Transmission line seismic communications system

A seismic communications system including a central control and recording station and a plurality of remote data acquisition stations, the stations being approximately in line and connected to each other by one or two transmission lines. One line is all that is necessary if the central station is at one end of the line of stations; however, if the central station has some acquisition stations upstream and some downstream therefrom, two lines are required. Coded central strobe bursts put on a transmission line at the central station successively cause insertion of digital seismic data from successively more remote acquisition stations, the data being time slotted after each central station strobe burst so that reverse transmissions do not interfere with subsequent signal propagations emanating from the central station. Twin lead is preferably used for the transmission lines. Power shut down, except for a pilot receiver, is accomplished at each acquisition station between recording cycles.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to the collection and transmission of seismic data 
and more particularly to a unified data acquisition system wherein the 
seismic data conventionally collected via an array of seismometers is 
recorded at a central station facility. 
2. Description of the Prior Art 
In seismic surveying it is common practice to generate seismic waves by the 
detonation of explosives located either on or above the surface of the 
earth or in holes drilled in the earth. Alternatively, seismic waves may 
be imparted via mechanical vibrators. Upon detonation of the charge or the 
imparting of a mechanical disturbance, the waves generated thereby travel 
into the earth and are reflected back to the surface from layers thereof 
which may be interfaces between earth's strata. The reflections are 
detected by seismometers, or geophones, which convert the detected seismic 
waves into electrical signals suitable, when amplified, for recording. 
Each reflection adds waves to the normally uniform trace of the electrical 
output from the seismometer. It is from the visual inspection of traces 
made from these waves that geophysicists are able to obtain desired 
prospecting data. 
It is conventional to develop simultaneous traces as received from multiple 
geophones spaced in a line at regular intervals. For example, 48 geophones 
may be spaced apart by about 500-foot intervals in a line along the 
terrain. A disturbance from a source also in the line with the geophones 
is then generated and the signals received from each of the geophones 
turned into 48 correlated traces. Additional data is obtained if the "shot 
points" (source locations of induced seismic disturbances) and the receipt 
of signals at the geophone stations are operationally advanced 
sequentially along the line. This is possible by providing for additional 
geophones and wiring for sequential electrical detonation or triggering of 
multiple sources or shots, a shot point being located in the near vicinity 
of each of the geophone stations. Sequential operation of the shots and 
detection stations provides seismic signals in so-called "roll along" 
fashion. 
It is also common somewhere along the line of geophones to experience a 
terrain condition where it is difficult or impossible to position a 
geophone, even though regular interval spacing dictates placement. Hence, 
to obtain 48 traces (in a 48 trace system), it is necessary to skip the 
inaccessible location. This practice is referred to as "gapping". 
Although the spacing intervals are normally on the order of about 500 feet, 
spacing intervals which are closer together or further apart than this are 
not unusual. 
In operation, it is desirable to correlate the geophone responses with a 
particular source shock or vibration impulse. That is, a shock which is 
initiated is sent out from a particular location and is received, although 
not simultaneously, as a reflection by each of the individual geophones. 
These received signals are treated by delay circuits and in other regards 
well known in the art to achieve correlation. Since source occurrences are 
happening in periodic time sequence and from sequential locations, as 
above described, the received signals at the geophones may be mistakenly 
correlated unless there are long delays or unless there is suitable data 
handling to perform the required correlations. One way of assuring 
suitable handling without initiating induced delays is by centrally 
recording the signals from the variously positioned geophones, as opposed 
to separately recording the individual signals for later matching. Various 
means have been used or attempted for this purpose including using radio 
transmission, multiple wires (one from each geophone or an array of 
geophones to a central facility), one or more co-axial shielded cables 
(each cable having a sufficient bandwidth that together they are able to 
accommodate the frequency multiplexed signals from the geophones, as well 
as the control signals for triggering the sources and the like). All such 
transmission media to date have been fraught with one or more 
shortcomings. 
Radio transmission requires expensive transmitters at each geophone, 
requires the use of frequency spectrum allocations for this purpose (in an 
already crowded spectrum), and is susceptible to variations in terrain and 
weather conditions that may have an effect on variation in quality of 
transmission from the various geophones. The quality effect may actually 
result in misinterpretation of the seismic data. 
