Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
INSTRUMENTATION FOR SEISMIC EXPLORATION FORGROUND WATER IN HAWAII
by
Leonard A. Palmer
Technical Report No .6
April 1967
Project Completion Report
for
FEASIB ILI TY OF SEISMIC EXPLORATION FOR GROU NDWATER
OWRR Project No. A-009-HI Grant Agreement No. 14-01-0001-781
Principal In vest igators : Leonard A. Palmer , Doak C. Cox, WilliamM. Adams
Project Period : July 1, 1965 to June 30, 1966
The programs and activities described herein were supported i n part by fundsprovi ded by the Unite d Stat e s Department of t he Interior a s authorized undert he Wat e r Resour ces Ac t of 1964, Publ ic Law 88- 379.
ABSTRACT
This study developed i nstrumentation and t echniques t o ·be use d
for seismic exploration of groundwat er in Hawaii. A t hree-stage field
test of instruments and methods was conducted to determine their
capability of recording and reproducing seismic data. Reproduci
bility was the main criterion to permit usi ng certain signal analysis
techniques.
Multichannel explosive tests utilizing dynami t e shots were car
ried out in waimanalo, Oahu during the first stage of t esting.
The second stage was the use of a two-channel magnetic t ape r e
corder converted to receive voice and up-hole geophone si gnals on one
channel and seismic signals on the other channel.
St age three tested the use of a "thumper" acoustica l source~
seismic filters~ and timers deve l oped f or the pro j ect .
The signals generated by explosives and recorded photographically
were very reproducible~ but this is a re lat ively expe nsive met hod and
analysis is slow.
Sei smi c data recorded on magnetic tape allows versati lity i n
analysis of recorded signals reproduced ei t her in wiggle or i nt ensity
contrast analog form. Sections can be selected fo r digi t al computer
analysis.
The findings from this pi lot phase of seismi c exp Lorat.ion for
groundwater does not indicate that seismology would not be an appropriate
tool for studying geological structure . Success in f ur t her work iai l. ]: be
implemented by this preliminary work i n solving i ns trument ation problems
and the number of personnel of the Water Resource s Research Center and
the Hawaii Institute of Geophysics who have been trained.
iii
CONTENTS
INTRODUCTION .................................................•..... 1
OBJ ECTI VES 2
PROCEDURES 3Field Operations 5Stage 1: Multichannel-Explosive Seismic Field Test, Waimanalo,
Hawaii 5
Stage 2: Magnetic Tape Seismic Data Recording Development 8Stage 3: Thumper Seismic Signal Source l?
TRAINING AND EDUCATION 23
SUMMARY AND CONCLUSIONS ..........•................................ 24
ACKNOWLEDGEMENTS 25
BIBLIOGRAPHY 26
LIST OF FIGURESFigure
1
2
3
4
5
6
?
8
9
10
"012
Location Map 6A Multi-Channel Galvanometer Type Seismic Recorder 9Seismic Recording at the University of Hawaii ExperimentStation in Waimanalo 10
Manual Digitization of the Seismic Records 11Utilization of Acoustical Energy to Test the MagneticTape Recording of Seismic Signals 12
Block Diagram of Seismic Signal Recording Apparatus 13Multiple Trace Time Analog Oscilloscope and Oscilloscope
Camera 14Schematic Diagram of Simple Seismic-signal Filter 15Design of Tripping Hook for Weight Dropping 16Seismic Control Timer 19Schematic Diagram of Thumper and Recording Equipment 20"Thumper" Weightdropping Equipment 21
v
2
·OBJECTIVES
The applicability of seismic techniques to the problem of detail
ing the structure of Hawaiian basaltic lavas at and below the water table
and the advancement of the state-of-the.art in seismology were the
principal concerns of this study . Multiple impulses and signal-averaging
techniques are being used to develop exploration capabilities at depths
of several thousand feet in other areas of the world. The utilization
of such techniques in the exploration for groundwater in the Islands was
evaluated. Extrapolation of porosity and permeability data from existing
wells by seismic techniques is contemplated.
