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get. 2%, 19456.
0. s. PETTY
Original Filed .Aug. 18, 1939
'2_ Sheets;Sheet 1
Patented Oct. 29, 1946
Olive S. Petty, San Antonio, Tex.
Continuation of application Serial No. ‘290,928,
August 18, ‘1939. This application May 27,
1943, Serial No. 4885764
4 Claims.
(Cl. 177-352)
This invention relates to methods of and ap
very much less thanthat of the direct waves
paratus for seismic surveying and deals partic
ularly with the treatment of the received shock
when travelling from .upper horizons, while those
waves after their conversion to electrical equiv
alents, whereby a recorded chart may be made
in which the maximum'amplitude swings through
out the length of the chart are generally main
tained below a predetermined level while weaker
more attenuated, having sometimes only 1/600 of
_ portions of the waves are ampli?ed to usable
levels. The invention contemplates especially_
certain improvements in methods and apparatus
for amplifying and recording the waves and the
application is a continuation of my ‘prior appli
cation Serial No. 290,928, ?led August 18, 1939.
Heretofore in making use of seismic methods .
and apparatus for surveying sub-surface geologi
cal formations, a great many di?iculties have
arisen because of the limitations ofv-equipment
available, and it has not always been possible to
make an accurate study of the records or charts
produced because of the intermingling of several
types of waves whose relative sizes may be on
the order of 600 to 1.
from progressively deeper horizons are more and
the .intensityof the direct waves.
It is-customary to record on a single chart .the
record traces from a plurality of detectors and
it is highly desirable to keep vthe physical size of
the chart within reason.
The waves are con
verted from seismic to electrical at the detector,
but there still remains the tremendous discrep
ancy in peak amplitudes between the direct and
the re?ected waves as Well as the problem of at
tenuation of the later arriving re?ected waves.
Most detectors seek to convert the seismic waves
to ,electro-motive-forces, the voltages of which
are indicative of the amplitudes of the corre
sponding seismic waves. The outputs ,from .such
detectors are usually at such low levels that they
are not adequate throughout the whole wave train
to operate the recorder which includes a galva
nometer having multiple moving elements each
responsive to the voltages of the waves from one
To understand some of these di?iculties a re
detector. Resort is therefore had to electric .am..
view of the general methods of use of seismic 25 pli?cation to raise ‘the level of the waves so that
recording apparatus may be brie?y made at this
the weakest produce a usable trace on the chart.
point. Such surveys are made by correlatingthe
results attained from a plurality of records each
If this is done with the conventional ampli?er
having uniform gain throughout, thetraces of
obtained by generating seismic waves, for instance
the direct waves become so large that they can
by ?ring a charge of dynamite below the surface 30 not be kept within the physical con?nes of the
of the earth at a position generally referred to
chart and furthermore those on adjoining traces
as the “shot point,” and detecting, at remote
tangle on the record. Likewise there is di?iculty
points, the time of arrival of radiating seismic
of entanglement of .the moving systems of a mul
Waves as well as vthe instant of the explosion and
recording the same on a seismogram, usually
by photographic methods.
The seismic waves generated in the earth by
the exploding charge travel in all directions and
some arrive at the receiving or detecting stations,
at remote points, by short paths of travel through
strata near the surface of the earth. These are
known as “direct waves” and because of their
relatively short path and major horizontal com
ponent of vibration, usually arrive ?rst .at the
detector. A second form of waves referred to '*
as “reflected,” travel down ‘to horizons at vari
ous distances below the surface of the earth and
at various inclinations thereto and are re?ected
back to the receiver. Such waves from the upper
horizons usually travel in longer paths than the .
direct waves, while those from the deeper hori
zons always travel in longer paths and take more
time to reach the detector than do the direct
waves. The re?ected waves consisting largely of
tiple galvanometer thereby vitiating the record
and probably damaging the instrument.
with excellent damping applied to the moving
systems of the galvanometer, the tremendous
swings ‘imparted by the oversized direct waves
prevent rapid subsidence and accurate recording
of the .earlier portions of the re?ected waves, and
it is highly important tojknow the instant the
first re?ected wave is received. ‘It is also im
portant to know the. instant of arrival of the
?rst direct wave.
