On-bottom seismometer electronic system

An ocean bottom seismometer unit records refraction waves from a seismic source. The refraction waves are digitzed and stored in a magnetic memory. Periodically, a tape recorder is started and digital samples are transferred from digital memory to tape during a time in which operation of the tape recorder will not generate spurious noise signals which interferes with the detection of the refraction waves. The water break is detected and the time of its occurrence is digitized. Only the digitized time of the water break occurrence is recorded thereby permitting shortened record length and increased record storage capacity.

BACKGROUND OF THE INVENTION 
This invention relates to an on-bottom seismometer system and, more 
particularly, to a system which will record seismic data and water break 
arrivals from repetitive seismic pulses. 
T. J. G. Francis et al, Ocean Bottom Seismograph Marine Geophysical 
Researches (1975), 195-213, describe three ways of retrieving seismic 
information from the ocean floor. A self-contained instrument may be 
attached by a rope to a surface ship or buoy; a transducer package may 
telemeter its data to a surface ship buoy or land station for recording; 
and a completely independent instrument of the "pop up" type may record 
data in its self-contained memory. The latter type instrument has been 
referred to as an on-bottom seismometer. These instruments have the 
advantage that they are easier to place on a particular bottom feature; 
the instrument is more secure from being tampered with than one which is 
connected to a surface buoy; and less system generated noise is associated 
with this type of instrument than one employing a mooring or a telemetry 
link. 
On-bottom seismometers have been extensively used to record explosion and 
earthquake data. Prothero, "A Free Fall Seismic Capsule for Seismicity and 
Refraction Work", Offshore Technology Conference Paper No. OTC2440, 
prepared for presentation at the 8th Annual Offshore Technology 
Conference, Houston, Texas, May 3-6, 1976; and, "An Operationally 
Optimized Ocean Bottom Seismometer Capsule", (Work sponsored by NOAA 
office of Sea Grant, under Grant 04-5-158-20), describe on-bottom 
seismometers which were developed at the University of California, Santa 
Barbara. In the former seismometer digital samples of seismic data are 
passed through a delaying shift register; in the latter seismometer they 
are stored in a microprocessor. When the amplitude of the signals is above 
the noise level in either seismometer, a tape recorder is started and the 
samples are transferred to magnetic tape. 
Attempts to use on-bottom seismometers for refraction surveys have 
generally used a continuously running tape recorder to record the data. 
Avedik et al, "Ocean Bottom Refraction Seismograph (OBRS)", Marine 
Geophysical Researches 3 (1978), 357-379, describe an ocean bottom 
seismometer used for refraction surveys. In this system the seismic data 
are encoded with pulse width modulation. A tape recorder is set to start 
and stop by a programmer synchronized with the ship-borne seismic 
programmer. As best shown in FIG. 8, the recorder is running during the 
time that the seismic signal is being detected. U.S. Pat. No. 4,138,658, 
Avedik et al, discloses and claims some features of this unit. 
Johnson et al, "A Direct Recording Ocean Bottom Seismometer", Marine 
Geophysical Researches 3 (1977), 65-85, describe another system using a 
continuously running tape recorder. 
One problem associated with such tape recorders is the noise generated by 
the tape drive motor. If the tape recorder is running during the time that 
the seismic signals are being detected, this noise is mixed with the 
signal. Another problem with continuously running tape recorders is that 
only a limited number of seismograms can be recorded with good resolution. 
If the tape is run at very low speeds, more seismograms can be recorded, 
but the resolution is poor. 
It is desirable to provide an ocean-bottom seismometer which can record a 
large number of refraction seismograms produced in response to repetitive 
shots of seismic energy. A system for producing repetitive pulses of 
seismic energy from air guns is shown, for example, in the Ritter U.S. 
Pat. No. 3,687,218. 
It is desirable to record these refraction seismograms in the format of 
conventional reflection seismograms with header information and the time 
of arrival of the water break. The water break is the arrival of the 
direct traveling wave at the seismometer. This wave travels from the 
source to the ocean bottom through the water. The water break comes in 
later as the boat with the seismic source steams away from the 
seismometer. Because of this, prior art techniques for recording 
refraction seismograms including th water break, have required an 
exceedingly long record length. This limits the number of seismograms 
which can be recorded on a given magnetic tape. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an ocean bottom seismometer 
records seismograms in a refraction survey in which repetitive pulses of 
seismic energy are produced. 
