@c?. ‘i, 39%. x 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.