US2383571A - Gain control for seismograph amplifiers - Google Patents

Description

Aug- 28, 1.945 E. M. sHooK 2,383,571

GAIN CONTROL FOR SEISMOGRAPH AMPLIFIERS Original Filed Feb. 1, 1938 4 Sheets-Sheet S Aug.v 2s, 1945. E M. SHOCK 2,383,571

' GAIN CONTROL FOR SEISMQGRAPH AMPLIFIERS Original Filed Feb. l, 1938 4 Sheets-Sheet 4 www@ www

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Patented ug. 28, 1945 OFFICE.

GAIN CONTROL FOR SEISMOGRAPH AIVIPLIFIERS Earley M. shook, washington-D. c., assigner, by 'mesne assignments, to Socony-Vacuum Oil Company, Incorporated, corporation of New York `New York, N. Y., a4

continuation or' applicati@ serial No. 188,067,

February 1,

1943, serial No. 4s3,z12

1`4 claims. (c l. 177-352) This invention relates generally to a .method and apparatus for surveying the subsurface strata of the earth by the use of articially created seismic waves, and more particularly to the method and apparatus for recording all the data on a single spread when shot in one direction by detonating a single charge of explosive, and is a continuation of my application Serial No. 188,067, illed February 1, 1938, for Gain control for seismograph amplifiers.

In the art of exploring subsurface strata by the use of articially created seismic waves, it is customary to detonate a charge of explosives at a point on or near the earths surface and record the seismic waves generated by the detonation of.

the explosives at predetermined points removed from the point of generation. From the data thus recorded it is possible to ascertain the depth of subsurface horizons, from which the seismic waves that are recorded have bee'n reflected. The depths of these subsurface horizons are computed from the velocity and time required for these seismic waves to travel down to the reflecting horizon and return to the detecting linstrument. This velocity will vary, dependent upon the density and elastic coeillcients of the materials through which it travels. i In addition to considering the velocity at which reflected waves will travel in subsurface strata, it is necessary to consider the Velocity of their transmission through the unconsolidated,-

` weathered, sedimentary surface of the earth.

The usual procedure when the exploration of an ,area is begun is to develop these velocities in the different strata by what is termed a velocity Program which entails recording records of seismic waves which will give directly the velocities in these particular strata. After having once ascertained these velocities, they can be used throughout the area in computing the depth of the partlcula`r reflecting strata. Due-to the fact that the weathered surface layer of the earth varies in thickness, it is necessary that the thickness of this rweatheredlayer be computed for each spread or location of geophones. To obtain the data from winch' the thickness of the weathered layer is computed, charges of explosives are detonated at the selected s'hot points and the velocity of the wave travel through the weathered or surfacelayer is determined. Due to the fact that the wave in which we are llrst interested travels substantially in a direct route to the detector, there is very little absorption present and an impulse e of great amplitude is recorded. It is necessary that the gain in amplication of a vacuum tube amplifier be at the maximum in order that the 1938. This application April 15,

break in the seismogram trace, resulting from the arrival at the geophone of the first impulse explosives are detonated Waves.

-as lmuch as 20 or 30 of the of energy, be very definite. As a consequence heretofore, it has been necessary to record these data on a separate seismogram from that on which reflected waves are recorded. Such a procedute would necessitate recording a plurality of seismograms to obtain the data in the weathered layer and records of the reflected waves from shallow subsurface strata. Since the'high cost of field operations makes the time factorrof paramount importance. it is desirable to record all of these data on a single record of standard width from thev detonation of a single charge of explosives. Such a procedure would result in tremendous savings in the amount of explosives required and in the time required by the field party to record the data necessary on a single spread, as Well as the time required for an interpreter to observe the data .on a plurality of seismograms.

With previous methods of recording these data by the use of a plurality of charges of explosives, from which seismic waves have been recorded on a corresponding number -of seismograms, it has been almost `impossible to duplicate the conditions under which each charge of explosive is detonated, due to several factors, the most,

important of which being the changes in the conditions affecting the medium in which the shot is planted When one charge of explosives is detonated in the bottom of a shot hole, a pocket or cavity is formed in some formations, while in others the bottom of the hole is lllled up, due to the fact that the medium forming the walls of the cavity is of loosely packed material. 'I'his sometimes results in a loss of as much as` 10.'I in the depth of the hole, and, in extremecases, where the bottom of the hole is in a quicksand material, hole are lost lby the detonation of a single charge in it. Such changes in conditions under which successive charges of introduce variable factors inthe apparent velocities of the recorded files commited fromv these data in error.

