Method and apparatus for increasing the dynamic range of a data acquisition system at low frequencies without reducing high frequency gain

A geophysical data acquisition system is disclosed having a dynamic range larger than that achievable with typical existing systems. The range of input signals that can be accommodated by a system are limited on the lower end by the noise level of the system and at the upper end by saturation levels of the system. High amplitude input signals can be accommodated by the system if certain unneeded frequency components are filtered off at a sufficiently early point in the signal flow path. In the present invention this desirable filtering is accomplished while maintaining a high gain level in the signal flow path ahead of those components which introduce high noise levels. As a result, the system has an expanded dynamic range and the needed capability to faithfully acquire low amplitude signals.

This invention relates generally to geophysical data acquisition systems, 
and more particularly to a geophysical field acquisition system having an 
expanded dynamic range. 
Recording systems for providing a permanent record, typically on magnetic 
tape, of geophysical data acquired in the field are well known. For the 
purpose of this disclosure the term "geophysical data" shall include both 
data acquired in a land prospecting environment where the sensors may be 
seismometers, geophones, or the like, as well as data acquired in a marine 
prospecting environment where the sensors may be hydrophones or the like. 
While such systems typically digitize the acquired data prior to 
recording, the invention disclosed and claimed wherein is not so limited. 
A typical digital field recording system is that disclosed in U.S. Pat. 
No. 3,819,864. 
An important parameter of such field acquisition systems is their dynamic 
range. Dynamic range is defined as the difference between the largest 
input signal that can be processed and recorded, i.e., the signal having 
an amplitude just below that which would cause saturation in some element 
of the system, and the smallest signal which can be properly recorded by 
the system, as limited by noise levels in the system. Dynamic range is 
commonly expressed as the ratio of the largest to the smallest signal and 
is typically expressed in decibels or dB. While recording systems such as 
that illustrated in U.S. Pat. No. 3,819,864 typically have large dynamic 
range, sometimes exceeding 100 dB, even these large dynamic ranges have 
proved inadequate in the demanding environment of present day oil and gas 
exploration. 
To some extent this inadequacy stems from the typical spectral 
characteristics of the data to be recorded. Partially as a result of the 
fact that the attenuation of elastic waves in the crustal material of the 
earth increases more or less logarithmically with frequency, the spectrum 
of the data to be recorded is typically dominated by low frequency energy. 
It therefore becomes necessary to filter off the low frequencies in order 
to be able to detect the high frequencies which are essential to high 
resolution analysis. It is also common to include high cut anti-alias 
filters and notch filters in such systems. 
Typically, each channel of data coming into prior art field recording 
systems is amplified in a preamplifier stage. The preamplifiers are 
followed by a filter bank comprising the low cut, high cut anti-alias, and 
notch filters. After the filters, further amplification is provided to the 
signals prior to digitization and recording on magnetic tape. 
In such prior art systems the preamplifiers, the filter banks, and the 
following amplifier stages all add noise to the signals. If the gain of 
the preamplifier is sufficiently high, even the smallest signal to be 
recorded by the system will have sufficient amplitude when it reaches the 
filters and following amplifier stages to be well above the noise level 
added by those components. In such case the noise, which determines the 
smallest signal that can be faithfully recorded by the system, is that 
added by the preamplifier itself. In such a system the high amplitude low 
frequency signals are filtered off after they have passed through the 
preamplifier. In some cases these low frequency signals may have 
sufficient amplitude to saturate the preamplifier. 
If the preamplifier gain is reduced so as to avoid saturation by low 
frequency signals, the noise added by the filters and the following 
amplifier stages becomes more important relative to the noise added by the 
preamplifier. As a result, reduction of the preamplifier gain results in 
increase in the size of the smallest signal that can be faithfully 
recorded. Thus, while reducing the preamplifier gain tends to increase the 
dynamic range because larger input signals can be accepted, the increase 
is not linear since the bottom end of the dynamic range is increasing to a 
certain extent. If the preamplifier gain is decreased sufficiently, the 
noise added by the filters and following amplifier stages becomes the 
dominant noise contributor and no further increase in dynamic range is 
achieved. Such systems are incapable of providing the required dynamic 
range while preserving the ability of faithfully record sufficiently small 
input signals. 
Other measures for resolving the problem have been attempted. In some cases 
large arrays of detectors have been interconnected so as to provide a 
signal having diminished low frequency energy. Such large arrays, however, 
are costly to acquire and to deploy. Further, for shallow reflecting beds, 
large arrays tend to reject even the desired signals. 
