, a pair of closely spaced condenser
and oscillating circuit elements in said casing
plates secured respectively to the said housing
and connected to said condenser plates, said tubes
motely from the seismometer.
‘
‘
and steady-mass for relative movement, a vacu
and elements being 50 positioned as to be shielded
um tube in said housing, impedance elements in
.from each other, from the condenser plates and
corporating said tube and plates into a circuit
external ?elds by the .casing, casting and steady
oscillating at a frequency dependent on the spac
mass.
ing of said plates, a source of plate current for
11. A portable condenser type seismometer in- '
said tube, said circuit being such that changes in
eluding, in combination, a tubular metal casing,
said oscillation frequency cause corresponding
a closure cap for the same, a metal backbone
changes in the plate current, an electro-magnet
casting immovably mounted in said casing and
mounted in said steady-mass, an armature ?xed
having a recess therein, a steady-mass and at
to said housing below said magnet, a source of
tached lever in said recess, means hinging said
current for said magnet, and means to adjust 70 lever to said casting, means suspending and pro
said current to vary the plate spacing to deter
viding a restoring force for said steady-mass, a
‘mine the repose value of said plate current.
pair of closely positioned condenser plates, one
supported by said casting and the other by said
'7. In a portable seismometer of the condenser
steady-mass, a thermionic tube and oscillating
type, in combination, a housing, a condenser plate
rigidly supported for movement with the housing, 75 circuit elements in said casing and connected to
15
said condenser plates, said casting having com
partments therein, certain of said circuit elements
being positioned in said compartments for elec
trostatic shielding.
12. A portable condenser type seismometer in
cluding, in combination, a tubular metal casing,
a closure cap for the same, a backbone casting
immovably mounted in said casing and vhaving
16
0nd element to permit such relative movement,
said means including a steady-mass carrying said
second element, said steady-mass being hinged
to the housing on an axis laterally spaced from
said elements, the hinged axis being disposed
substantially in the horizontal plane containing
the center of gravity of the steady-mass, where
by the seismometer is unresponsive to horizontal
components of seismic waves.
a recess therein, a steady-mass and attached
16. A portable seismometer responsive to ver
lever in said recess, means hinging said lever to 10
said casting, means suspending and providing a
restoring force for said steady-mass, and a pair
of closely positioned condenser plates, one sup-
tical displacements comprising, in combination,
13. In a portable seismometer of the condenser
type, in combination, a housing, a condenser plate
the supporting member with said elements in
a supporting member adapted to be moved by
seisms, a ?rst element carried by said member, a
steady-mass, a second element on said mass,
ported by said casting and the other by said
15 means movably suspending said steady-mass from
steady-mass.
rigidly supported ,for movement with the hous
ing, a second condenser plate adapted to have a
close repose spacing adjacent the ?rst, a steady
mass mounting said second plate and being
close proximity, an electric circuit so associated
with said elements as to be in?uenced by their
spacing, means in said circuit to record the in
stantaneous spacing of said elements, means in
a said circuit independent of said last named means
to indicate the repose spacing of said elements,
and means remote from the seismometer to ad
just the repose spacing of said elements.
tion at right angles to the planes of the plates,
17. A portable seismometer responsive to dis
the axis of the hinge being in the same horizon 25
placements, comprising a casing, a steady-mass
tal plane as the center of gravity of the steady
supported from said casing, and means for genmass movable-plate assembly, whereby the seis
erating a voltage which is a function of the in
mometer is unresponsive to horizontal compo
stantaneous displacement between said casing
nents of seismic waves.
and said steady-mass, separate means for re
14. A portable condenser type seismometer re
cording
said voltage, separate means for indi
sponsive to vertical displacements comprising, in .
cating at a remote place the average spacing be
combination, a supporting member adapted to be
tween said casing and said steady-mass, and
moved by seisms, a condenser plate carried by
means to adjust the average spacing between said
said member, a steady-mass, a second condenser
casing
and said steady-mass from a remote place.
plate on said mass, means movably suspending
18. In a portable seismometer responsive to
said steady-mass from the supporting member
vertical displacements, in combination, a housing,
with said condenser plates in close proximity,
a steady-mass hinged to said housing, means for
said suspending means being resilient and pro
generating a voltage which is a function of the
viding a restoring force for said steady-mass
instantaneous displacements between said hous
after displacement, an electric circuit so asso
ing and said steady-mass, the axis of said hinge
ciated with said condenser plates as to be in
being in the same horizontal plane as the center
fluenced by their spacing, means in said circuit
of gravity of said steady-mass, whereby the seis
to record the instantaneous spacing of said plates,
mometer is unresponsive to horizontal move
means in said circuit independent of said last
named means to indicate the repose spacing of * ments.
19. In a portable seismometer responsive to
said plates, and means remote from the seis~
vertical displacements, in combination, a housing,
mometer to adjust the repose spacing of said
hinged to the housing at a distance laterally of
the plates for small range movement in a direc
plates.
15. In a portable seismometer of the type em
ploying elements which are readily displaceable
on application thereto of ‘seismic wave energy, in
combination, a housing, an element rigidly sup
ported in the housing, a second element sup
ported in the housing for movement relative to
said ?rst element, and means mounting said .sec- ..
a steady-mass hinged to asid housing, means for
generating a voltage which is a function of the
displacements between said housing and said
steady-mass, the axis of said hinge being in the
same horizontal plane as the center of gravity of
said steady-mass, whereby the seismometer is
unresponsive to horizontal movements.
OLIVE SCOTT PETTY.
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