Therefore, the primary object of this invention is the provision of a method and apparatus whereby all the data that is required to be recorded on a particular spread when shot from one These variable-factors would make pro- Still another object of this invention is in the provision of means whereby the vacuum tube amplifier is allowed to remain at or'near its maximum sensitivity until the first impulse of the direct traveling waves has been recorded; .then instantaneously suppressing the gain in amplification and allowing it to expand eir-A ponentially with time.

This invention further contemplates means ofexplosives.

Still another object ofl this invention resides in the provision of a method for controlling the gain in amplification in a predetermined manner, without sacrificing any of the available gain present in the amplifier at the time the first impulses are recorded from direct traveling waves or at the time the latest reflected waves are recorded. I

Still another object of this invention is in the provision of a means for initiating the predetermined control which can be accomplished automatically by the operation of a gaseous discharge tube,` or by relay', or manually by switch.

'Ihis invention also contemplates the provision of means which will operate on either the control or suppressor grid of a conventional vacuum tube in one or more stages of the conventional amplifier to efiect the above mentioned results.

Another object of the `invention is in the provision of a unit for effecting these results which can be utilized with a conventional amplifier.

Still another object of the invention resides in rthe provision of a unit which will operate with low energy consumption.

Other objects and advantages will become apparent from the following detailed description, when considered with the attached drawings, in

which:

Figures 1a and 1b.' placed end to end, illustrate a seismogram recorded in the conventional manner;

Figures 2a and 2b similarly illustrate a seismogram of the type that can be recorded by the method and with the apparatus of this invention;

Figures 3a and 3b, placed end to end, illustrate a voltage wave that would result from passing a wave of constant amplitude through an electric seismograph amplifier employing the control forming the subject matter of this invention;

Figure 4 is a curve which has been plotted with gain in amplification as ordinates and time las abscissae representing the instantaneous Figure 8 is the circuit diagram shown in Figure 7 showing additionally the use of a gas triode vacuum tube for the purposeof initiating the operation of the control;

Figure 9 is a partial block diagram of an electric seismograph showing the application of the circuit forming the subject matter of this invention to a conventional amplifier.

Referring to the drawings in detail, particularly Figures 1a and 1b there is illustrated a seismogram-which shows the' manner in which the direct traveling waves, as well as the reflected wavesI from five geophones or detectors are conventionally recorded on' a single seismogram. The transverse lines indicate denite and equal small intervals of time, such, for example, as .01 second. It will be noted that there is an indication known in the art as a time break on the middle trace at X- indicating the instant at which the charge of explosive was detonated. The period of time elapsing between the instant at which the charge was detonated and the arrival of the waves generated thereby at the geophone is represented by the number of spaces and fractions thereof between the indication X, denoting the time break or instant of explosion, and the point where a discontinuity occurs on the respective traces. To record a record such as that illustrated the charge of explosive and the five geophones are located in the same vertical plane and the geophones are lequidistantly spaced one from the other. The discontinuity in the respective traces is indicative of the instant at which the first energy strikes the geophone. Although this first initial impulse is generated by direct traveling waves of abrupt wave front, there are many other Waves of different frequencies and velocities that follow the wave which delivers the initial impulse to the detector. Due to the fact that these direct traveling waves are of so much greater amplitude than reflected waves, their attenuated wave train would tend to obscure reflected waves from shallow horizons that would arrive at the geophone or detector a relatively short interval of time after' the first impulse. As shown by the illustration in Figures 1a and 1b, it is impossible to distinguish any of the reflected waves from shallow horizons, from direct traveling Waves. This is partly due to the fact that the direct traveling waves completely obscure the reflected waves, and partly due to the fact that there is less absorption and resulting loss of energy in waves which have been reflected from shallow horizons than those which have been reflected from deep horizons. From this 'it readily becomes apparent that some means must be provided for controlling both the amplitude of the direct traveling waves after the first impulse has been recorded and the amplitude of reected waves from shallow horizons. Such control of amplification is illustrated in Figures 2a and 2b. These figures illustrate a seismogram such as would be recorded by the method and apparatus forming the subject matter of this invention. In this illustration it will be noted that the first arrival of energy to the detector or geophone is recorded on the seismogram at an amplitude corresponding to that disclosed in Figure 1a. Additionally, it will be noted that, after the last first break has been recorded, the gain in amplification in all amplifiers has been suppressed to some small value, which, in some cases, can be zero. Then, from this point the gain in amcorrelate or proiile. In some areas the weathered v layer or overlying unconsolidated stratum is relay tively' thin and reflections arerobtainabie from shallow horizons underlying the weathered layer. If it is impossible to instantly suppress the gain in amplification after the direct traveling waves have been recorded, these shallow reections will be' completely obscured. Therefore, the utility for this particular response is readily apparent. A record such as that illustrated in Figures `2a and 2b in addition to giving-edenite data from which the thickness of the weathered layer can be checked at each'spread, also gives the reflections from all horizons that it is desired towork or prole. Since these are all the data that are required from -seismic waves which have been recorded in one direction on a particular spread, there is no necessity for detonating more than one charge of explosive and recording more than one seismogram. This obviously results in a tremendous saving of time, explosives, and a saving in depreciation of the equipment.