Stringing and moving multiple telephone wires to each geophone is 
cumbersome and time consuming. Moreover, keeping track of multiple wiring 
connections is inherently exacting and the possibility of making a mistake 
is large, even with highly qualified field crews. Hence, not only is 
having to cope with a large number of separate wires a nuisance, it is 
both a material and labor burden, introducing large possibility for error. 
Use of one or more shielded cables and multiplexing equipment has proven 
operationally satisfactory. However, shielded cables having wide bandwidth 
characteristics and suitable for carrying a large number of multiplex 
signals are heavy and are expensive. 
Therefore, it is a feature of this invention to provide an improved means 
for transmitting data from a plurality of seismometers or geophone 
stations to a central location over a relatively inexpensive single 
transmission medium. 
It is another feature of this invention to provide an improved seismic data 
transmission means for carrying signals both to and from a central station 
and a plurality of seismometers or geophones in a time sharing manner, 
thereby minimizing bandwidth requirements of the medium. 
It is still another feature of this invention to provide an improved 
seismic data transmission means operating in time-sharing fashion, the 
return signals from the seismometers or geophone stations each being timed 
by the same strobe signal sent out from a central control and recording 
station, the strobe signals synchronizing clocks at the seismometers or 
geophone stations to positively assure an absence of interference between 
signal insertions. 
SUMMARY OF THE INVENTION 
A preferred embodiment of the present invention comprises a transmission 
line of television-type, twin-lead strung between a control and data 
recording station and sequentially positioned remote data acquisition 
units (RDAU), each such unit being connected to a plurality of individual 
geophones or seismometers. The control station may be located somewhere in 
the middle of the string of RDAU stations; therefore, there may be an 
upstream and a downstream transmission line. Typically, there are 18 RDAU 
stations with three separate geophone channels (single geophone or an 
array) connected into each such station. This provides a capacity of 16 
active RDAU stations, one RDAU station for providing "roll along" and one 
RDAU station for "gapping". 
The coded strobe pulse sent from the control station may be coordinated 
with a particular signal for triggering a particular source so that the 
strobe pulse is identified with a particular "shot". This coded stroke 
pulse from the control station as it passes each successive RDAU station 
synchronizes the crystal clock located in the RDAU station and gates on in 
sequential time slot order a first set of simple memory units to record 
the data being received at the three geophone channel locations. This 
timing of the station memory units synchronizes the geophone responses 
with the particular shot. At the next strobe pulse, the first set of 
memory units are again gated on to insert the previously recorded data 
from this set of memory units onto the transmission line. At the same time 
a second set of memory units are gated on to record new data. 
For convenience and simplicity, the signals from the three channels at the 
RDAU stations may be time multiplexed and digitized before recording. 
The data insertions, being sequential, do not interfere with the strobe or 
with each other. Further, the timing is all with respect to the same 
strobe as it passes along the transmission lines one station to the next. 
Since there is new synchronizing with each pulse, extreme accuracy in the 
timing components is not required. 
It has been discovered that header information in bi-phase-mark code and 
data insertion in Miller code allows the compression of the most data into 
a unit of time without information interference or ambiguity. Typically, a 
strobe may be sent out every two thousand microseconds and still allow 
sufficient transmission time for header and ending pulses, data pulses 
from 18 RDAU stations of three channels each (or equivalent), necessary 
guard spaces between data pulses with a little spare time left over. 
The twin-lead may be used rather than shielded cable as the transmission 
medium since the bandwidth requirements are not extreme. This is not only 
an economic and handling advantage over the heavier and bulkier shielded 
cable, but even if large lengths of twin-lead are left on the ground after 
use, the expense is minimal. 
BRIEF DESCRIPTION OF THE DRAWINGS 
So that the manner in which the above-recited features, advantages and 
objects of the invention, as well as others which will become apparent, 
are attained and can be understood in detail, more particular description 
of the invention briefly summarized above may be had by reference to the 
embodiments thereof which are illustrated in the appended drawings, which 
drawings form a part of this specification. It is to be noted, however, 
that the appended drawings illustrate only typical embodiments of the 
invention and are therefore not to be considered limiting of its scope for 
the invention may admit to other equally effective embodiments.