3
PROCEDURES
The original procedures included (i ) construction of a weight
dropper or vibrator, CiiJ recording of repeated drops on the Enhancatron
or equivalent, (iii) application of the techniques over known structures
having high velocity contrast such as tunnels and (i v) application of
the techniques over aquifers having known porosity contrast.
The initial procedure emphasized processing seismic signals in
electrical form on an Enhancatron. The Enhancatron is a single-purpose
digital computer. It performs time-averaging by breaking the time axis
into 1,024 segments and striking an average within each segment. Al
though it works very well, its inflexibil ity was soon apparent . Analo g
magnetic tape data recording was selected, ut ili zing an appropriately
modified Ampex tape deck for recording seismic signals. (The modifica
tion is described in Stage 2, Magnetic Tape Seismic Data Recording
Development .) The data recorded on the magnetic tape can be processed
either by analog or digital methods. Analog processing is done with a
data presentation system developed in cooperation with several groups of
the Hawaii Institute of Geophysics at the University of Hawaii.
For experimenting with various analysis techniques , the analog in
formation is digitized and processed by a digital computer. This project
is contributing the format interface between the analog-to-digital con
verter and the digital tape recording units. The interface permits
analog tape seismic data obtained in the field on the Ampex magnetic tape
recorder to be inserted into the data-reduction system and obtain output
on magnetic tape suitable for direct input in the IBM 7040, or IBM 360
computer.
4
To permit development of the digital computer programs for pro
cessing the seismic data, i. e., summation, cross-correlation" spectral
analyses, etc., some of the paper seismograms have been hand-digitized.
This process is extremely tedious and results only in marginal repro
ducible data. Although it does permit test data analysis for computer
program development, hand-digitizing will not be extensively used.
There are numerous reasons for the switch from the on-line Enhanca
tron processor doing only straight summation. Conversion to analysis by
digital computer has added unlimited flexibility of processing. For
instance, seismic signals can be 'evaluated in each acoustical band pass
spectrum. Seismic data are recorded in a form suitable for direct input
to an Enhancatron processor, if one is available, and if such signal sum
mation is desired.
The analog playback equipment will permit a quick look at the data
to indicate zones which should have more intense study. The corresponding
data from these zones can be processed on the digital computer after an
analog-to-digital conversion and the results plotted back on a digital
plotter for interpretation.
The decision to change from the Enhancatron processor does not divert
from a basic policy of using analog equipment for processing when the op
timum technique has been selected. In the current situation, the pro
cessing procedure is still in a state of flux. Once the study is able to
fix upon the optimum processing method, then purchase of the necessary
analog hardware would be appropriate.
The approach of this effort is verified, to a great extent, by that
used by the U. S. Geological Survey for the National Aeronautics Space
Administration (Watkins, et aZ.~ 1966).
5
Field Operations
Field operations progressed through a series of stages during
which changes and improvements in instrumentation wer e required. Pre -
liminary tests were carried out with conventional multichannel s ei smi c
equipment available at the Hawaii Institute of Geophysics. In later
stages, the need for less expensive and more versatile methods led to
the development of magnetic tape data recording from a thumper source.
Description of the various field operations are gr ouped into stages ac-
cording to the type of equipment and procedure used.
STAGE 1: MULTICHANNEL-EXPLOSIVE SEISMIC FIELD TEST,WAIMANALO, HAWAII
I ntroduct i on. Preliminary seismic field investigations were carri ed
out at Waimanalo, Oahu. This site was chosen not only for its geol ogi c
setting, but because the seismic work would contribute new information
concerning the underground structure of the area. Convent ional s eismic
equipment available at the Hawaii Institute of Geophys i cs of the Univer -
sity of Hawaii was utilized. Reflection and refraction profiles were
made in order to obtain informat ion to be used in the signal-averaging
method.
Location. The location of the town of Waimanalo and the site of the
field test is shown in Figure 1. The first seismic array was set up on
Waikupanaha Street between Kumuhau and Kakaina Streets , the second, on
the University of Hawaii Exper iment Station property.