To obtain a seismogram such as that illustrated in Figures 2d and 2b, it is desired that the amplification characteristic of the electric or vacuum tube amplifier be as disclosed in Figures 3a and 3b' and in Figure 4 in which there is shown a curve having the gain in amplication as ordinates plotted against time as abscissae.

A fundamental electrical circuit which may be used in connection with the control or suppressor grid of a. conventional vacuum tube in one or more stages of a vacuum tube ampliiier, is shown in Figure 5. This circuit, although a simple circuit, serves very well vto illustrate the fundamental principles of this invention. In this circuit there is illustrated, connected in series, a source of potential I, a resistance d, a condenser 5 and a switch 2. Then, connectedin parallel with the series combination of resistance d and capacity 5, there is a. second resistance 3. At thepoint 6 in the circuit a ground wire is attached, bringing this point in the circuit tozero potential. When the switch 2 is closed, a source Cof potential l will cause a current to flow in the circuit through the resistance 4 until the condenser 5 is fully charged. The intensity of the `current flowing during the charging process of ing tiine for thecondenser 5 is controllable by the. Aresistance 4.- From the fundamental elements of electric circuits it is well known that a curv rent flowing through a resistance is in time phase mum negative value instantly, and then from its maximum negative value to zero exponentially with time. In order to complete a discharge circuit for'the condenser 5, a resistance 3 is connected as described above and shown in Figure 5. This discharge circuit becomes effective to restore the circuit to original conditions and makes it ready for a repetition of the cycle described above when the switch 2 is open.

The varying potential at the point B can be utilized as a variable grid bias voltage on a vacuum tube in an electric seismograph amplier.

It is well known that the gain *n amplification plitlcation of this vacuum tube. the battery S is connected in series with the point produced by a vacuum tube can ne controlled by varying the grid bias voltage. Additionally, it has been found that if a sumciently large negative value of voltage is impressed across the grid of a vacuum tube, it can be biased completely out of its characteristic and no appreciable gain can be obtained from that tube. Dependent upon the 'type and make of a particular vacuum tube used for purposes of amplification in an amplifier, it will function to produce the greatest gain at a zero grid potential4 or some small negative-value of grid bias voltage. Additionally, it has been found that -by varying this grid bias from avzero or small negative value to a relatively large negative value, the amplification can be reduced proportionally, and this principle is utilized in con-l trolling the instantaneous values of gain in amplication in the seismograph amplifier.

In Figure 'l there is shown this fundamental circuit connected to the control grid of a conventional vacuum tube of a conventional amplifier. The simple circuit explained above has now been modified in that the resistance has been divided into two variable resistances 4 and l in series.

The point B is made variable Ion the resistance d in order to select' the desired negative potential and the pointA is made variable in order to control the charging time for the condenser 5. By connecting the point B tothe control -grid of a vacuum tube, the variable potential described above can be utilized to control the gain in am- Additionally, if

B and the control grid of a vacuum tube 9, one is enabled to control the initial and/ or maximum `grid bias voltage, which, in `turn functions to control the gain in amplification produced by the tube 9. As described above, when the switch 2 in this circuit is closed, the point B is immediately placed at a negative potential with respect to the ground 6. This negativepotential is se-v lected suliciently large that the grid is biased out of the characteristic of the tube suillciently far to reduce the gain in amplification to a predetermined value. This is effected instantly lon closing the switch 2. As the condenser 5 aprelationship with the voltage across the resistance, y