It is also possible to configure the sensors themselves so that they tend 
to reject low frequency energy. However, the spectral sensitivity of such 
sensors is fixed at the time of manufacture and cannot then be changed. 
It is therefore an object of the invention to provide a geophysical data 
acquisition system having a wide dynamic range. 
It is a further object of the invention to provide a wide dynamic range 
system while preserving the ability to record low amplitude signals. 
It is another object of the invention to provide a system wherein the 
dynamic range and/or the equivalent input noise of the system may be 
switchably selected. 
Briefly, these and other objects of the invention are provided by a system 
wherein the low cut filter is preceded by a pre-filter amplifier stage and 
succeeded by a post-filter amplifier stage. The post-filter amplifier 
stage is succeeded by other filters in the system and by further gain 
stages. Proper selection of the pre-filter and post-filter gain results in 
the desired dynamic range while preserving satisfactory equivalent input 
noise levels. In one embodiment of the invention the pre-filter and 
post-filter gains are selectable so as to provide selectable tradeoffs 
between the system dynamic range and equivalent input noise level.

FIG. 1 is a block diagram of a typical prior art geophysical data 
acquisition system. The input to such a system is normally provided by a 
cable comprised of a plurality of twisted pair signal conductors. In the 
case of land seismic prospecting, each twisted pair is connected at some 
remote location to a geophone or geophone group. In the case of marine 
prospecting, the twisted pairs are enclosed in a streamer which is towed 
behind the prospecting boat and are connected at remote locations to 
hydrophones or hydrophone groups. In either case a plurality of channels 
of data are available to be recorded by the data collection system. Each 
twisted pair provides a differential signal which is not referenced to 
ground. 
With reference to FIG. 1, one such signal is coupled to a channel of the 
data collection system by lines 1 and 2. The signal enters differential 
amplifier 3 which in turn provides a differential output on lines 5 and 6 
to differential amplifier 7. Amplifier 7 functions to provide a high 
common mode rejection and to convert the differential signal to a single 
ended signal on line 8. Amplifiers 3 and 7 comprise a preamplifier. 
Preferably amplifier 3 should have a low equivalent input noise and a high 
gain. The high gain will amplify the received signals to a sufficiently 
high level that the noise contributed by the following devices is 
insigificant relative to the noise contribution of amplifier 3. Amplifier 
7 typically has a gain near unity. 
Other means for coupling the cable signals to the data collection system 
are known including those disclosed in U.S. Pat. Nos. 3,778,759 and 
3,972,020. 
The preamplifier output is coupled by line 8 to low cut filter 9. The 
function of low cut filter 9 is to attenuate low frequency signals. The 
use of an active filter as contrasted with a passive filter provides 
substantial savings in size, weight and cost. Typically, one or more 
three-pole active filters such as that illustrated in FIG. 2 is/are used. 
The input signal is coupled by the series combination of capacitors 24, 26 
and 28 to the non-inverting input of amplifier 30. The junction between 
capacitors 24 and 26 is returned to ground through resistor 32 while the 
non-inverting input of amplifier 30 is returned to ground through resistor 
36. One hundred percent negative feedback is effected by coupling the 
output of amplifier 30 through line 38 to the inverting input of the 
amplifier. In addition, the amplifier output is coupled by line 38 and 
resistor 34 to the junction between capacitors 26 and 28. Such active 
filters are well known and in a typical embodiment will have component 
values as follows: 
Capacitor 24: 2 microfarads 
Capacitor 26: 1 microfared 
Capacitor 28: 1 microfared 
Resistor 32: 8.236K 
Resistor 34: 7.56K 
Resistor 36: 63.22K 
With the values given above, the three-pole active filter has a cut off 
frequency of eight hertz and a slope of 18 dB per octave at frequencies 
below the cutoff. 
Returning to FIG. 1, line 10 couples the output of the low cut filter to a 
high cut filter 11. The high cut filter attenuates high frequencies and 
typically has a slope of 72 dB per octave. Since the analog signals are to 
be sampled and digitized at a later point in the system, the steep slope 
of high cut filter 11 is used to assure that frequencies above one-half 
the sampling frequency are attenuated by at least 70 dB. 
The output of high cut filter 11 is coupled by line 12 to notch filter 13. 