DESCRIPTION OF PREFERRED EMBODIMENT 
Now referring to the drawings and first to FIG. 1 an example of a typical 
seismic surveying system is illustrated. Geophones 1-54 are spaced along 
the terrain 56 at regular intervals, insofar as possible. Each geophone 
may be an equivalent seismometer or an array, but will be referred to 
normally hereinafter as merely a geophone. 
The string of geophones are also approximately in line with one another, 
again insofar as terrain 56 will permit. 
In performing seismic exploration a number of shot points are located in 
close proximity with at least a number of the geophone stations, perhaps 
each station. One such shot point location 58 is shown adjacent geophone 
38. 
With respect to the surface of the earth, the geophones may be placed on 
the surface of the terrain, mounted above the terrain or placed in a hole 
drilled beneath the surface. Likewise, the shot points may be similarly 
positioned with respect to the earth surface in accordance with the 
preference of the seismic surveying crew. 
Each successive three geophones are connected to a remote data acquisition 
unit (RDAU) station located in the field. For example, RDAU-1 station 60 
is connected to geophones 1, 2 and 3, RDAU-2 station 61 is connected to 
geophones 4, 5 and 6, and, in like manner, RDAU-3 station 62 through 
RDAU-18 station 77 are respectively connected to three successive 
geophones. RDAU-18 station 77 is connected to geophones 52, 53 and 54. 
Central record and control station 80 is connected to each of the RDAU 
stations 60-77 by either one or two transmission lines. If the central 
station is located near one end of the line (i.e., near RDAU-1 station 60 
or RDAU-18 station 77), then only one common transmission line is 
necessary. In FIG. 1 central station 80 is located near RDAU-8 station 67; 
therefore, there are effectively two transmission lines connected to 
central station 80. Upstream transmission line 82 is common to RDAU-1 
station 60 through RDAU-8 station 67 and downstream transmission line 84 
connects central station 80 to each RDAU-10 station 69 through RDAU-18 
station 77. A more complete description of these transmission lines is set 
out below. 
Even though geophones 1-54 are spaced uniformly approximately at equal 
distances along the line, there is no requirement to connect each of the 
RDAU stations to transmission lines 82 and 84 at uniform spacings. 
Therefore, connections are made to these lines whereever it is physically 
convenient. Now referring to FIG. 2, a simplified time-distance diagram 
for the system is shown. In this diagram, it is assumed that control 
station 80 is located at RDAU-9 station. For convenience in this diagram, 
the station positions are identified by their RDAU numbers. In the 
diagram, RDAU-9 station is deactivated for recording seismic data. 
The FIG. 2 timing diagram corresponds with the block diagram shown in FIG. 
1 and therefore stations 1-8 are located upstream of control station 80 
and RDAU stations 0-18 are located downstream of control station 80. At 
each RDAU station there is a remote data acquisition unit comprising a 
receiver, a crystal-controlled timer or clock, a transmitter and data 
acquisition and storage means. The data acquired and stored at each RDAU 
station is sent back to control station 80 along the transmission line to 
which it is connected following a scan or strobe signal set out from the 
control station along the transmission line. A single strobe from the 
control station interrogates and actuates each of the RDAU stations in 
turn. These RDAU stations may be considered to be randomly disposed along 
their respective transmission lines. As above described, the actual 
positioning is dependent on physical conveniences not related to data 
acquisition. 
At control station 80 there are two receivers, one for receiving the 
signals from the upstream stations and one for receiving the signals from 
the downstream stations. In the diagram, the heavy bars underneath the 
signal time slots indicate when data bursts are inserted on the 
transmission lines. 
Assuming that initial set-up and addressing have been established to 
initiate interrogation or operation of the RDAU stations, a scan or strobe 
burst S is inserted at control station 80 on both the upstream and 
downstream transmission lines 82 and 84. The strobe burst propagates along 
a transmission line, coming first on downstream line 84 to RDAU station 
10. At station 10, two basic functions are initiated by receipt of the 
strobe burst: the RDAU internal clock is synchronized (this is preferably 
a crystal-control clock capable of control within +0.1 microseconds) and 
the RDAU time slot counter at the station is started so as to insert at 
the proper time the RDAU data burst from station 10. Since station 10 is 
the station nearest control station 80 on the downstream side, the time 
slot allocated for RDAU data burst from station 10 is the one adjacent to 
strobe burst S. 
