US2317334A - Master control for electric seismographs - Google Patents

Description

April 20, 1943. E. J. SHIMEK 2,317,334

MASTER CONTROL FOR ELECTRIC SEISMOGRAPHS I Filed Feb. 21, 1940 v 2 Sheets-Sheet i [iv/A5751? con reoL if; RECORDEA? I [K I LAh'fi A 1A 2T mrhru 2 Z T/ME April 20, 1943. E. J. SHIMEK MASTER CONTROL FOR ELECTRIC SEISMOGRAPHS 2 Sheets-Sheet 2 Filed Feb. 21, 1940 Patented Apr. 20, 1943 UNITE s'mres MASTER CONTROL FOR ELECTRIC SEISMOGRAPHS of New York Application February 21, 1940, Serial No. 320,132

4 Claims. (Cl. 177-352)' This invention relates to electric seismographs and more particularly to a method and apparatus for controlling the gain in amplification as derived from a thermionic tube amplifier throughout the period of time during which seismic waves which have been created in the earth's surface are being detected and amplified.

Another feature of this invention resides in the provision of means whereby a geophone that is used to detect waves which travel vertically to the surface from the explosive charge, can be automatically cut out of the circuit so that the trace on which the vertically traveling waves has been recorded, can be used to record signals which have been generated by one of the geophones forming the spread.

Heretofore, it has been necessary to use a separate trace on the seismogram for recording the uphole or vertically traveling waves. This difiiculty has been overcome by the provision of a circuit that is energized by the time break for operating a delayed acting relay to disconnect this geophone, which has been connected in parallel with one of the geophones forming the spread.

A number of methods for controlling the gain in amplification as derived from athermionic tube amplifier while amplifying seismic waves are known in the art. These methods, however, have not proven entirely satisfactory, since they did not control the amplification characteristics of the amplifier in such a manner that all the necessary data from a single spread, when shot in-one direction. could be recorded on a single seismogram. 'The master control forming the subject matter of this application makes such a record possible. The gain in amplification as derived from the amplifier is allowed to remain at a maximum value until signals corresponding to the first direct traveling wave energy have been recorded. Then the gain is automatically reduced to a predetermined value so that shallow reflections carrying high energy can be recorded on a limited width of recorder strip. The gain in amplification then is caused to increase with time -as deeper and deeper reflections are detected andamplified.

Therefore, the primary object of this invention resides in the provision of a master control for electric seismograph amplifiers. in conjunction with auxiliary circuits, by means of which it is possible to record all the data required from a single spread when shot in one direction on a single seismogram.

Another object of this invention resides in the provision of means whereby an uphole geophone can be cut out of the circuit after the vertically traveling waves have been detected at the surface.

Still another object of the invention is the provision of means for controlling the gain in amplification as derived from an electric seismograph amplifier in such a manner that it will remain at a maximum until signals corresponding to the first direct traveling seismic waves have been recorded, after which it is immediately reduced to a predetermined low value and then automatically expanded with time in substantially inverse proportion to the envelope of the detected signals.

Another object of this invention is the provision of means, initiated by the detonation of the explosive charge, for placing the master control in operation.

This invention also contemplates a master control for controlling the gain in amplification as derived from a plurality of amplifiers.

Other objects and advantages of the invention will be apparent from the following detailed description when considered in the light of the drawings in which:

Figure 1 is a diagrammatic illustration of a complete reflection seis'mograph system showing the location of the geophones relative to the explosive charge;

Figure 2 is a composite circuit diagram showing the invention forming the subject matter of this application incorporated in the circuits of an electric seismograph;

Figure 3 is a curve which has been plotted with negative suppressor grid potential as ordinates and time as abscissae, showing the manner in which the suppressor grid potential as applied to a thermionic tube is caused to vary to effect the desired variations in the gain in amplification as derived from the amplifier.

Figure 4 is a second curve that has been plotted with time as abscissae and negative suppressor grid voltage as ordinates showing in addition to the disclosure of Figure 3. an additional step in the voltage control;

Figure 5 is a curve which has been plotted with ain in amplification as ordinates and time as abscissae showing the manner in which the gain in amplification is caused to vary while recording seismic waves.

Referring to the drawings in detail, particularly Figure 1, there is illustrated the usual seismic survey operation using the reflection method by means of which data can be recorded from which to compute and plot profiles of the subsurface geological strata. These data are in the form of wave travel velocities, the waves bein created in the earths surface by the detonationmeasuring the velocity of wave travel in the unconsolidated weathered surface layer or the earth. Various detailed methods of accomplishing this result are well-known to those skilled in the art and need not be discussed here.