The reject band of notch filter 13 is centered at the power line frequency 
so as to attentuate any pickup from nearby power lines. High cut filters 
and notch filters are well known in the art and require no further 
discussion here. In some systems it has been known for the notch filter to 
preceed the high cut filter or for the high cut filter to precede the low 
cut filter. 
The output of the notch filter is coupled by line 14 to the input of a low 
drift DC amplifier 15. Typically, the amplifier has a gain of unity and a 
low output impedance. The low drift requirement stems from the fact that 
the remainder of the data acquisition system is DC coupled. The output of 
amplfier 15 is coupled by line 16 to multiplexer switch 17. Multiplexer 
switch 17 is typically a semiconductor switch which is closed for a very 
brief period so as to provide a sample of the data from the illustrated 
channel via line 18 to the input of floating point amplifier 19. 
That portion of the data collection system described to this point, that is 
the channel between the input signal and line 18 is duplicated for each 
channel of input data to be recorded by the system. The various 
multiplexer switches such as switch 17 are successively closed for brief 
periods so as to provide on line 18 a continuous succession of analog 
samples from the various data channels. 
These various samples, after amplification by amplifier 19, are coupled by 
line 21 to analog-to-digital converter 20. The digitized output signals 
are coupled by line 23 to a recorder 22 which in the preferred embodiment 
comprises a magnetic tape recorder. The geophysical data recording system 
thus provides on magnetic tape digitized samples of the multichannel 
analog data received from the cable. 
Most high quality analog-to-digital converters in current use have a 
dynamic range of 80 to 90 dB. This is not sufficient to accomodate the 
dynamic range of typical geophysical input signals which may exceed 120 
dB. Floating point amplifier 19 has the capability to automatically adjust 
its gain for each new data sample as the sample is received on line 18. 
The gain is adjusted so as to insure that the amplified signal appearing 
on line 21 will fall within the acceptable range of the analog-to-digital 
converter. The actual gain setting of amplifier 19 is provided to recorder 
22 on line 25 for recordation along with the corresponding digitized 
sample appearing on line 23. One example of a suitable floating point 
amplifier is that disclosed in U.S. Pat. No. 3,684,968. Specific 
embodiments of various components illustrated in FIG. 1 appear in a 
digital field system supplied by Texas Instruments Incorporated of Dallas, 
Texas under the trademark DFS V. 
In the discussion to follow the noise amplitudes introduced at various 
points in the system will be referred back to the system input and 
expressed as equivalent input noise. To do this, the amplitude of the 
noise at the point where the noise is introduced is divided by the total 
gain of all the amplifiers preceding that point in the system. The 
resultant equivalent input noise is that noise amplitude that would have 
to appear at the input of a noise free system to result in the noise level 
that is in fact generated at the point in question in the actual noisy 
system. Noise amplitudes will be expressed in terms of their RMS value. 
Also, it will be assumed that the nature of the noise is such that the RMS 
value of the total noise from a plurality of noise sources can be 
expressed as the square root of the sum of the squares of the noise from 
each of the sources. 
In a typical prior art system amplifier 3 has a gain of 320 and an 
equivalent input noise of 0.103 microvolts RMS. As a result of this gain 
level, the noise introduced by devices in the system is divided by 320 to 
obtain their equivalent input noise. Amplifier 7 typically has a noise 
level of 0.64 microvolts. It will be assumed that two 18 dB per octave low 
cut filters are used each having a typical noise level of 0.88 microvolts. 
The high cut filter, notch filter, low drift amplifier, multiplexer and 
floating point amplifier typically have a noise level of about 11 
microvolts. The equivalent input noise then resulting from all sources is 
given by Equation 1. 
##EQU1## 
It will be seen that because of the high gain of amplifier 3, the dominant 
noise contributor in the system illustrated is amplifier 3 itself. 
The maximum peak output signal that can be handled by amplifier 7 without 
saturation is largely dictated by the available power supply voltage and 
typically has a value of about 9.3 volts. Converting the 9.3 volts peak to 
a corresponding RMS value and dividing by the gain of amplifier 3 yields 
20.48 mv RMS as the maximum input signal. Therefore, the dynamic range of 
the illustrated system is given by equation 2. 
##EQU2## 
A dynamic range of 105 dB sometimes proves to be inadequate since input 
signals larger than 20.48 mv are received resulting in saturation of the 
system. To allow for such large signals the gain of amplifier 3 may be 
made manually adjustable. Typically, the gain can be adjusted in 4 to 1 
gain steps. For the various gain levels available in such a system. Table 
1 gives the corresponding equivalent noise levels and dynamic ranges. 