In a similar manner, the clock internal to RDAU station 11 assures that the 
RDAU data burst at that station is inserted on transmission line 84 in the 
time slot second closest to strobe burst S (i.e., next to the data burst 
from station 10). At each successive RDAU station 12-18, internal timing 
operation inserts the data burst from that station at the next successive 
time slot so that the data burst at station 18 is placed as shown in FIG. 
2, namely, after the strobe and the data bursts from stations 10 through 
17. 
In each case, the internal clock at the station which is interrogated by 
the receipt of the strobe burst is synchronized and the timing for 
insertion of the station data burst is initiated. Timing errors are not 
cumulative, since the timing for each station is independent of the timing 
at the other stations, only being dependent on the time of receipt of the 
strobe burst at that station. 
In similar fashion to what has been just described, data bursts are 
inserted at the upstream RDAU stations on transmission line 82, the RDAU 
station 8 data burst occupying the station time slot nearest to the strobe 
burst, the RDAU station 7 data burst occupying the next station time slot, 
and so forth for RDAU stations 6 through 1. 
The return propagation of the data bursts from the RDAU station does not 
interfere with the strobe burst from control station 80 or with the 
insertion of data bursts so long as the period of elapsed time for RDAU 
data burst 18 to be initiated and propagated to control station 80 is 
longer than the time interval between the strobe bursts initiated from 
control station 80. That is, the period between strobe bursts must be 
longer than the sum of: the strobe burst, twice the propagation time from 
the control station to the RDAU station furtherest therefrom, and the 
total time of the time slots for each of the RDAU data bursts in the 
direction along transmission line having the greatest number of RDAU 
stations. Of course, if the control station is located at RDAU stations 1 
or 18, the maximum time is defined for the system. 
It should be observed that because there are two receivers at the control 
station, there is no conflict in receiving data bursts from RDAU stations 
upstream and RDAU stations downstream at overlapping times (e.g., 
simultaneously receiving data bursts 3 and 14 in the diagram). 
Now referring to FIG. 3, a partial, but expanded time-distance diagram is 
shown. In this instance, RDAU stations 10-13 are shown, all on the 
downstream side of control station 80. In FIG. 3, time slots are 
illustrated which are sufficiently long to accommodate not only a data 
burst from each channel geophone (three to a station), but also a guard 
time for each channel data burst. Data bursts may be of slightly varying 
duration for reasons hereafter explained. However, in each case the time 
slot is the same. Also, a slightly longer time slot as represented by the 
guard time provides for positive recognition of one data burst from 
another as they are received at the control station, as well as providing 
additional means for maintaining separation between the respective station 
data bursts as they are inserted on the transmission line. 
All of the RDAU stations are illustrated for insertion of data bursts from 
three geophone channels. Each RDAU station has its three geophone data 
bursts applied in time consecutive order. To accomplish this, internal 
clocks at the respective RDAU stations are preset so that the data burst 
from its first channel is inserted in a first time slot, the data burst 
from its second channel is inserted in the second time slot for that 
station and the data burst from its third channel is inserted in the third 
time slot for that station. In effect, the RDAU station is allocated three 
consecutive time slots, one of which is filled by the data burst from each 
geophone channel. 
Finally, it may be observed that the full electronics at each RDAU station 
is not needed all of the time. Therefore, for each recording cycle of 
data, a pilot signal may be sent out to actuate the stations. At other 
times, the stations may remain dormant except for the operation of a pilot 
receiver or detector. Such pilot receiver and an accompanying switch has 
much less power consumption than a full data receiver. After the 
transmission has been achieved, the transmitter portion also may be turned 
off until required by operation to be again turned on. 
The turning off of the electronics when not needed has an additional 
benefit to merely saving power consumption. Various noise signals on the 
transmission line are prevented from inadvertently initiating data burst 
inserts from the RDAU station out of turn. The station must first be 
turned on by an appropriate pilot signal and then address programmed by an 
initial strobe burst before the station is operational from its dormant 
state. Once activated the station remains so until there is an inordinate 
gap between strobe bursts. This means the cycle is over and the station 
should again be dormant. 