Repetitive reflection shooting was done at the center of the array
with 3 geophones on each side. From 10 to 30 explosions were used wi t h
the same shot point and geophone to obtain repeat data for signal averag-
....9 1900 20PO 3~00 40100
5~Oqmile
FEET
Figure 1. LOCA TION MAP
,,I
I,,,, ,<, / Univ. of Hawa ii..... •"-1., Exp.erimental Slo.w ....,.
J. •, ..SEIsMIC • ***
/ FIELD TEST NO.2,,,,.• •
LEGENDGeophone Spread
Shot Points
N
o-
7
ing. The geophone array in both instances was set up with one geophone
every 100 feet. The six geophones extended over a distance of 500 feet.
A refraction seismic traverse was made with the same geophone array. Shot
points were located every 200 feet extending beyond one end of the geophone
array. "Dupont" Boosters and "Nitromon" explosives provided the energy
source for the refraction shooting.
Equipment. Equipment used included: seismic truck with a multi-channel
galvanometer-type seismic recorder with signal monitoring on photographic
paper, developing equipment, two-way radios, and one operator for record
ing and developing seismic records; a blasting truck carried the explosives
and a shot box with all accessory explosive materials. Radio communica
tion between the shockpoint and the seismic truck was used for timing
(Figures 2 and 3). Analyses of the data from these seismic shots re
quired manual conversion of the photographic seismic records to digital
data on IBM cards. Photographic data were recorded at 4 inches per second
with timing marks each 1/100 of a second and a darker time line marked
every tenth line. By use of the IBM OSCAR, the photographic records were
read each 1/200 of a second for 1.5 seconds after the shot instant (three
inches of record on the photographic paper). Digital data on IBM cards
are punched by the OSCAR which are processed by computer averaging to pro
duce a composite signal trace on which signal noise ratio is considerably
improved (Figure 4).
The results of the Waimanalo seismic operation were utilized by the
Division of Water and Land Development of the Department of Land and
Natural Resources of the state of Hawaii in their consideration of the
construction of an infiltration well. The effort has provided data con
cerning the depth of the coastal sedimentary fill underlying the Waimanalo
8
area. Geologic data on the depth of sedimentary formations at Waimanalo
is also valuable basic information to the scientific community at large.
STAGE 2: MAGNETIC TAPE SEISMIC DATA RECORDING DEVELOPMENT
The magnetic tape data recording capability was tested at the
University of Hawaii by utilizing a seismic source generated by dropping
a SOOO-pound foundation exploration cable drill tool rig from a height of
24" (Figure 5). Two channels of data were recorded by placing one geo
phone at the acoustical source and the other away from it. The former
provided the time source for the seismic data; the latter recorded reflected
signals.
The seismic signals were satisfactory on the geophone placed away
from the drilling rig. However, the up-hole geophone produced distorted
signals due to overdriving and drilling-rig vibrations.
A block diagram of the seismic signal recording apparatus is shown
in Figure 6. Geophones with a natural frequency of 4.5 cycles per second
fed signals into a simple seismic filter consisting of balancing potentio
meters. An isolation transformer was added because of the radio frequency
hum and spark interference. The filtered signal was amplified to a level
suitable for recording on magnetic tape by an Ampex model 860 dual
capstan drive unit with built -in audio amplifiers.
Playback of the seismic magnetic tape data on the same Ampex 860 tape
deck is monitored on a Tektronix Type 453 oscilloscope which contains time
delay capability (Figure 7). Reproducibility of the seismic signals was
judged as excellent. Since overdriving of the up-hole geophone destroyed
the usefulness of the timinggeophone, signal averaging was not possible.
The filter circuit is shown as Figure 8.
FI ~URE 2. A MULTI-CHANNEL GALVANOMETER TYPE SEISMIC RECORDER WAS UTILIZED TO PRODUCE REPEATEDREFLECTION RECORDS ON PHOTOGRAPHIC PAPER.
\D
FIGURE 3. SEISMIC RECORDING AT THE UNVERSITY OF HAWAII EXPERIMENT STATION IN WAIMANALO. THE EXPLOSIVESTRUCK IS IN THE FOREGROUND AND THE SHOTPOINT IS IN THE BACKGROUND.