To overcome the various di?iculties outlined
above, the present invention contemplates the
provision of an ampli?er, to be inserted inter
mediate the detector and the galvanometer of
the recorder, which ‘has such characteristics as
'to substantially eliminate the di?iculties and to
provide on the chart a trace of the seismic waves
having a general average level of maximum swings
which is substantially uniform .or increases to
ward the end of the trace.
vertical components‘ have an intensity usually 55 .To obtain these ‘improved results it is an 0b
ject of the present invention to provide an ampli
?er for use between a seismic detector and re
without the overlapping of the traces.
If this
same large amplitude were used for the high fre
quencies, it would be difficult to follow them since
they would be moving so rapidly. It would also
difficult to follow the high” frequency energy
wave size.
at large amplitudes because there would not be
Another object of the invention consists in
the duplication of the traces from one to the
the arrangement of an ampli?er whereby it pro
next since the normal step-out of the reflected
duces at all times such gain as tends to level out
energy from shallow beds (which produces high
the general average of the maximum swing
10 frequency energy) ’ may be a half a cycle or more;
heights of the response curve of the detector.
in other words, instead of the traces being merely
An important feature of the invention com
identical, one above the other, they will be shifted
prises a gain control responsive to the output
by one-half cycle or more. For this reason it is
of the ampli?er in which the gain remains con
extremely desirable that the traces not overlap
stant until the output passes a predetermined
level after which the gain is maintained at such 15 when high frequency energy is being recorded.
It will readily be noticed that the instant applica
value as to substantially hold this level during
tion shows a method whereby all of the above
the tendency to excess output and thereafter
advantages are obtained. Whenever the low fre
slowly returns to normal over a period of several
quencies predominate, on the contrary, very high
seconds whereby compensation for attenuation
corder which is provided with an automatic gain
adjuster at all times under the control of the
at the extreme end of the wave train is made.
Another important feature of the invention
comprises an ampli?er gain control operating
by superimposing portions of the plate current
gain is helpful in emphasizing the peaks of the
low frequency cycles and there is no confusion
in the record since the separate cycles are more
widely spaced.
It is therefore an object of the invention to
of an automatic volume control tube onto the grid
provide automatic gain control which tends to
of the ampli?er tube or tubes and in using a 25 suppress the ampli?cation of higher frequencies
condenser charged from this plate current to de
and to favor the amplitude of lower frequencies
liver the same over a period of time to said grid,
within a usable frequency range. Ordinarily the
whereby no abrupt increase in gain is effected
effect of such an arrangement in practice is to.
after any decreases.
Still another important feature comprises ap 30 establish increased amplitude or sensitivity
toward the end of the record since seismic waves
paratus for regulating the rate of discharge of
re?ected from the deeper beds and arriving later
said gain control condenser, the time over which
usually tend to be lower in frequency as well as in
the discharge takes place,,and the type of dis
charge-time curve resulting.
Consequently, in an ampli?er in
A further important feature of the invention 35 which higher frequencies are relatively more ef
fective in reducing the gain, the automatic gain
comprises means for determining the threshold
value below which the gain normally remains
control is, generally speaking, more effective near
the beginning of the record than toward the end
constant and above which any increase in out
of the record.
put causes a corresponding ‘decrease in gain.
Other and further features and objects of the
This threshold value may itself remain ?xed or 40
including the provision of various
may vary throughout the taking of a record in
?lters and the like for improving the operation
any predetermined and desired manner. In gen
of the whole ampli?er will be more apparent to
eral, I prefer that the threshold value shall not
those skilled in the art upon a consideration of
be ?xed, as regards the amplitude of energy su?i
cient to initiate functioning of the gain control, 45 the accompanying drawings and following speci?
but that it shall vary in accordance with the fre
quency of the received signals.
For example, I have found that it is desirable
to maintain a predetermined average ampli?er
output for the highest usable frequencies, and a .
somewhat larger average output of the ampli?er
when the energy is of lower frequencies. This
would preferably vary continuously from the
highest to the lowest usable frequency. Thus
where high frequencies predominate the peaks of .
the recorded traces are quite sharp and a large
amplitude is not required for readily locating
cation wherein are disclosed two exemplary em
bodiments of the invention together with various
curves representing the operation of important
portions of the circuits.