Consequently, the IR drop across the resistance 4,'while the condenser 5 is charging, has similar lcharacteristics to that of the current flowing through it. Therefore, the potential at the point B with respect to the pointv 6, which is grounded, goes through the same values relative to time that the 1R drop across the resistance ,6 does. That is. this potential will go from -zero value to its maxiproaches full charge, the current in theresistances 4 and 1 approaches zero, and the potential from the ground, of the point B as a result approaches zero, causing the grid bias voltage on the tube 9 to assume the initial value. The position o f the point A on the resistance I can be so yadjusted as to control the time required to asthe time required to record a record, such as that disclosed in Figures 2a and 2b. ComparingA the curve to the seismogram illustrated in Figures 2a and 2b, it will be noted that between the points G and` H on the curve, the gain in amplification is at a constant predetermined value. 'I'his period of time represents the time required for the initial breaks due to the direct traveling seismic waves to be recorded. At the instant at which the last initial break occurs on the trace from the geophone most distant from the shot, the gain in amplification immediately drops to a value represented by the point K. This drop in. gain of amplification occurs instantly with the closing of the switch 2 discussed above. Dependent upon the value of the resistances 4 and 1, the capacity of the condenser 5 and the magnitude of the potential I, the gain in amplification is then caused to expand exponentially with time as vthe condenser 5 is fully charged during an interval of time corresponding to the distance between the points K and L on the curve.

In Figure 8 there is illustrated a circuit which corresponds to that in Figure '7, with the exception that the operation of the circuit to control the gain in amplification of the vacuum tube 9 is initiated by agas triode tube I3, such as the tube commonly sold on the market under the trade-name of Thyratron The-characteristics of a. vacuum tube of this type are well known to one skilled in the art; that is, that a grid placed between a cathode and anode of a hot cathode tube containing gas, if sufficiently-negative, will prevent the arc from striking, because under these conditions, there is no accelerated field around the cathode. -The electrons, therefore, cannot acquire sufiicient velocity to ionize. When the grid is made positive, electrons are accelerated toward it. They collide withgas atoms, ionizing them and the arc strikes. Therefore, it can be seen that a negative grid prevents the arc from striking, and a positive grid allows it to strike, effecting a flow of current from the cathode to the anode of the tube.

After the arc has struck, it presents the peculiar feature that the grid has no more control of it, and the current will continue to flow until the plate circuit is broken. As shown, the grid is negatively biased with respect to its cathode- A by the bias battery C.

The arc in the gaseous discharge tube can be caused to strike in many ways. For example, the impulse of current flowing in the blaster circuitv that is dependedv upon for the time-break signal, can be used to change the grid bias voltage of the gaseous discharge tube from a large negative value to a positive or small negative value, or the initial impulse of the wave train after sufficient amplification, can be used for the same purpose In other words, a potential from any source, just so long as it has a suicien'tly high positive value, can be used t'o initiate the flow of current in the plate circuit of the gaseous discharge tube.

After the arc is struck in the tube, the current from the battery I or plate supply source which flows/in the plate circuit causes an IR drop across resistance 3. This IR drop is impressed across l. scribed above in completing the charging circuit for the condenser 5, switches l0 and Il are p roy vided. Switch I0 is used for opening the gas triode plate circuit, to stop the flow of current in the plate circuit, for it is a characteristic of the gas triode tube that once the arc is struck, the grid has no more control. v Switch I I is used for removing the bias manually from the grid of the gas trlode tube to cause current to flow in its plate circuit for test purposes. A resistance IIa is included in the circuit completed by switch I I. Anfter all adjustments have been made and a seismogram is to be recorded, the 'normal position forv the switch Il) is closed and the switch II is open.

When a positive potential resulting from an impulse is impressed on the grid of the gas triode tube to reduce the negative bias from battery C, the full plate current starts to flow instantly. This plate current develops an IR drop across the resistance 3 instantly. This IR drop ls also impressed across the resistances 4 and 'I and condenser 5, that is connected in series with them.

This results in the point B going instantly to a.

maximum negative value with respect to the ground 6. Since the control grid has ths value of IR drop impressed upon it, the gain in amplication instantly drops to a predetermined value.

Then, as the current continues to flow through the resistances 4 and 1, the condenser 5 will become fully charged and the gain in amplification, controlled by the IR drop in the res'stance, will increase exponentially with time.