One active system in accordance with the invention is illustrated in FIGS. 
1 and 2. However, in accordance with the need for using the "roll along" 
method of advancing the active sensor stations while advancing the shot 
point sequence, a number of inactive RDAU stations may be included in the 
line of such units, thus allowing considerable physical advance in 
prospecting using the line without requiring excessive repositioning of 
the RDAU stations. As already noted, 18 active stations in the system 
previously described includes provisions for allowing a gap in the 
sequence of active sensor stations to accommodate terrain or other 
restrictions and to allow disenabling of one station for a shot point, 
referred to as allowing for "roll along". One workable configuration 
including inactive RDAU stations includes 26 RDAU stations (18 active, 8 
inactive) each having three geophone channels or seismometer sensors. 
Another workable configuration includes 20 RDAU stations (14 active, 6 
inactive) each RDAU station having four geophone channels. When such a 
configuration is used 12 of the active RDAU stations are used for the 
required channel recordings, one is used for "roll along" purposes and 
one allows for gapping. 
As also previously noted, the length of the line needs to be approximately 
five miles long to be consistent with normal usage of sensor-stations 
spacing. Increase in this length due to terrain of 6 percent produces a 
5.3 mile cable length. A 5.3 mile round trip for signals on the 
propagation medium which is disclosed hereinafter takes approximately 74 
microseconds. 
It has been discovered that a bit cell rate of the serial data transmitted 
from the RDAU stations in a block may conveniently be 640 kHz, although 
bit rates slightly less than or in excess thereof are equally desirable. A 
bit cell is the time required to send one bit of data or, in other words, 
the time between two, voltage transitions, one up and one down. A 640 kHz 
bit rate has been determined to be well within the bounds imposed by the 
deterioration of rise time over a 5.3 mile length of transmission line, as 
that line is hereafter described. 
It has been further discovered that the use of Miller coding produces 
transition spacings at 1, 11/2 and 2 bit cell intervals, with 1.5625 
microseconds per bit cell. 
A discussion of Miller coding is found in Electromechanical Design, dated 
March 1971, beginning on page 6 Digital Design. In the Miller Code, a 
transition in the middle of a bit cell represents a one and a transition 
at the end of or in phase with the first zero's bit cell represents a zero 
followed by a zero. As further explained in the above referenced article, 
the Miller Code also requires a "101" sequence to clear the decoder. In 
order to isolate each successive block with a detectable gap and thus 
facilitate reassembly of the received data, a detected transition interval 
of at least 21/2 bit cells is required as an inter-block gap. Also 1/2 bit 
cell before the first full transition of a block, there is an offset step 
from zero line voltage that guarantees the next transition (first bit cell 
center) is detected by the receiver. The ending step that returns the line 
voltage to zero occurs 1/2 bit cell after the final bit cell of the block. 
Thus the start, synchronization, end, and gap functions take up six bit 
cells of time per active RDAU station transmission. 
Data samples of each channel of an RDAU station typically consist of a 
4-bit gain code and a mantissa of 14 or 15 bits, depending on a possible 
line-length/information-capacity tradeoff. At least one odd-parity check 
bit may be used with each block of data samples. The digitizing for each 
individual seismic signal to obtain their individual information and 
handling bits is well known in the art. 
Data block format for each RDAU station may thus be expressed for the 
three-channel-per-RDAU-station system, as follows: 3 bit cells for header 
information for synchronizing purposes, 57 bit cells for data (i.e., 
3.times.(4+15)), 1 bit cell for block parity and 3 bit cells for end step 
and gap. This amounts to 64 bit cells per block. As each bit cell is 
1.5625 microseconds long, this constitutes 100 microseconds per block. 
Multiplying 18 active blocks per scan (between storbe burst) this totals 
1800 microseconds per scan. 
For a four-channel-per-RDAU-station system, 3 bit cells may be used for 
initial synchronizing, 76 data bit cells (4.times.(4+15)) may be used for 
data, 1 bit cell may be used for block parity and 3 bit cells may be used 
for end step and gap. Multiplying the total of 83 bit cells per block by 
1.5625 microseconds per bit cell develops a total of 129.69 microseconds 
per block. Since there are 14 active channels per scan, this amounts to 
1815.6 microseconds per scan. 