In said drawings:
Figure 1 is a wiring diagram of an ampli?er
constructed according to the present invention
shown connected to the output of a detector;
Figure 1A is a fragmentary view of a modi?ed
form of the last stage of ampli?cation;
Figure 2 is a curve showing the ampli?er gain
at various frequencies without the use of a ?lter;
Figure 3 is a similar curve showing the gain
with a ?lter in circuit;
the peak. If the same amplitude is used whenever
the low frequencies predominate, on the con
Figure 4 shows similar curves with possible
trary, the peaks are not sharp and are difficult 60
?lter adjustments;
to readily locate. This di?iculty can be over
Figure 5 is a characteristic curve of the auto
come by using a larger amplitude for the low
matic volume control tube showing how the grid
frequencies. If a large- number of traces are
bias can be adjusted to cut off plate current;
placed on a single record this would mean that
Figure 6 is a curve showing how the grid
one trace would deviate beyond the rest position 05
of the A. V. C‘. tube is modi?ed by the
of one or more of the traces near it. This is not
ampli?er output;
objectionable for low frequencies since it is much
Figure '7 is a corresponding curve illustrating
easier to follow the traces when they deviate
the resultant plate current ?ow;
slowly. The low frequency energy is usually from
Figure 8 is a set of curves‘ showing various
the deep reflecting beds, so there is very little 70
possibilities for changing the rate, time, and style
step-out from one trace to the next trace; in
of current discharge from the gain control con
other words, each trace nearly duplicates the
denser; while
trace above. it where low frequencies exist. This
Figure 9 shows the resultant ampli?er gain;
means that this similarity of adjacent traces
permits the use of appreciably larger amplitude 75
Figure 10 is a set of curves showing the effect
of the ?lter resistance value on the time of cur
rent leak off.
Referring now to the drawings for a better
understanding of an actual embodiment of the
invention, there is shown in Figure 1 a detector
The control grid
43 of the second stage ampli?er is coupled by they
_ .ter automatic volume control.
condenser 37 to the plate of the ?rst stage and
receives ‘its bias through resistor 45 from a tap
on battery 46.
The constants of the coupling condenser 31
from the ?rst stage to the second stage, of the
grid coupling resistor 45 of the second stage, and
or seisometer I5 of any suitable type adapted
‘to convert seismic waves to wave-form electrical
energy. The detector shown happens to be
ofthe plate coupling resistor 35 of the ?rst stage
‘equipped with a single stage pre~ampli?er and ilO are all predetermined or adjusted to give a se
therefore the cable leading to the ampli?er of the
lective frequency response as desired. The in
present invention includes four conductors I5,
coming signal in most cases is of a frequency
‘11, i8, and IS, the last two of which conduct ?la
varying from 20 to 100 cycles, whereas a number
ment heating current from a suitable “A” battery
of unwanted noises are at higher or lower fre
as represented at the left-hand side of the ?gure. v15 quencies and may thus be partially or completely
The conductors l6 and I? carry the 'B-supply
eliminated. To provide a further ?lter action,
from battery 20 and return the output of the
an adjustable coupling condenser generally de
single stage ampli?er to the ampli?er of the in
noted 4'! provides the coupling between the sec
vention. Each of these signal leads contains a
ond stage plate 38 and the control grid 48 of the
radio frequency choke coil 2| and one of these is 20 third stage ampli?er valve 49, functioning along
connected to the —A conductor by means of a
with the plate resistor 50. As shown, condenser
condenser 22 for ?ltering out alternating cur
41 comprises a plurality of small condensers, any
rents which may be picked up by the conductors
number of which may be placed in circuit by
leading to the detector. These conductors are
appropriate connections or links. A switch 5!
sometimes several hundred yards in length and 25 permits removing the condenser bank from the
may, by inductance, pick up hum from A. C. power
circuit by shunting the same. This condenser
lines and the like. A milliameter shown at 23
acts in the nature of a ?lter, ?rst to keep the
indicates the plate current of the tube in the
plate current of the tube from ?owing through
detector while the variable resistance 215 permits
the primary winding 52 of the interstage cou
regulation of the detector ?lament current as in
pling transformer 53, thus improving the trans
dicated by the milliameter 25. ‘This is essential
former life and permitting its design to be more
because of the diiferent lengths of conductor
eflicient. In addition, the condenser is used to
cable which might be used.
vary the frequency response of the ampli?er,
giving it the highest gain on the desired fre—
It is not uncommon to employ groups of de
tectors or seismometers in which the outputs of . i
the seismometers in each group are combined, the
quencies and very low gain on undesired fre
quencies, which e?ect comes from resonating the
seismometers in the group being so spaced asto
transformer primary.