In carrying out the invention, the seismic signals are converted to digital 
samples which are stored in a digital memory. After the seismic energy has 
been detected, a tape recorder is started and the digital samples are 
transferred from memory to the tape. The system operates in a repetitive 
cycle which is synchronized with the generation of the seismic pulses. For 
each seismic pulse generated, the detected seismic energy is converted to 
a digital sample and the digital samples are transferred to tape after the 
detection of the seismic energy. This technique decreases the noise in the 
recorded signal. The tape recorder is turned off before the next seismic 
pulse and does not generate noise which will interfere with the recording 
of refraction waves from this pulse. 
In accordance with an important aspect of this invention, the time of 
arrival of the water break signal is digitized and recorded. This obviates 
the necessity of having a record length which is sufficiently long to 
record the arrival of the water break. By shortening the time length of 
the seismic records in this manner, the capacity of the tape recorder for 
recording seismograms is very much increased. 
The foregoing and other objects, features and advantages of the invention 
will be better understood from the following more detailed description and 
appended claims.

In accordance with the invention a determination is made of which signals 
meet the criteria of a valid water break. The times of all valid water 
break signals are digitized and recorded. Later consideration of all 
recorded water breaks will show the true water break pattern whereas some 
true water breaks may be missing if only one is recorded for each seismic 
pulse. 
DESCRIPTION OF THE SPECIFIC EMBODIMENT 
Referring to FIG. 1, an exploration vessel 11 tows a source of seismic 
pulses 12. Typically, this source and its control system may be of the 
type disclosed in the aforementioned Ritter patent wherein an array of 
airguns is used to periodically produce pulses of seismic energy. 
Typically, the exploration vessel 11 tows a streamer of hydrophones which 
detect seismic energy reflected from the subsurface. 
An on-bottom seismometer is depicted at 13. This on-bottom seismometer 
contains a geophone which detects refraction waves such as that indicated 
at 14. A refraction survey includes records of the detection of the 
refraction waves for each shot of seismic energy. After a number of such 
refraction records have been recorded, an acoustic command actuates a 
release so that on-bottom seismometer 13 returns to the suface. There it 
is retrieved and the tape recorded records are processed. 
In a refraction survey, it is desirable to record the arrival time of the 
water break signal produced by the seismic energy which travels directly 
through the water along the path 15. Because the seismic energy travels 
more slowly through the water, this signal may occur much later than the 
arrival of the refraction waves depending upon vessel location. 
FIG. 2 shows a block diagram of the on-bottom seismometer of this 
invention. A geophone 16 (or geophones) produces a signal representing the 
detection of the refraction waves. Conventional filter and gain changing 
circuits 17 and gain range circuitry 18 shape the seismic signal to be 
recorded. This seismic signal is sampled at a preselected rate (once every 
4 milliseconds in the example being discussed) and the samples are 
digitized in the analog to digital converter 19. These digital samples are 
stored in memory 20 as they are produced. After the refraction waves have 
been detected and digitized, tape recorder 21 is turned on and the digital 
samples from memory 20 are transferred to the tape recorder 21. Also 
transferred to the tape recorder 21 are digital samples representing 
header information. This includes the usual information recording time and 
location and it includes a digitized sample representing the time of the 
water break. 
The water break is detected by hydrophone 22. Water break detector 23 
separates the water break from other noise detected by hydrophone 22. 
Timing and control logic 24 and clock 25 generate the signals producing 
the header information, including water break time, and transfer the 
digital samples from memory 20 to tape recorder 21. A register 26 receives 
the digital samples from timing and control logic 24 and clock 25 and 
stores them until they can be inputted into the memory 20 at the proper 
storage location. 
Before the on-bottom seismometer is deployed, it is rendered operational in 
response to an output of the clock 25. At a predetermined time, clock 25 
acts through gate 28 and flip flop 29 to generate the signal OPNL. This 
controls the application of power to the seismometer unit. Alternatively, 
the unit can be rendered operational in response to an acoustic pulse from 
the exploration vessel. Such an acoustic pulse is detected by the acoustic 
transceiver 27 which acts through gate 28 to set the flip flop 29. All 
timing is carried out with respect to an initialization pulse which is 
generated aboard the vessel prior to deployment. After the on-bottom 
seismometer has been retrieved a debrief pulse is applied to timing and 
control logic 24. The times recorded by the internal clock circuitry can 
be compared to the actual time represented by the debrief pulse to 
determine how accurately the timing and control logic performed during 
deployment. 