In Figure' 9 there is shown a further modification of this invention, in that the fundamental circuit is used to control the grid bias voltage on vacuum tubes in a plurality of amplifiers. In this illustration. of the circuit diagram the potential at point B through the resistances I2 is appl'ed to the grids of one or more vacuum tubes in aplurality of amplifiers. The resistances I2, in addition to serving as means whereby a' .connection to a plurality of amplifiers can be made, are made variable so that the maximum gain in amplification for each amplifier can be controlled independently of the others.

The battery'IS in this circuit is used to adjust the initial and/or maximum amplification of all the amplifiers controlled. The potential of the battery 8, by means of the variable resistances I2, is used to control the initial and/or maximum gain in amplification of the individual amplifiers. With such an arrangement it is possible to record on one seisrnogram several traces having equal amplitudes throughout their length. Although the illustrative circuits by which the control of the amplifier is accomplished, are described in connection with the suppressor grid of a conventional vacuum tube, it is obvious that equal or even better c'cntrol ,can be had by ,applying this principle to the ordinary control grid of three element tubes.

, ducted through the conductor I4l to` the trans-v Referring to Figure 9, the operation of this system is as follows: A signal suchas that delivered by the impulse recorded on the last trace of the seismogram due to directtraveling waves is conformer T and is impressed upon the grid of a gas 'triode tube I3, which has been adjusted to some negative value.

closed when this impulse is impressed upon the grid of the gas trode, the arc will be struck, due i to the fact that the grid has been made sufficiently positive to cause it to lose control. A flow of plate current results through the circuit, which will in turn flow throughvthe resistances 4 and 'l to charge the condenser 5. At the time the plate current begins to now inthe plate' circuit of this Assuming that the switch I0 is some predetermined value.

tial at the point B will go from some maximum negative value to ground potential at the time the condenser becomes fully charged. This vari able potential produces a change in the grid bias on the vacuum tube 9. There will result a corresponding change in the gain in amplification. In Figure 4, the instantaneous values of amplication, from the instant at which the time break occurs until the entire seismogram is recorded, are illustrated. Utilizing a system having a characteristic curve of this nature will produce a seismogram such as that illustrated in Figures 2a and 2b. A study of this seismogram will indicate that no more than one record is required to record all the data necessary on a profile that is shot in one direction.

In Figure 9 it will be noted that by connecting the conductors l to the time break and telephone circuit that the operation of this circuit can be initiated by the time break. With the circuit in an electric seisrnograph connected in this manner it will function as an ordinary expander at the time the time break impulse is received at the transformer I. The grid of the gas tricde I3 will immediately lose control and plate current will begin to flow. `As described above, this plate current will develop a large IR drop across the resistances d and to place a relatively large negative bias upon the vacuum tube 9 in the conventional amplifier shown. This results in a suppression in the gain in amplication instantly to As this occurs the gain in amplification is then allowed to build up exponentially with time as the condenser 5 becomes fully charged.

Although specic arrangements of elements are herewith described, it is obvious to those skilled in t the art that other arrangements of elements in an electric circuitwould produce the same result. t Although the circuit of Figure 9 shows the control acting on a grid of one tube, it is equally adaptable to the control of the grids of more than one tube of a seismograph amplier.

I claim:

l. Apparatus for making records of seismic waves generated at a shot-point and received at a station remote therefrom,'comprising a seismometer at the said station adapted to convert said waves into electro-motive-forces o-f voltages indicative of the amplitude and in sympathy with vsaid waves,- an amplifier for magnifying said voltages, a-galvanometer-recorder for recording the same in respect to time intervals, and means responsive only oncey during the receipt of said waves to an amplifier loutput above a predetermined minimum valuefto reduce the gain of said amplifier, and means to gradually return said gain to normal over a period of the order of the duration of the waves.

2. Apparatus for making records of seismic vv'aves generated at a shot-point and received at a station remote therefrom, comprising a seismometersat the said station adapted to convert said waves into electro-motive-forces of voltages indicative of the amplitude and in sympathy with the same in respect to time intervals, means responsive only once during vthe receipt of said Waves toan amplifier output above a predetermined minimum value, to reduce the gain of said amplier, means to gradually return said gain to normal over a period of the order of the duration of the waves, and means to adjust the rate of return oi said gain to normal.