minimize the e?ect of ground roll. It will be
Figure 2 shows the result of operating the
appreciated that such groups of seismometers act,
ampli?er with the ?lter cut out, and it will be
for the purpose of the instant invention, as sep 40 seen that the frequency response for the main
arate units and it will be understood that when
portion thereof has a fairly uniform gain over
ever one seismometer or detector is referred to
a wide range. Figure 3 is a similar View with
hereinafter, such groups having combined out
the ?lter cut in, showing in solid line 54 a ?at
puts are considered as embraced within such ref
topped, sharply cut off response over a short fre
An incoming signal passes through the pri
mary 25 of a suitable transformer, the secondary
'2'? of which is shunted by a pair of variable re
sistors 28 and ‘29, one for coarse and one for
?ne adjustment. These controls are for manu
ally setting the level of the signal applied to the
ampli?er of the present invention. They merely
waste a portion of the signal from a detector
and hence control the maximum gain of the am~
pli?er. They are set at one value for anyone
record and remain unchanged during the taking
45 quency range.
The size of ‘the plate coupling
resistor 50 may be'adjusted to give this response
‘the flat top and much greater sharpness of reso
nance as shown in dotted line 55 in Figure 3.
Figure 4 shows how the ?lters may be adjusted
to maintain the same sharpness of resonance
within various frequency ranges.
The secondary winding 56 of the interstage
transformer 53 connects between the control grid
G8 and the bias battery 46. The screen grid of
this tube is connected by wire 51 to the conductor
supplying “B” battery to the plates 3| and 38,
‘although it might be connected to conductor 4|
record to the next.
supplying potential to the screen grid of the
The secondary 2'! feeds into the cathode and
second stage ampli?er. The output from the
grid in the right-hand portion of the thermionic 60 third stage is fed through the primary 58 of
tube 30 which constitutes a simple triode ampli
transformer 59, one secondary 60 of which de
?er having the plate 3|, grid 32, and heated cath~
livers the ampli?ed waves to the moving element
ods 33 which it shares in common with the ele
of a galvanometer associated with and forming
ments of the second stage ampli?er indicated at
part of a recorder 6 l. Another secondary 62 takes
34. A suitable biasing battery 35 is arranged in 65 off a portion of the output of the ampli?er for
the grid circuit of the triode ampli?er. This am
supply to the automatic volume control tube as
pli?er is coupled to the second stage ampli?er by
will 'be further described hereinafter. The con
means of a plate coupling resistor 36 and con
denser 63 between the plate 64 of the third stage
denser'3'l. The plate 3|. as well'as the plate 38
tube and ground serves to by-pass high frei
of the second stage tube receive their “B” bat
quencies and to resonate the output transformer
tery supply through the common conductor 39
to assist in the desired ?lter action. At the same
connected to the positive end of the battery 40.
time, this condenser performs the important
'A tap 4| in this battery supplies the screen grid
function of assisting in the damping of the mov
of the same but are often changed vfrom one
42 of the second stage ampli?er with la somewhat
iower vpotcr'itial, permitting more gain'and “bet;
ing element of the galvanometer.
_ _
‘IniFigure 1A ‘is shown an optional way of con
necting the third stage tube 49’ to the output
transformer 59'. A second transformer 592 has
a primary in series with that of 59’ and which
is fed from the plate 64' of the last ampli?er
stage by means of the resistance 65 and con
denser 66, thus giving a resistance coupling in
which the values of the condenser and resistance
are critical, again assisting in frequency selec
rent flow as indicated in Figure 7, where milli
amperes of plate current are indicated by the
The curves 89 in solid lines indicate
the corresponding plate current flow for the three
swings 88 of Figure 6 which cross the position of
cut-off bias.
The curves of Figures 6 and ‘7 indicate, in gen
eral, the manner of controlling the gain of the
ampli?er in accordance with its output for, as
tion, this time, however, to determine those fre
later be described in more detail, the plate
quencies on which the automatic volume control
current from the automatic volume control tube
will function. Hereinafter the expression “auto
determines the total amount'of bias on the con
matic volume control” will be designated by the
trol grids of the second and third stage ampli
characters A. V. C. for convenience. The sec
ondary of transformer 59’ feeds the galvanometer.