The operation of the system can be summarized with reference to the wave 
forms of FIGS. 3 and 4. The ocean bottom seismometer is initialized on 
board ship before being deployed on the ocean bottom. As shown in FIG. 3A, 
an initialization pulse synchronizes the electronic circuitry of the ocean 
bottom seismometer with the times of firing of the seismic guns. After 
this initialization pulse, the ocean bottom system is in a STANDBY MODE 
during which it is deployed. At the START TIME the ocean bottom system is 
switched into a DATA ACQUISITION MODE, during which time it is in 
condition to detect and record refraction records. If the unit is not 
released by acoustic commands, at a predetermined time determined by a 
clock, the ocean bottom seismometer is automatically released to the 
suface where it is retrieved. 
FIGS. 4A-4F depict a typical shot interval where the interval between shots 
is 20 seconds and a 6 second refraction record is recorded. FIG. 4A 
represents the seconds in the shot interval. FIG. 4B shows the pulses 
designated T SHOT which are 20 seconds apart and which coincide with the 
time of the shot. After each shot, there is a 6 second record interval 
shown in FIG. 4C during which the signal from the geophone is sampled and 
digitized. FIG. 4D indicates the sampling interval. FIG. 4F shows the 
interval during which the water break detection is enabled. In the 
specific example under consideration, the system is capable of detecting a 
water break which occurs anywhere up to 18 seconds after the shot. (It is 
posible to detect beyond 18 seconds.) The record lengths are programable, 
in this example, only 6 seconds long. This results in a considerable 
increase in the capacity for records. If the record length were long 
enough to record water breaks, far fewer records could be recorded on the 
same tape. As depicted in FIG. 4E the power for the tape recorder is 
turned on after 18 seconds. During a 1.2 second interval the header 
information and digitized samples are transferred to 9 track magnetic 
tape. 
The system is shown in more detail in FIGS. 5-8. FIG. 5 shows the details 
of the water break detection circuit. Referring to FIG. 5, water 
disturbances are monitored by hydrophone 72 and examined for proper 
frequency and duration by the water break detection circuitry. If the 
disturbance satisfies the requirements for a water break, the time of the 
occurrence is written into memory 20. Preamp 73 and amplifier 74 provide 
the necessary gain. Filter 75 is a high pass filter. Limiter 76 produces 
an output of +5 or 0 volts. This limiter converts the water break into 
square waves. Pass filter 77 has a pass band centered near the waterbreak 
signal frequency. Comparator 78 compares the incoming signal to a 
threshold. If it is above the threshold it triggers a retriggerable 
monostable multivibrator 79 which has a duration of 1.1 milliseconds. One 
of the outputs of multivibrator 79 triggers the one shot multivibrator 80 
which has a period of 3 milliseconds. If the output of multivibrator 79 is 
still up at the end of the period of one shot multivibrator 80, the flip 
flop 81 is set. This signifies the presence of a water break signal WBE. 
Monostable multivibrators 79 and 80 and flip flop 81 form a circuit which 
detects the presence of at least three pulses, or excursions. These three 
excursions are the minimum number of events which are normally present in 
a water break signal. This discriminates against a spurious pulse of the 
right frequency content triggering the water break detector. If there are 
at least three excursions, the flip flop 81 is set, signifying a water 
break event. When a water break event occurs, the Q output of flip flop 81 
disables the monostable multivibrator 79. 
The operation of the circuit FIG. 5 can be better understood with reference 
to FIGS. 5A-5E. The output of comparator 78 is shown in FIG. 5A; the Q 
output of multivibrator 79 is shown in FIG. 5B; the Q output of 
multivibrator 80 is shown in FIG. 5C; the Q output of flip flop 81 is 
shown in FIG. 5D and the reset pulse is shown in FIG. 5E. The pulse 31 
from comparator 78 triggers multivibrator 79 and multivibrator 80. 
However, it does not generate a water break event signal (WBE) because 
there was not a repetition of the triggering during the 3 millisecond time 
that the multivibrator 80 was triggered. However, the pulses 32, 33 and 34 
do result in the production of a valid water break signal. The pulse 35 
does not retrigger the multivibrators 79 or 80 because multivibrator 79 is 
disabled during this time. The flip flop 81 is reset by a pulse from the 
timing and control circuit and is to be subsequently described. 