The sequence of events occurring at the geophone after the detonation of an explosive charge is first the detection of the first direct traveling waves, followed by the detection of waves which have been reflected from the interfaces of substrata that it is desired to profile.

The geophone G can be of any type adapted to convert seismic waves impinging on it into electrical signals. These electrical signals that are generated by the geophone are amplified through as many stages of amplification as required and recorded by means of a recording galvanometer on a sensitized paper or photographic film in the form-of traces, there being a trace for each geophone station in the spread.

Seismic surveying by the reflection method depends upon an analysis of the records of the waves recorded and the determination of the instant of arrival of these waves at the geophone or geophones. Additionally, in'order to make corrections for weathering to the times of arrival of the reflected waves, it is necessary to determine the instant of arrival of the first direct traveling waves to reach the geophone or geophones. This means that the record must have an indication of the instant of detonation, some means for indicating on the record the passage of time, a clear record of the instant of arrival of the first direct traveling waves and the instant of arrival of the succeeding reflected waves. The direct traveling waves and those waves which have been reflected from relatively shallow interfaces carry a high amount of energy and if the sensitivity of the apparatus was allowed to renot go of! of the recorder strip, then automatically increasing the sensitivity of the apparatus as the energy of the reflected .waves from the deeper interfaces decreases, so that all of the sig nals representing reflected wave energy ,will be recorded at substantially the same amplitude, regardless of the depth of the interface from which they have been reflected.

As an aid to the determination of the velocity of wave travel in the weathered layer, a geophone is placed adjacent the mouth of the shot hole for detecting waves traveling vertically upward from the explosive charge. The record from a geophone so placed heretofore has been recorded on a separate trace. In a recording galvanometer having a predetermined number of vibrating elements, this procedure would obviously limit by one the number of geophone stations that could be used in the spread. This difficulty can be overcome by connecting the uphole geophone in parallel with one of the geophones in the spread through the medium of means whereby it can be disconnected from the recorder after it has served its purpose. The other geophone then will.

function in the normal manner. The result would be that one of the traces would have not only the usual recorded data that would be similar to that of the other traces but would have in addition thereto a record of the arrival of the uphole waves at the geophone that was located near the mouth of the shot hole.

Referring to Figure 2, the explosive charge S is detonated in the conventional manner by a blaster B. Rupturing the blaster circuit by the detonation of the explosive charge generates a transient voltage in the circuit that will pass through the transformer T1 to the grid circuit of a gas triode tube ID. The grid of the gas triode tube I0 is negatively biased by the battery II to prevent it from firing in the manner well-known in the art. The resistance I2 is placed in series with the battery I I to prevent short-circuiting the battery when the switch I3 is closed for test purposes. The transient voltage from the transformer T1 causes the negative potential on the grid of the gas triode to become less negative to a point where the grid no longer has control and plate current will immediately begin to flow in the plate circuit; the switch l4, provided for test purposes, is normally closed. The plate current from the gas triode flowing through the resistance produces an IR drop across it. This voltage, by means of the conductors I6 and I1, I8 and I9, is conducted to the input transformer T2 of the amplifier A through which it passes to reach the recording galvanometer R, where it is recorded as an indication of the instant of detonation of the explosive charge S. The plate current from the gas triode also passes through the resistance producing an IR. drop across this resistance that serves as the charging potential for the condenser 2|. The voltage from this condenser, by means of conductors 22 and 23, operates a multiple'contact relay 24. The capacity of condenser 2| and the value of resistance 20 is so selected that ample time will elapse between the instant of detonation of the explosive charge, which coincides with the time that the condenser begins to receive its charge, and the time at which the relay 24 operates to disconnect the geophone 25from theinput transformer of the amplifier.

' The operation of the relay 24 accomplishes this tional.

by opening contacts f-g. It will be noticed that Additionally, the varying potential when applied to an even number of stages in the amplifier, will block out distortion produced by varying the potential on these grid elements.