The features of Figure 1A can be substituted in 15
Figure 1 where desired without any other changes.
The second half of tube 49 is a triode contain
ing plate 67, grid 68, and the common heated
?ers, functioning in conjunction with battery 46.
With the automatic volume control‘ tube inop
erative, i. e. when insu?icient output from the
last stage of the ampli?er is available to reduce
the tube bias to permit plate current ?ow, the
gain of the ampli?er is ?xed and constant irre
cathode 69 which functions also with the ele
ments of the third stage ampli?er. The plate of 20 spective of the size of the output as long as it
remains beneath the value predetermined by the
this tube is connected by wire 10 to a separate
setting of the potentiometer 18. This setting
“B” battery H, the negative pole of which leads
determines the threshold value above which any
through variable resistors 12, 13, and 14 back to
output from the ampli?er tends automatically
wire 15 connected to the negative “A” battery and
common ground terminal. It will be noted that 25 to reduce the gain thereof. The ampli?er may be
said to have a variable gain, the value of which
bias battery 46 is connected by wire ‘I5 to the
is ?xed until the output crosses the threshold
slider of the resistor 13 and hence is also con
after which the gain is reduced in accordance
nected to the ground through resistor ‘Hi shunted
with the tendency to increase the output. This
by condenser 93. The battery ‘H may have a
potential of about 45 volts for the type tube 30 has a levelizing effect on the maximum amplitude
of voltage swings delivered from the ampli?er
shown, while resistors ‘l2, ‘l3, and ‘M are of the
without, however, changing the relative size of
order of 200,000 ohms each.
the swings beneath the threshold value except as
The grid 68 of the A. V. C. tube is fed from
hereinafter described.
the secondary 62 of the output transformer 59,
The amount of plate current ?owing to the
the opposite end of which is connected by con 35
automatic volume control tube is determined not
ductor ll’ to the slider of a potentiometer ‘l8
alone by the setting of the bias resistor 18 but
energized from the portion of “B” battery 40 be
by the setting of the resistor 12 which is directly
tween negative terminal 19 and adjustable tap
in the plate circuit. This resistor may be set, for
80, through the contacts 8| of a relay, the wind
ing 82 of which is in series with the several tube 40 instance, to obtain plate current swings of the
size indicated at 89 in Figure '7 or to obtain small~
heaters. This ensures the contacts 8| remain—
er swings as are indicated by dotted lines 90. The
ing open until the ampli?er is turned on, thereby
settings of the resistors 13 and ‘M, in series with
not draining the lower portion of the “B” bat
72 and ground, are not so effective in regulating
tery through the potentiometer. The main por
tion of the “B” battery is shunted by ?lter con- ~ the plate current since they are shunted by ?xed
condenser 9! of considerable capacity, for exam
denser 83.
ple of the order of 1 mf., which by-passes the
The slider on the potentiometer 10' is set to
A. C. peak impulses to ground and at the same
supply a sufficiently negative bias to the control
time is charged with a substantially pure direct
grid 38 of the A. V. C. tube to normally prevent
the ?ow of any plate current in that tube. ‘ A ref 50 current voltage. The upper plate of condenser
9| is grounded and the potential of the lower
erence to Figure ‘5 will make this clear, where
plate is negative, feeding through conductor 92
and the portion of the resistor 13 set by means
between grid voltage and plate current. The cut
of the slider to the conductor 18 and into the
off point is at 86 when the grid is to that extent
negative. The grid, however, is carried negative 55 positive side of battery 46, increasing the nega
tive. potential supplied to the grids of the second
to the extent indicated, for instance, at 81 and
and third ampli?er tubes, thereby reducing the
will have to become positive by an amount indi
gain of these ampli?ers in accordance with the
cated by the distance between 86 and 8‘!- before
size of the current swings on the A. V. C. plate.
any currentv will ?ow in the plate circuit. The
Since the charging current for the condenser
position of 81 is adjustable by moving the slider 60
9| consists of pulsating D. C. it is not suf?ciently
on the potentiometer 18.