As previously described, the central station propagates along the 
transmission line a scan strobe which is received by each RDAU station and 
used to start the internal operating sequence with each RDAU station. Each 
RDAU station responds with a new block of data being inserted in its own 
assigned time slot within the 2 millisecond separation between strobe 
bursts. The strobe burst may consist of 7.5 bits coded into bi-phase-mark 
for simplicity for decoding circuits in the RDAU station electronics. Each 
bit of this code takes 3.125 microseconds, so that the transition spacing 
is either one or two 1.5625 microsecond intervals. The detected code is 
thus a 7-bit pseudo-random bit pattern and the total time that is taken is 
23.5 microseconds. The time taken by an RDAU station assigned to the first 
time slot from the scan or strobe burst detection until the start of its 
data block transmission is the acquisition delay and in a preferred 
embodiment consists of the following: 15 data bits for multiplex switching 
and gain determination, 5 bit cells for sample settling prior to holding, 
15 bit cells for digitizing bits while holding, 1 bit cell for encoder 
delay, 2 bit cells for register transfer delays and 0.5 bit cell for 
internal logic propagation. This amounts to 38.5 bit cells or, at 1.5625 
microseconds per bit cell, 60.2 microseconds of acquisition delay. 
The strobe burst or scan may be typically on the order of 23.5 
microseconds, as hereinafter discussed. A summary of the total scan timing 
for the three-channels-per-RDAU-station system is as follows: 23.5 
microseconds for strobe bursts, 60.2 microseconds for acquisition delay, 
1800 microseconds for 18 data blocks with gaps, 74 microseconds for round 
trip line propagation over 5.3 miles, and 2 microseconds for decoder 
delay. This amounts to 1959.7 microseconds or 40.3 microseconds of spare 
time in a 2 millisecond scan interval. 
Similarly for a four-channel-per-RDAU-station system, the following summary 
of scan timing applies: 23.5 microseconds for the strobe burst, 60.2 
microseconds for acquisition delay, 1815.6 microseconds for 14 data blocks 
with gaps, 74 microseconds for round trip propagation over 5.3 miles of 
transmission line and 2 microseconds for decoder delay. This totals 1975.3 
microseconds or allows 24.7 microseconds of spare time in a 2 millisecond 
scan interval. 
If an RDAU station does not receive a strobe within a 3 millisecond scan 
interval, then it shuts off or becomes dormant, having negligible power 
drain. A return to "standby" condition with the receiver and controller 
portions of the electronics active occurs when a low-power pilot receiver 
or detector circuit senses the presence of a pilot frequency that may be 
present on the transmission line before the start of the next recording 
cycle. Detecting the pilot keeps the RDAU stations in "stand-by" until 
call up is over. 
The first strobe of a recording cycle is treated differently from the other 
strobes by the RDAU stations. This strobe allows inputs to the previously 
cleared address registers within the RDAU stations to establish the 
addresses and associated time slot numbers, which together constitute the 
callup list. the time slot number is retained with the address and is used 
to determine the delay within each scan that is a multiple of the data 
block with gap time as required for orderly time multiplexing of the data 
onto the transmission line, as above described. 
The first strobe burst as it advances along the transmission line 
accumulates a data block with gap for each RDAU station. Each RDAU station 
may have predetermined the order of its own channel data, so that data 
block with gap that is inserted for each RDAU station is for the entire 
station. The address at each subsequent station is determined by how many 
data blocks with gap (time slots) are used up by the time the first strobe 
burst and the accompanying signals arrive at that station. 
Now referring to FIG. 4, a block diagram of a typical RDAU station is 
shown. Individual channel geophones or seismic sensors 102, 104 and 106 
are applied to input check unit 108 and also respectively to appropriate 
preamplifiers and filters 110, 112 and 114. Input check unit 108 is 
connected to header formatter unit 116 and both units 108 and 116 are 
connected to receive control commands from format programmer 118, for a 
purpose to be hereafter explained. 
The channel preamplifiers and filters are connected to time multiplexer 
120, whose output is connected to automatic gain range amplifier 122, 
which, in turn, is connected to both digitizer 124 and data formatter 126. 