FIG. 6 shows the clock circuit. The basic clock 36 is a 1 MHz crystal. The 
output is applied to serially connected counters 37-42. These counters are 
started at the initialization. The pulse outputs of counters 38 and 39 
represent milliseconds, low and high order respectively. The pulse output 
of counter 40 represents seconds, the pulse output of counter 41 
represents minutes, and the output of counter 42 represents hours. 8-byte 
registers 43-47 receive the pulse outputs of counters 38-42 respectively. 
These 8-byte registers contain a count representing the time after 
initialization time. The registers 43-47 make up the register 26 shown in 
FIG. 2. A digital time sample is stored in registers 43-47 until it can be 
transferred to memory. 
The time latch control (TLC) signal updates the contents of all 8-byte 
registers 43-47. When a water break event (WBE) occurs, the contents of 
milliseconds and seconds registers, 43-45 are outputted to the write data 
bus. When a water break event occurs, controller 48 is enabled to produce 
the signals RSEC, RMSH, RMSL and RFF. These signals are shown in FIGS. 
6A-6D respectively. These signals cause the contents of registers 43-45 to 
be applied to the write data bus, (WDAT). 
The format in which digitized water break times are stored in memory 20 is 
shown in FIG. 7. Assume that two water breaks were detected, one occurring 
at 21.9327 seconds and one occurring at 23.4715 seconds. The digits 21 are 
written into memory location 1C, the digits 93 are written into memory 
location 1D; the digits 27 are written into memory location 1E. The 
letters FF are written into memory location 1F after the first water break 
write. This indicates detection of a valid water break. Upon detection of 
the next valid water break the letters FF in location 1F are written over 
by the digits 23. Digits 47 are written into memory location 20, digits 15 
are written into memory location 21 and the letters FF are written into 
memory location 22. The appearance of the letters FF indicates that there 
are no further valid water breaks. The portion of header memory depicted 
in FIG. 7 has memory locations for 12 water breaks. 
The first twenty-eight bytes of the header contain information related to 
the record such as date, time, crew number, line number, OBS unit number, 
and schedule information. The remainder of the 64 byte header contains a 
time record of events that appear to be water break events. The time of 
arrival of the water break is recorded in seconds to the nearest tenth of 
a millisecond. Three bytes of data are written for each water break event. 
A maximum of 12 water break type events can be recorded in the header. By 
recording the times of all apparently valid detected water breaks it is 
posible for the interpreter to later discern a pattern of true water 
breaks. Water break events occur in a consistent pattern and it is 
possible for the interpreter to determine this pattern if the times of all 
valid water breaks are recorded. The present invention provides the 
capability for recording the times of all valid water breaks. 
Referring again to FIG. 6, the registers 43-47 are used to temporarily 
store the time of the debrief pulse until this digital sample can be 
recorded. The debrief pulse acts through gate 82 to set the time of the 
debrief pulse into the registers 43-47 from which the contents are 
transferred to memory 20. 
In accordance with a feature of this invention, timing marks are recorded 
on each of the refraction records in a manner similar to the recording of 
timing marks on seismic signals. In the example under consideration, a 
timing mark is inserted into the data field every 100 milliseconds. 
The last water break event recorded in a particular record is followed by 
the characters FF. The remainder of the header field will not be cleared 
of previous header writes, and will be irrelevant. 
FIG. 8 shows the timing and control logic which generates the signals for 
digitizing the refraction records, transferring the digitized samples to 
memory, turning on the tape recorder, and transferring the digital samples 
from memory to tape. 8-bit register 49 is loaded during initialization 
with the record length, which in this case is six seconds. 8-bit register 
50 is loaded during initialization with the shot interval which is in this 
case 20 seconds. 8-bit register 51 is loaded during initialization with 
the sampling rate which in this case is 4 milliseconds. 
After each shot (every 20 seconds in this example), the down counter 52 is 
loaded with the contents of register 49. Counter 52 is counted down by the 
seconds output of the clock, which is applied through gate 53. Flip flop 
54 is set at the start of the shot interval and it is reset when the 
counter is counted down to zero. The Q output of flip flop 54 is the 
signal RECINVL which is up for the length of the record interval, in this 
case 6 seconds. The signal RECINVL enables the analog to digital converter 
19 (FIG. 2). 