3. Apparatus for making records of vseismic waves generated at a shot-point and received at a surface station remote therefrom, comprising a seismometer at the said station adapted to convert said waves into electro-motive-forces of voltages indicative of the amplitude and in sympathy with said waves, an amplier for magnifying said voltages, a galvanometer-recorder for recording the same in respect to time intervals, means responsive only once during the receipt of said waves, to an amplifier output which is greater than a predetermined value, to reduce the gain of said amplifier, means to gradually return said gain to normal over a period of the order of the duration of the waves, and means to nonlinearly change the rate of return of said gain to normal.

4. A recording system for seismic waves occurring in a train extending over a period of several seconds in which thejrst part of the train has a general amplitude level several times Ygreater than that of the remainder and in which the general level of the remainder gradually decreases, comprising in combination, a detector for converting said waves into voltages indicative of the amplitude and in sympathy with said waves, an amplier for said voltages associated with said detector, a recorder connected to the output of said amplifier, said amplier including a tube having a control element, a control tube having a control element associated with said output and a cathode, a source of negative bias voltage for said second mentioned element having its positive side at 'cathode potential and being adjusted in value to prevent currentA `ilow in the anode circuit of the control tube until said output exceeds a predetermined value, a resistor -in the said anode circuit the drop across which increases the amount of negative bias on said amplifier tube control element when current flows through the resistor, and means in said anode circuit to effect an initial high current flow through the resistor followed by a gradual diminution in the current.

, 5. In methods of seismograph prospecting wherein a series'o seismic vibrations of amplitude initially high and decreasing with time are detected, converted into electrical signals and the signals are amplified electrically and recorded,

. energy the controller acts to lower the gain and thereafter gradually and progressively restores the gain: the .improvement which comprises cutting off the effect of the signal energy on the -gain controller immediately after receipt of the initial high amplitude signals and keeping such effect cut on throughout receipt of the remainder of the seriesof vibrations, whereby the gain increases smoothly without falling off to lower values, subsequent to receipt of the initial high amplitude signals, uninuenced byl subsequent fluctuations in amplitude of signal energy.

.6. An apparatus-for seismic prospecting comprising a detector of; seismic waves adapted to convert said waves into electrical signals; a

ther-imams tube ampnfief for ampnfying said electrical signals, said tube amplifier having a rcontrol grid; a recorder for recording said electrical signals in correlation with time; a control circuit including a Thyratron tube, a resistance and a condenser in series in the plate circuitl of said Thyratron tube; means for impressing a negative bias on the control grid of said amplifier upon the flow of current in said plate circuit whereby the gain of said amplifierv Thyratron control grid to prevent the flow of current in the plate circuit of the Thyratron tube; and means for reducing the bias on said Thyratron control grid to permit the ow of current in said Thyratron plate circuit.

7. An apparatus for seismic prospecting comprising a detector of seismic waves adapted to convert said waves into electrical signals; a thermionic tube amplier for amplifyingr said electrical signals, said -tube amplifier having a control grid; a recorder for recording said electrical signals in correlation with time; a control circuit including a Thyratron tube, a resistance and a condenser in series in the plate circuit of said Thyratron tube; means for impressing a negative bias on the control grid of said amplier upon ,the ow of current in said plate circuit whereby the gain of said ampliiier is reducedA substantially instantaneously upon the flow of current in said plate circuit and is gradually restored upon the completion of the charging of said condenser; a control grid for said Thyr'atron tube; means for normally biasing said 'I'hyratron control grid to prevent the now of current in the plate circuit of the Thyratron tube; and means responsive to the initial output of signals from said amplier for reducing the bias on said Thyratron control grid to permit the i'low of current in said Thyratron plate circuit.

i3. Apparatus for seismic prospecting comprising a detector of seismic waves adapted to convert said waves into electrical signals; a thermionic tube amplifier to amplify the electrical signals, said tube ampliier having a suppressor grid; a recorder to make a record of said electrical waves in correlation with time; a control circuit including in series a condenser, a resistance, a source of current and a Thyratron tube; a connection between said suppressor grid and said circuit to impose a negative bias on said grid when the circuit is initially closed by the striking of an arc across said 'I'hyratron and thereby 'substantially instantaneously reduce the gain of said thermionic tube amplifier, said ,neg`

ative bias gradually decreasing with the redu'c tion in the current in said. circuit as said condenser becomes charged and thereby gradually restoring the gain of said thermionic tube amplifier; and means responsive to the seismic operations for causing an arc to strike across the Thyratron tube.