the characteristic curve of the tube is shown as
free from ripples to be fed directly back into
the grids of the ampli?er stages. It is the func
tion of the resistor 13 and the condenser 93 to
ous wave represents the output from the last 65 form a ?lter smoothing out these ripples and
causing substantially pure direct current to be
ampli?er stage as applied to the grid 68 in series
applied to the grids. The setting of the resistor
with its negative bias. It functions, of course, on
T3 and the size of the condenser 93 affect the
both sides or in other words is additive to or
manner in which the charge on the condenser
subtractive from the bias setting, again indicated
Bl leaks off to the grids. As hereinbefore men
in, this ?gure by 87. The amount of bias required
tioned, condenser 9| may well have a capacity of
for cut-off is shown at 85. Whenever the positive
the order of 1 mi. If these ?lter elements were
potential supplied from the transformer winding
not present, the charge would leak off from con
62 exceeds the difference between 86 and 81 as
denser 8! to the grids as shown‘by the curve H10
shown in the three swings 88', the grid is forced
Referring now to Figure 6 the operation of the
grid bias in preventing and permitting current
flow in the plate circuit is illustrated. The sinu
to a sufficiently. reduced bias to permit-plate cur
in Figure 8.
Appropriate settings of the con»;
g ..
denser and resistor permit the current to leak 01f.
in the manner indicated by such curves 'as I01
and “I2, increasing the time of discharge and
changing the slope thereof in accordance with
certain». characteristics desired to be accentuated
on the record chart. The resultant effect on the
gain of the ampli?er is indicated in Figure 9
where the curve 103 indicates the operation With
out the ?lter elements 13, 93, while curves I 04
and H15 correspond to the condenser and resistor
settings indicated by curves IUI and I02 respec
The variable resistor 14 a?ects the rate of
ferent frequencies so as to suppress to.- a greater
extent waves of high amplitude and high fre
quency than waves of comparable amplitude of
low frequency. To this end I may employ the
arrangement illustrated more particularly in
Figure 1A of the drawings in which, as herein
before pointed out, a requency- responsive ?lter,
comprising, for example, resistance 65 and con
denser 66, is associated with the source of supply
to the automatic volume control tube. By proper
selection of the values of these ?lter elements,
signals of high frequency may be favored and
greater reduction of the gain may thereby be
obtained whenever the higher frequencies are
leakage from the condenser 93 and indirectly
from condenser SI and various settings thereof 15 being received. For example, satisfactory results
result in curves such as shown at I06, I01, and
may be achieved if the impedance of the con
H18 in Figure 10. This resistor 74 is also eifec
denser 66 at 50 cycles is substantially equal to
tive to select the total voltage available for re
the impedance of winding 582 of transformer 592
turn to the main ampli?er tube control grids. If
which is comparable to the value of resistance 65.
resistance of resistor 14 is equal to resistance of
If the highest usable frequency is higher than 50
resistor 13, approximately 50% of total voltage
cycles it may be desirable for this relationship to
developed across M is fed back into lead 16 to
exist at this higher frequency.
reduce the gain of the ampli?er. However, if
Under these conditions the threshold value for
resistance of 14 is four times resistance of 13,
the automatic gain control is in effect substan
approximately % of total voltage of condenser 25 tially higher for signals in which low frequencies
9| will be fed back into lead 16 to vary the gain
predominate than for signals of comparable am
of ampli?er, hence there will be a large variation
plitude in which high frequencies predominate.
in gain for a slight excess in signal level.
Operation of the automatic gain control is par
Thus whenever high frequencies predominate,
die out over a period of several seconds and it is
predominantly low, and the peaks are fairly wide
ly separated, ampli?cation is not closely con
trolled by the automatic gain circuit, the gain is
higher for signals of comparable amplitude, and
and the signal peaks on separate cycles of the re
ticularly effective for seismic recording because 30 corded trace are therefore quite close, the sensi
the customary output from the detector com
tivity of the ampli?er is reduced on the occur
prises, ?rst, waves of large amplitude directly
rence of relatively small increase in signal am
received; second, waves of much less amplitude
plitude, so as to minimize confusion on the rec
re?ected from sub-strata. These second waves
ord. When, however, the signal frequencies are
highly desirable that their amplitudes be main
tained at usable size to the end of the chart.
The setting of the resistor determining the bias
on the A. V, C. tube should be such that each of
the clarity of the record is improved without any
the swings of the direct wave voltages causes the 40 apparent crowding of the successive peaks.