The varying suppressor bias potential is supplied by a novel circuit arrangement, to be described hereafter, whose operation is initiated by the relay 24. Before the detonation of the explosive charge S, the relay contacts ae, h--i and fg are closed, and contacts be are open. With this condition existing, the tube 21 is blocked by the potential that is supplied by the battery 28. With contacts be open, the grid 29 of tube 30 is at cathode potential but grid 3| of tube 30 with contacts a-e closed, is biased by means of the batteries 32 and 33 to a point considerably beyond the cut-off value by proper choice of these voltages. With this condition existing, there will be no plate current flowing in the plate circuit 34 of tube 30 and therefore no drop produced in the resistance 35. As a result, tubes 36 and 3'! in the amplifier will operate to produce maximum gain. This condition exists until after detonation of the explosive charge and the condenser 2| becomes charged to the relay actuating potential. When this value has been reached, relay 24 will operate to open contacts a-e, bi and f--g and close contacts be. When contacts a-e open, the charging potential is removed from condenser 38 and it begins to discharge through the resistances 39 and 40 to cause the negative bias on grid 3| of tube 30 to vary toward zero as the condenser 38 discharges. The rate at which this bias be comes less negative and eventually reaches zero -is controlled by the resistors 39 and 40 and is rapid relative to the rate at which the potential of the condenser 4| changes. However, until the instant that the potential on grid 3| reaches the point of cut-01f from a point below cut-oif,'the tube 30 is still blocked and there will be no voltage applied to the suppressor grids of tubes 36 and 31 in the amplifier. The time required for this potential to reach the cut-off potential is so selected by varying the resistance 40 that signals corresponding to the first direct traveling Waves that have been detected can be recorded before any suppression of the gain in amplification as derived from the amplifier is effected.

Since at the time contacts a-e opened, the contacts be closed, the potential as supplied by the condenser 8| to the grid 29 of tube 36 begins to become more and more negative at a rate determined by the resistance 42. This change in potential on grid 29 though is slow as compared to the rate of change of potential on grid 3|, but this change in the grid voltage on grid 29 will eventually block the tube and bring the potential on the suppressor grids of tubes 36 and 31, which was permitted to increase as the potential on grid 3| became less negative, back to zero at which time the amplifier is again operating at full sensitivity.

With the circuit described thus far, the potentials on the suppressor grids of tubes 36 and 31 will be caused to vary in the manner illustrated in Figure 3. This curve has been plotted with negative suppressor grid potential as ordinates and time as abscissae. From the time to to ii, the bias on grid 3| of tube 30 is becoming less nega tive to the point of cut-off. From the time ii to t. the potential on grid 3| is becoming less negative and the potential on grid 29 is becoming more negative but at a much slower rate of change than the potential on grid 3|. This will result in a suppression in the gain in amplification during the time 1 to 2. From is to t: the biasing potential on grid 29 is controlling and this potential will become more and more negative until tube 30 has again become blocked. During this time t2 to t: the negative potential supplied to the suppressor gridsis becoming less and Jill less negative and will reach zero when tube 30 has been blocked by the potential on the grid 29.

Due to the fact that the energy carried by the waves that have been reflected from deep interfaces is relatively low, it is desirable to have the gain in amplification as derived from the amplifier from a certain time on to the end of the record. increase more rapidly. This result is accomplished in the manner hereafter described.

During the time when the above described sequence of events were occurring, the voltage across the condenser 43, due to the fact that the contacts h i were opened by the operation of the relay 24. has been increasing positively. but at this instant the tube 27 is still blocked. This positive increase in the voltage across condenser 43 is produced by its discharging through the resistance 46. Therefore, the voltage, due to the absence of plate current in the plate circuit 45 of the tube 21, across the condenser 38, remained at. zero potential. mined by the choice of resistance 44, the tube 21 becomes conductive and the voltage across the condenser 38 begins to increase negatively. This rate of change will be reflected by means of the grid 3| in the tube 30 to cause the rate of change of suppressor grid bias that is being applied to tubes 36 and 31 in the amplifier to suddenly increase in such a manner that it will approach zero much more rapidly. The curve in Figure 4 represents the manner in which the suppressor grid voltage, as supplied to the tubes 36 and 31 in the amplifier, is caused to behave over the 'period of time during which seismic waves are being recorded. This curve, like that in Figure 3. has been plotted with negative suppressor grid potential as ordinates and time as abscissae, the periods of time to to t1 and ii to it: being the same as described in connection with Figure 3. In Figure 3. however, the period of time from t2 to is is limited by the time at which the tube 21 becomes conductive. The time t: to t4 represents variation in suppressor grid potential from the time that tube 21 became conductive to the time that tube 30 was blocked and the suppressor grid potential again became zero,

A variation of the gain in amplification as derived from tubes 35 and .31 during the same periods of time indicated in Figure 4 would be as shown in Figure 5. During the period to to 11. while the suppressor grid potential is zero. the amplifier is operating at maximum sensitivity. As the suppressor grid potential becomes more and more negative during the time tl to n, the

gain in amplification is suppressed. However, as

the suppressor grid potential varies toward a less negative value during the period 2 to is, the gain in amplification is caused to increase and from the time t; to is. during which the negative However, at a time detercontacts 48'; can be used to simultaneously control the gain in amplification as derived from a plurality of amplifier channels.