TABLE I 
______________________________________ 
Amplifier 3 Equivalent Dynamic 
Gain Input Noise Range 
______________________________________ 
320 .109 .mu. v 105.5 dB 
80 .173 .mu. v 113.5 dB 
20 .564 .mu. v 115.3 dB 
5 2.22 .mu. v 115.4 dB 
______________________________________ 
As the gain of amplifier 3 is decreased there is a corresponding increase 
in the amplitude of the input signal that can be handled by the system 
without going into saturation. Thus, reductions in the gain of amplifier 3 
tend to result in increases in the dynamic range of the system. The 
increase is not linear, however. The reason for this is that decreases in 
the gain of amplifier 3 result in corresponding increases in the 
equivalent input noise levels for noise sources appearing after amplifier 
3 in the system. This results in increases in the total equivalent input 
noise of the system and the amplitude of the smallest signal that can be 
faithfully recorded by the system increases. Ultimately noise sources 
appearing after amplifier 3 in the system become the dominant noise 
sources and further reductions in the gain of amplifier 3 result in no 
perceptable increase in dynamic range. This is illustrated graphically in 
FIG. 3 where curve 40 shows the value of dynamic range corresponding to 
various values of amplifier 3 gain. For convenience of illustration the 
abscissa of FIG. 3 is the inverse of the amplifier 3 gain. It is seen that 
as the gain is reduced more and more the dynamic range asymptotically 
approaches the limiting value of 115.45 dB. In many cases this limiting 
value of dynamic range and the associated high total equivalent noise 
levels are inadequate. 
In the geophysical data acquisition system of the present invention, the 
input portion of each data channel has the configuration illustrated in 
FIG. 4. The preamplifier comprising amplifiers 3 and 7, the low cut filter 
9, and high cut filter 11 may each be similar to the corresponding 
components appearing in the prior art structure of FIG. 1. In particular, 
amplifier 3 preferably has a gain which may be changed by an operator. In 
the structure of the present invention the output of low cut filter 9 is 
not coupled directly to the input of high cut filter 11. Instead amplifier 
42 provides additional gain to the signal coming from low cut filter 9. 
The actual gain of amplifier 42 is selected through the expedient of 
positioning manual or electronic selectable switch 44, thereby 
establishing the amount of negative feedback utilized around the 
amplifier. The resistance R of resistor 52 in the preferred embodiment is 
one thousand ohms. If resistors 50, 48 and 46 have resistances of 3R, 12R 
and 48R respectively, it will be recognized that for the four switch 
positions illustrated in FIG. 4, the gain of amplifier 42 will be 1, 4, 
16, or 64. In the practice of the invention, the gain settings of 
amplifiers 3 and 42 may be independently selectable. Alternatively, the 
switching may be coordinated so that the product of the gains of 
amplifiers 3 and 42 will always have a constant value, i.e., 320. In such 
a case when the gain of amplifier 3 is 320 the gain of amplifier 42 will 
be 1. Other combinations of the two gains are 80/4, 20/16 and 5/64. In 
explaining the advantages of the present invention, the coordinated method 
of gain switching will be assumed. 
In the prior art structure of FIG. 1, with the gain of amplifier 3 set to a 
high level, i.e., 320, large signals received at the input would tend to 
saturate amplifiers 3 or 7. It may be expected that it would be the final 
stages of the amplifiers which would saturate. 
In the structure in the present invention as illustrated in FIG. 4, the 
preamplifier is split into two sections. Amplifiers 3 and 7 will be 
referred to as the pre-filter gain while amplifier 42 will be referred to 
as the post-filter gain. Then when the pre-filter gain is set to something 
less than 320, i.e., 20, the post-filter gain would be 16. In such a case 
it might be expected that saturation, if it were to occur, would occur in 
amplifier 42. However, prior to reaching amplifier 42 the large low 
frequency components are removed by low cut filter 9. The remaining high 
frequency components in amplifier 42 are normally not of sufficient 
magnitude to saturate the amplifier. As a result, saturation is avoided 
even in the presence of large low frequency components while maintaining 
the total preamplifier gain at a high level of 320. It is important to 
maintain the total preamplifier gain at a high level so that the 
equivalent input noise contributed by the high cut filter and succeeding 
components is negligable in comparison with the equivalent input noise 
contributed by amplifier 3. It will be recalled from the discussion in 
connection with equation 1 that this noise at the point in the system 
where it is generated has an RMS level of about 11 microvolts. When the 11 
microvolts is divided by the preceding gain of 320 the equivalent input 
noise corresponding thereto is small in comparison with 0.103 microvolts. 