Units 120, 122, 124, and 126 are all connected to receive control commands 
from format programmer 118. The seismic signals pass from the time 
multiplexer and undergo treatment in the gain range amplifier, digitizer 
and data formatter in a manner well known in the art. Typically, gain 
information will be in the first four bit cells and the mantissa will be 
in the next 15 bit cells. 
Input check unit 108 and header formatter unit 116, upon control command, 
monitor the input from the three channel geophones and determine an 
overall gain code in appropriate format to be associated with the data. 
Switch 128 receives the header code and the data blocks and alternately 
allows the data to be stored in memory A 130 or memory B 132. Output 
switch 134 connected to both memory units permits alternate call up of the 
information stored in the memory units. Note that units 128, 130, 132 and 
134 are all connected to receive control commands from format programmer 
118. 
The output from switch 134 is applied to Miller encoder 136 and the encoder 
is connected to transmitter 138, the output of which is connected to 
transmission line 140. 
The illustrated RDAU station is activated by the presence of an enabling 
signal present on the line which is detected by low power pilot receiver 
142, which activates latch 144 for turning on data receiver 146. At the 
end of the transmission cycle, latch 144 is operated by a control command 
to turn off the data receiver. Latch 144 also causes power to be applied 
to bi-phase-mark decoder 148 and command decoder 150. 
It may be recalled that the strobe burst is coded in bi-phase-mark code and 
therefore bi-phase-mark decoder 148 allows data receiver 146 to be 
sensitive to such a strobe burst for triggering, via the command decoder, 
cycle controller 152. Cycle controller 152 also receives an input from 
crystal clock 154. Upon command from decoder 150, it synchronizes the 
operation of the station by selecting the next appropriate cyclical output 
from clock 154 and, in accordance with the time delay address of the 
station, produces via format programmer 118 the control commands at the 
station. 
Cycle controller 152 is programmed with the proper address as described 
above by receipt of the strobe burst and the accompanying time slotted 
data from the previous stations. Thereafter, each time a strobe burst is 
sensed the timing automatically operates to put the data from the station 
onto the line during the proper time slot, as above discussed. 
Now referring to FIG. 5, a block diagram of the central or control and 
recording station is illustrated. In this diagram, it is assumed that the 
control station is connected to downstream line 210 and to upstream line 
212. Hence, there are two transmitters, 214 and 216, respectively, and two 
receivers, 218 and 220, respectively. The receivers are connected to 
processor 222, which, in turn, is connected alternatively to memory A 224 
and to memory B 226. 
Memory units A and B are connected to multiplex formatter 228, which, in 
turn, is connected to multiple track tape recorder 230. Both the multiplex 
formatter and the tape recorder are controlled by tape controller 232. 
So that the information recorded on the tape recorder may be played back, 
tape recorder 230 is connected to play back unit 234 comprising a 
digital-to-analog converter and demultiplexing unit. Play back unit 234 is 
then connected to camera 236. So that information data can be taken 
directly from the multiplex formatter, rather than from the tape recorder, 
a direct connection is made from multiplex formatter 228 to play back unit 
234. 
The functional heart of the central or control station is arithmetic 
calculator 238, which functions as an electronic bookkeeper for the 
station operator. Calculator 238 is connected to command and control unit 
240, by which the operator identifies the RDAU station by number at which 
the control station is located, the number of the RDAU station closest to 
the station of the upstream line and whether the RDAU numbers are in 
ascending or descending sequence (determined by whether the line is moving 
north or south, for example). Similarly, the operator may identify the 
number of the RDAU station closest to the station on the downstream line 
and whether the RDAU numbers are in ascending or descending sequence. 
The arithmetic calculator determines which RDAU stations are called up and 
assigns the proper time slot numbers to each upstream and downstream group 
via formatter 242, which is connected to each transmitters 214 and 216. 
Note also that the arithmetic claculator is connected to memory address 
unit 244, which in turn is connected to memory units 224 and 226. 
When rolling one RDAU position, arithmetic claculator 238 automatically 
advances the number assignment and time slot assignment on command by the 
operator, or redetermines the RDAU station positioning by the operator 
reidentifying his beginning RDAU station and upstream closest RDAU 
station. The system positioning and timing assignments are displayed on 
system display unit 246 connected to calculator 238 so that the operator 
has a continual visual description of the deployment of his system. 