A. V. C. tube to function and therefore the con
It will be appreciated that the ?ltering action
denser 9| is maintained charged until the last of
just described may be obtained in some other
the direct waves ceases. The ?rst of the reflect
point in the circuit. For example, a suitable ?l
ed waves is normally too small to cause function
ter might be associated with the secondary wind
ing of the A. V. C. tube but the gain of the am 45 ing 62 of the output transformer with similar
pli?er does not immediately return to normal
since the charge on condenser 9| leaks off slowly
Having thus described the invention, what is
and hence holds the ampli?er grids at excess
claimed as new and desired to be secured by
bias. The time required for this charge to leak
off can be set by appropriate adjustment to ex
tend substantially for the time during which it
is desired to record the re?ected Waves, the gain
slowly increasing as the strength of the waves is
reduced because of the lowering voltage on 9|,
Letters Patent is:
1. In apparatus for use in seismic surveying,
the combination with a seismometer for convert
ing incoming seismic energy into electrical wave
form signals, and means for amplifying and re
cording such signals, said amplifying means in
producing a chart of uniform character. The 55 cluding at least one thermionic valve ampli?er,
total time over which the automatic volume con
of a thermionic gain control valve for applying
trol functions is controlled by resistor 14 as pre
to a control grid of said valve ampli?er a more
viously mentioned. The size of the middle por
negative bias in response to increase in amplitude
tion of the A. V. C. discharge curve is largely
of incoming energy above a predetermined am
controlled by resistor 73. The size thereof im
mediately after the automatic volume control
has functioned is controlled by resistor 12 while
resistor 18 controls the size which thefoutput
plitude, means initially biasing the grid of said
gain control valve beyond cut-off, and means ap
plying to the grid of said gain control valve a
voltage derived from the incoming energy such
signal must reach for the A. V. C. to function
as to render the grid potential less negative than
at all. Thus the operator can, by appropriate 65 said initial bias, said last named means includ
settings of the various variables obtain the type
ing a frequency selective device favoring the
of chart trace most suitable for the terrain being
higher frequencies within a usable band of fre
explored and for the use desired to be made
thereof. By making trial charts he can compen
2. In apparatus for use in seismic surveying,
sate for various unexpected factors and achieve 70 the combination with a seismometer for convert
superior results, obtaining a chart useful through
ing incoming seismic energy into electrical wave
out its full range.
form signals, and means for amplifying and re
As has been pointed out hereinbefore, it is
cording such signals, said amplifying means in
highly desirable that the gain control be selec
cluding at least one thermionic Valve ampli?er,
tively responsive to the arrival of waves of dif 75 Of a thermionic gain control valve recti?er for
applyingto a control grid of said valve ampli?er
a more negative bias in response to increase in
amplitude of incoming energy above a prede
termined amplitude, means initially biasing an
element of said gain control valve recti?er be
yond cut-01f, and means applying to an element
of said gain control valve a voltage derived from
the incoming energy such as to render the ele
ment potential less negative than said initial
bias, said last named means including a ?lter for 10
emphasizing energy of higher frequency within
the range of useful frequencies, whereby the volt
age of the gain control valve is varied to a greater
degree by incoming energy of predominantly
high frequencies than by incoming energy of pre
dominantly low frequencies of comparable ampli
tude of incoming energy above a predetermined
amplitude, means initially biasing the grid of said
gain control valve beyond cut-off, and means re~
sponsivein greater degree to high than to low
frequencies within the usable frequency range for
applying to the grid of said gain control valve a
voltage derived from the incoming energy such as
to render the grid potential less negative than
said initial bias.
4. In apparatus for use in seismic surveying,
the combination with a seismometer for convert
ing incoming seismic energy into electrical wave
form signals, and means for amplifying and re
cording such signals, of means operable in re
15 sponse to increase in signal energy above a pre
determined ampliutde for decreasing the gain of
said amplifying means, said last named means
including a device for initiating operation of said
3. In apparatus for use in seismic surveying,
means to decrease the gain, said device affording
the combination with a seismometer for convert
ing incoming seismic energy into electrical Wave 20 ?ltering action favoring the higher frequencies
within the usable range, whereby the gain at all
form signals, and means forv amplifying and re
frequencies is decreased in greater degree by si_g_
cording such signals, said amplifying means in
nals of large amplitude and predominantly high
cluding at least one thermionic valve ampli?er,
frequency than by signals of comparable ampli
of a thermionic gain control valve for applying
to a control grid of said valve ampli?er a more 25 tude and predominantly low frequency.
negative bias in response to increase in ampli
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