The control voltage has been described as applied to the suppressor grids of the pentode type tubes. It is obvious to those skilled in the art that this varying potential may equally as well be applied to the control grids ofothertypes of tubes.

In the above specification the term uphole waves has been used to identify those waves which travel vertically upward from the explosive charge to the surface of the earth. The g'eophone which detects these waves is referred to as the uphole geophone.

The detailed circuits of the various elements used in the process and apparatus above have not been specifically described inasmuch as they are commonly known and may be varied within relatively wide ranges without departing from the principles of this invention.

I claim:

1. A method of recording seismic waves that comprises the steps of creating seismic waves in the earth's surface at a point vertically spaced below the surface, generating a signal coincident with the creation of the seismic waves, recording the generated signal on the trace of a seismogram as an indication of the instantof creation of the seismic waves, detecting the first impulses of waves which have traveled vertically upward from their point of creation, amplifying and recording the first impulses on the same trace on which the signal indicative of the creation of the seismic waves has been recorded, rendering inoperative said detecting means after the first impulses of the waves traveling vertically upward have been detected, amplified and recorded, detecting waves which have traversed paths substantially directly from their point of creation to a plurality of detecting stations that are collaterally spaced from said point, amplifying and recording the first impulses of these waves on separate traces one of which being the trace recited above, reducing the gain in amplification imparted to the detected direct traveling waves after their first impulses have been recorded, detecting, amplifying and recording on the separate traces and the first-mentioned trace waves which have traveled downwardly from their point of creation and have been reflected from the interface of the substrata, increasing the amplification imparted to the detected refiected waves at a predetermined rate for a predetermined time, independent of signal strength, then accelerating the rate of increase in amplification imparted to the detected reflected waves independent of signal strength during the recording of the remainder of the seismogram, whereby the reflected waves will be recorded with substantially -equal amplitude.

2. An apparatus for recording seismic waves comprising in combination, a plurality of means for detecting said waves by generating electrical signals corresponding to them, separate means for respectively amplifying and recording said signals in coordination with time, a single gain control for controlling the gain in amplification imparted to the signals by each amplifier, said gain control comprising means for reducing the gain in amplification after signals corresponding sponding to direct traveling waves are recorded with maximum gain in amplification and signals corresponding to detected reflected waves are recorded with substantially equal amplitude.

3. An apparatus for recording seismic waves comprising in combination, means for detecting said waves by generating electrical signals corresponding to them, means for amplifying and recording said signals in coordination with time, a gain control for controlling the gain in amplification imparted to the signals by said amplifier, means for initiating the operation of the gain control a predetermined period of time after the creation of said seismic waves, said gain control comprising means for reducing the gain in amplification after signals corresponding to detected direct traveling waves have been amplified and recorded, means for thereafter increasing the gain in amplification at a predetermined rate throughout a predetermined period of time, and separate means for accelerating the rate of increase of the gain in amplification throughout the period of time while the remainder of the record is being recorded, whereby signals corresponding to direct traveling waves are recorded with maximum gain in amplification and signals corresponding to detected refiected waves are recorded with substantially equal amplitude.

4. An apparatusfor recording seismic waves comprising in combination, means forming a conventional spread for detecting said waves by generating electrical signals corresponding to them, means for amplifying and recording said signals in coordination with time, an additional detecting means adapted to be positioned adjacent the shot point for detecting waves that have traveled vertically upward from their point of creation, means'for connecting said additional detecting means to amplifying means common to one of the first mentioned detecting means, a gain control for controlling the gain in amplification imparted to the signals by said amplifier, common means for rendering the additional detecting means inoperative and for initiating the operation of the gain control a predetermined time after the creation of the seismic waves, said gain control comprising means for reducing the gain in amplification after signals corresponding to detected direct traveling waves have been amplified and recorded, means for thereafter increasing the gain in amplification at a predetermined rate through a predetermined period of time, and means for accelerating the rate of increase of the gain in ampl fication throughout the period of time Whi e 'the remainder of the record is being recorded, whereby signals corresponding to direct traveling waves are recorded with maximum gain in amplification and signals corresponding to detected refiected waves are recorded with substantially equal amplitude.

EDWIN J. SHIMEK.

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