Arithmetic calculator 238 also furnishes information about the system 
positioning and timing the memory address unit 244, which uses this 
information to place each returning data burst in the proper place in a 
random access memory, either memory A or memory B, thus performing the 
"remapping" function. While the data is being stored in memory A, the 
previous memory B is read out of the memory, remultiplexed from a 54 to a 
48 channel system and recorded on tape. Note that command and control unit 
240 is connected to auxiliary channels unit 248. This connection 
determines which of the six active channels are not used at a particular 
time. Note also that auxiliary channels unit 248 is connected to system 
display 246 to provide a complete visual presentation of the system, 
including stations which are found included in the multiplex recording at 
any one time. 
Blaster control unit 250 is connected to command and control unit 240 so 
that impulses may be sent by radio 252 to trigger the source shots. 
Alternatively, the blaster control signals may be sent via a transmitter 
along suitable transmission lines, the blaster control signal being time 
shared coordinated with the strobe signals sent from the control station 
in the manner previously described. Perhaps different lines are used to 
trigger the shots. If not, the blaster control signals would be coded so 
as to actuate the shots, but so as not to actuate the RDAU stations until 
the proper delay and the subsequent transmission of the strobe burst is 
transmitted for that purpose. 
Transmissions lines used in seismic signal transmission in the prior art 
have been shielded co-axial cables capable of carrying a wide frequency 
spectrum and shielded against interference from outside disturbances. Use 
of frequency multiplexing, even of digital signals, requires wide band 
capability. 
However, such cables are expensive, heavy and for long-range transmission 
have fairly high attenuation, on the order of 15 db per mile. Such high 
attenuation requires the use of booster amplifiers or repeaters when used 
in the transmission line on the order of the lengths referred to above. 
It has been discovered that compared to a 50-ohm co-axial cable, a twin 
lead, such as used for television antenna systems, has superior 
characteristics. These twin leads normally carry two parallel copper 
conductors, each about 0.07-0.08 inches in diameter, separated by about 
0.4-0.5 inches. The cover for the conductors is typically insulated 
polyethylene. Many ruggedized twin leads are on the market capable of 
withstanding the pulling, whipping and twisting that a twin lead would be 
subjected to in use as transmission lines 82 and 84. 
A twin lead exhibits about a 0.9 db loss for one mile, a 1.8 db loss for 
two miles, a 4.7 db loss for five miles, and a 9.4 db loss for ten miles. 
Reflections from a terminated end varies from -30 db to -32 db, depending 
on the value of the termination. Optimum terminating resistance for the 
line has been discovered to be 285 ohms, although any value between 
275-300 ohms is totally acceptable. 
When the line is reeled out, it rests on rocks, sand, dirt, bushes and even 
hangs from tree limbs. The effect of varying materials adjacent to the 
line produces a noise on the line caused by minute reflections of these 
points where the material changes (from rocks to trees, for example). The 
noise varies from -32 to -42 db. In addition, two lines running parallel 
to each other for a distance of five miles and separated by 25 feet will 
produce cross talk between the two lines of -31 db. Such a parallel line 
may be a nearby power transmission line located in the area. In either 
event, the characteristics of the twin lead is suitable so that the 
systematic noise levels are still far below those which are a result of 
attenuation of the line. For example, the receiver detection level may be 
set at -18 db and the transmitted signal applied to the line at 25 volts 
peak-to-peak. As stated above the signal level at five miles will only be 
down 4.7 db, well detectable by the receiver and capable of filtering or 
other discrimination from the systematic noises. 
The central or control station operator can test the integrity of the 
transmission line with an ohm meter. The resistance of the line is 60 ohms 
per loop mile and hence with foreknowledge of the length of his line, he 
can determine whether or not the line has any breaks. Line breaks are 
readily located visually or may be located by making resistance 
measurements along the line. Field splicing of a line break of the twin 
lead is quite simple and requires only about 30 seconds. 
While particular embodiments of the invention have been shown, it will be 
understood that the invention is not limited thereto since many 
modifications may be made and will become apparent to those skilled in the 
art.