The down counter 55 is initially loaded from the register 50 by the 
initialization pulse, IEXT, which is shown in FIG. 3A. Thereafter, the 
contents of register 50 are transferred to counter 55 at the start of 
every 20 second interval. Counter 55 is counted down by the seconds output 
of the clock. Every 20 seconds the counter 55 is counted to zero. The 
C.sub.o output triggers the one shot multivibrator 56 which produces the T 
SHOT signal shown in FIG. 4B. This one shot in turn triggers one shot 
multivibrator 57 which produces the signal TRST which loads counter 52 and 
sets flip flop 54 as previously described. 
The signal TRST also enables memory 20 (FIG. 2) to receive an update of the 
header information. 
Counter 58 is loaded with the contents of register 51 every 4 milliseconds. 
Counter 58 is counted down by the millisecond output of the clock. The 
output of counter 58 is applied through gate 59 to the one shot 
multivibrator 60 which produces the signal TS. This signal is ANDED with 
the signal RECINVL in the gate 61. This sets the flip flop 62 which 
produces the signal ADCY (shown in FIG. 4D). 
The down counter 63 is loaded with the shot interval from register 50 at 
the same time that the counter 55 is loaded. The tape recorder in this 
example is started two seconds before the end of the shot interval so as 
to completely record the data stored in the 8K byte memory 20 before the 
next shot. Therefore, two clock pulses are added through gate 64 to 
advance counter 63 by two counts. Because of this, the signal ENREC is 
produced 2 seconds before the end of the shot interval. This triggers the 
one shot multivibrator 64A which produces the signal TRSP. This starts the 
tape recorder as will be described with reference to FIG. 9. 
In accordance with an important feature of the present invention, the 
registers 49, 50 and 51 are loaded with programmable representations of 
shot time interval, sampling interval and record length respectively. In 
this manner, any selected shot time interval, sampling interval or record 
length can be selected. 
The tape recorder 21 (FIG. 9) may be a conventional high density recorder. 
One example which is suitable for use is the Genisco ECR 10 which can 
record the data in the 8K byte memory 20 in less than 2 seconds. 
The signal TRSP from FIG. 8 sets the flip flop 65 in FIG. 9 to generate an 
enable tape write signal, ETAPWT. This acts through gate 66, flip flop 67, 
and switch 68, to turn on the power. 
The output of memory 20 (FIG. 2) is on the 8-bit bus 69. A parity bit is 
generated by parity generator 70 as is conventional. The 8 bits plus 
parity are recorded each time the WRITE STROBE input is enabled. This 
input is enabled by the signal TAPWSTR which is generated every 83 
microseconds after the memory to tape transfer is initiated. 
The address in memory 20 (FIG. 2) of the last byte written is set into the 
memory byte counter 71. Every time the signal TAPWSTR occurs, this counter 
is counted down. Counter 71 keeps track of the number of bytes transferred 
into the tape recorder. When an entire record has been transferred to the 
tape recorder it is turned off by circuitry which responds to a 0 count, 
MEMZRO, from counter 71. 
Data is recorded on tape in two bytes per sample, formatted as in FIG. 10. 
The mantissa is in 2's complement format where bit 11 is the sign and bit 
0 is the least significant bit. The exponent is an unsigned magnitude 
where bit X.sub.0 is the least significant bit. The maximum length of a 
data record is 8192 bytes, the capacity of the 8K byte memory 20. 
FIG. 11 shows the power supply for the on-bottom seismometer unit. It 
includes an internal battery supply 84 and a power supply regulator 
circuit 85. The unit operates from the internal battery supply 84 when the 
unit is deployed. During warm-up and initiation, the power buses are 
automatically switched to the exploration vessel. The power supply 
regulator circuit 85 selects the highest voltage available. Therefore, 
when the external supplies are removed, the unit buses are switched to the 
battery supply 84. The power supply regulator circuitry also responds to 
the operational command OPNL to render all circuits in the unit 
operational. 
Eight parallel strings of cells make up the low voltage battery, the 
strings 86, 87, and 88 being shown. Each string has a diode and a resistor 
in series with it to prevent failure of the entire battery should a single 
cell short. 
While a particular embodiment of the invention has been shown and 
described, various modifications are within the true spirit and scope of 
the invention. The appended claims are, therefore, intended to cover all 
such modifications.