In the configuration of FIG. 4 the only equivalent input noise component 
that varies as a function of the preamplifier gain setting is that 
component contributed by the low cut filter. It will be recalled that each 
low cut filter stage has an RMS noise value of about 0.88 microvolts which 
is quite small in comparison with the 11 microvolt value for the high cut 
filter and succeeding system components. The low cut filter noise does not 
become an appreciable component of the total equivalent input noise until 
the gain of amplifier 3 has been reduced to an extremely low level. 
Table II summarizes the total equivalent input noise and the dynamic range 
of the system for various combinations of prefilter and postfilter gain. 
Comparison of the data of Table II with that of Table I shows that by 
means of the present invention the dynamic range is substantially 
increased over that achievable with prior art systems while the total 
equivalent input noise is maintained at an acceptable level. It should be 
noted that the dynamic range figures of Table II are applicable only for 
low frequency signals, that is signals which are substantially filtered by 
the low cut filter. This is not an objectionable limitation since signals 
at frequencies not removed by the low cut filter are rarely if ever of an 
amplitude that would saturate the system. 
TABLE II 
______________________________________ 
Pre-Filter 
Post-Filter 
Equivalent 
Gain Gain Input Noise Dynamic Range 
______________________________________ 
320 1 .109 .mu. v 105.5 dB 
80 4 .110 .mu. v 117.4 dB 
20 16 .129 .mu. v 128.1 dB 
5 64 .301 .mu. v 132.8 dB 
______________________________________ 
It will be seen that typical prior art structures include a relatively low 
noise pre-amplifier. These structures also include a filter section which 
may generate noise of a level comparable to or slightly greater than that 
of the pre-amplifier section. Finally, the prior art structures include a 
high noise data processing section which generates noise levels 
significantly greater than either those of the preamplifier section or the 
filter section. In the cascade combination of the present invention the 
preamplifier section is split into a pre-filter amplifier and a 
post-filter amplifier. The pre-filter gain is chosen to be sufficiently 
high so that the equivalent input noise contribution of the filter section 
is smaller than that of the pre-filter amplifier itself. However, the gain 
of the pre-filter amplifier is chosen to be sufficiently low so that the 
low frequency components which are ultimately eliminated by the low cut 
filter do not saturate the pre-filter amplifier itself. The post-filter 
gain is selected so that, when taken in combination with the pre-filter 
gain, the overall gain preceding the high noise data processing section is 
sufficiently high to ensure that the noise contribution of the high noise 
data processing section when referenced to the input of the post-filter 
amplifier, is less than that of the filter section. 
In this description of the invention and in the claims, the term "low noise 
amplifier" is intended to mean an amplifier having a noise level equal to 
or less than that of the low cut filter. Similarly, the term high noise 
data processing means is intended to include that portion of the system 
which generates noise substantially greater than that generated by the low 
cut filter. 
While the advantages of the invention have been disclosed under the 
assumption that changes in prefilter gain are coordinated with changes in 
postfilter gain so as to maintain a constant overall preamplifier gain of 
320, the invention is not so limited. In fact, in the preferred embodiment 
of the invention the pre-filter gain is fixed while the post-filter gain 
is variable. Also by way of example the pre-filter gain can be set to 20 
and the post-filter gain set to 4. In this case with the total 
preamplifier gain at a value of 80, the total equivalent input noise will 
be found to have an RMS value of 0.186 microvolts. This is just slightly 
larger than the corresponding 0.173 microvolt level in the prior art 
structure with the preamplifier gain set to 80. On the other hand the 
dynamic range available in the new structure will be found to be 124.9 dB 
as contrasted with 113.5 dB for the prior art structure. 
Also, while the invention has been disclosed with only the low cut filter 
located prior to the post-filter gain, it may sometimes be desirable to 
also include part of the high cut filter ahead of the post-filter gain. 
Such would be the case, for example, if high levels of high frequency 
energy are present in the input signal. It is sometimes possible to move 
part of the high cut filter further forward in the amplifier chain without 
substantially degrading the total equivalent input noise. 
While there have been described what are presently considered to be the 
preferred embodiments of this invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention, and it is therefore, 
intended to cover all such changes and modifications as fall within the 
true spirit and scope of the invention.