9. Apparatus for seismic prospecting comprising a detector of seismic waves adapted to convert said waves into electrical signals; a thermionic tube amplifier to amplify the electrical signals, said tube ampler having a, suppressor grid; a recorder to make a record of said electrical waves in correlation with time; a control circuit including in series a condenser, a re,-A

sistance, a source of current and a Thyratron tube; a connection between said suppressor grid and said circuit to impose a negative bias on said grid when the circuit is initially closed by the striking of an arc across said Thyratron and thereby substantially 4instantaneously reduce the gain of said thermionic tube amplifier, said.

10. An apparatus for seismic prospecting com` prising a detector of seismic vwaves adapted to convert said waves into electrical signals, a thermionic tube amplifier for amplifying said electrical signals, said amplifier having a control grid, a recorder for recording said electrical signals in correlation with time, a control circuit for said ampliiiei` including a resistance and a condenser in series, means for applying a potential difference across said series-connected resistance and condenser thereby to produce current flow through said resistor which is initially of high magnitude and as said condenser is charged is gradually reduced, and means for applying to said control grid a negative bias derived from the potential' difference produced across said resistance by the current ow therein, whereby the gain of said amplifier is reduced substantially instantaneously upon the flow of current through said control circuit and is gradually restored upon the completion of the charging of said condenser.

l1. An apparatus for seismic prospecting com prising a detector of seismic Waves adapted to convert said waves into electrical Signals, a thermionic tube amplifier for amplifying said electrical signals, said amplifier having at least one gain-controlling grid, a recorder for recording said electrical signals in correlation with l time, a control circuit for said ampliiier including a resistance and a condenser in series, means operable during receipt of said waves for applying a potential difference across said seriesconnected resistance and condenser thereby to produce current iiow through said resistor which is initially of high magnitude and as said conl denser is charged is gradually reduced, and means for applying to said control grid a negative bias derived from the potential diierence produced across said resistance, whereby the gain of said amplifier is reduced substantially instantaneously upon the ilow of current through said control circuit, and is gradually restored to its original value upon the completion of the charging of Vsaid condenser.

12. In an apparatus for seismic prospecting having a detector of seismic waves adapted to convert said waves into electrical signals, a thermionic tube amplifier for amplifying said electric signals, said ampliiier having at leastI one gain-controlling grid and a recorder forI ing on said gain-controlling grid of said amplifiera negative bias whose magnitude varies with that of said current flow, thereby quickly to reduce by a substantial amount the gain of said amplifier and thereafter gradually to increase the amplification to its initial value upon the completion of the charging of said condenser.

13. In an apparatus for seismic prospecting` having a detector of seismic waves adapted to convert said waves into electrical signals, a thermionic tube amplifier for amplifying said electric signals, said amplifier having at least one gain-controlling grid and a recorder for recording said electrical signals in correlation with time, the combination of means for controlling the amplification of said amplifier comprising a control circuit including a resistance and a condenser in series, means operable at a predeterminedtime during the detection of said waves for applying a substantial potential difference to said control circuit to produce a current flow therein which is initially of high magnitude and which decreases with the, charging of said condenser, means for producing on said gain-controlling grid of said amplier a, negative bias whose magnitude varies with that of said-current now, thereby quickly toreduce by a.

fication to its initial valueupon the completion of the charging o-f said condenser.

14. In an apparatus for seismic prospecting' having a detector of seismic waves adapted to convert said lwaves into electricalv signals, a thermionic tube amplifier for amplifying said electric signals, said amplifier having at least one gain-controlling grid and a recorder for recording saidelectrical signals in correlation with time, the combination of means'for controlling the amplification of said amplifier 'comprising a control circuit including a resistor and a condenserA in series, means :for applying a substantial potential diierence to said control circuit.

to produce a current ow therein which is initially of high magnitude'and which decreases with the charging of said condenser at a predetermined rate fixed by the capacity ,of said condenser and the resistance of said resistor, means for producing .in response to said current flow afnegative bias'on said gain-controlling grid of V`said ampliiier therebyquickly to reduce by a predeterminedamount the gain of said amplifier, and thereafter gradually to increase the amplification to its initial value upon the completion of the charging of said condenser, and means for adjusting the extent of said reduction in amplification without changing the resistance 0f M. sHooK.

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