.....o
FIGURE 4. MANUAL DIGITIZATION OF THE SEISMIC RECORDS. THE OSCAR AUTOMATICALLY PUNCHES IBM CARDS FORUSE IN COMPUTER AVERAGING.
~
~
FIGURE 5. ACOUSTICAL ENERGY FROM A CABLE TOOL DRILL RIG WAS UTILIZED TO TEST THE MAGNETIC TAPE RECORDING OF SEISMIC SIGNALS.
......N
GEOPHONE SEISMIC FILTER AUDIO AMPLIFIERMAGNETICTAPE RECORDER
.. .. 0_0~ II~
V
FIG . 6. Block Diagram of Seismic Signal Recording Apparatus
~
~
FIGURE 7. MAGNETIC RECORDING OF SEISMIC DATA IS ANALYSED ON A MULTIPLE TRACE TIME ANALOG OSCILLOSCOPEAND RECORDED BY AN OSCILLOSCOPE CAMERA.
.......,.
GEOPHONE
INPUT
.01
.01
LINE TO GRIDTRANSFORM:R
AUDIOAMPLIFIEROUTPUT
a:.,~' . "".'.-.. ..;.;.._.. ,. c -,. ......""....= ;. .""'"' -~--
Figure 8. Schematic Diagram of Simple Seismic - signal FiIter Found Effective in Reducing
Radio Frequency Spark and Hum from the Recorded Signal
......U1
16
Figure 9. Design of Tripping Hook for Weight Dropping
17
STAGE 3: THUMPER SEISMIC SIGNAL SOURCE
A surplus jeep was renovated into a wei ght-dropping vehi.c l e with
a steel top installed for protection of the seismic equipment. A winch
with power takeoff from the jeep and a two-pole A-frame with a pulley
at the top was attached to the front of the j eep wi t h a reinforcing frame
work at the rear of the jeep. Voltage outlets were provided on the jeep
for 12 volt power sources to the electronic equipment . A 300 lb. weight
was constructed in 50 lb. sections so that it could be handled more eas i
ly. The jeep, winch, and A-frame, however, can lift several thousand
pounds more than 10 feet above the ground surface. Release of the 300 lb .
weight from the A-frame was done by a specially constructed release hook
styled after marine "pelican" hooks (Figure 9). The wei ght was dropped
on a steel plate and the contact impact started the time signal for the
seismic record.
Seismic signals generated by the weight impact were picked up by
either one or an array of geophones and recorded on magnetic tape. The
signals were directed through the filter described in the previous sec
tion (Stage 2 , Magnetic Tape Seismic Data Recording Development ). The
filtered seismic signals were put into the tape recorder amplifer. The
dual-capstan tape-drive of the Ampex tape r ecorder mechanism improves
tape transport velocities and stability with reasonable audi o frequency
response and comparable transport. Hence, the inexpensive Ampex 860 ,
originally a four-track stereo system in which two tracks recorded in
each direction on a magnetic tape, has been ideal for this study. In
initial tests, one track was utilized to record voice data and seismic
timing signals and the other track on the magnetic tape was used to re
cord seismic data.
18
A special in-line four track head was purchased from Applied Mag
netics Corporation of Santa Barbara, California for installation on the
tape recorder to enable it to record voice data of field operations and
record time signals on separate channels, and seismic signals on the re
maining two channels.
An oscillator has been constructed so that the timing signals
triggered by the contact of the weight striking the metal plate will
start and run for approximately two seconds and stop. It is reset and
triggered when another seismic impulse is initiated. The timing signal
itself is a 300 cycle oscillator available from Accutronics, Incorporated
in Batavia, Illinois. Circuit for the oscillator and trigger mechanism
is shown as Figure 10. The limited duration of the time signal permits
utilization of this signal by digitizing equipment in the laboratory to
analyze only those portions of the magnetic tape which contain the timing
signal. The wave form of the timing signal not only indicates the time
of arrival of seismic signals, but also is used to set the sampling rate
at which the seismic wave form is converted to digital form for analysis
with digital computers. The weight dropper, the geophone, seismic filter,
magnetic recording, and accessory timing device are shown diagramaticall y
in their operational configuration in Figures 11 and 12.
Magnetic tape recordings of seismic signals allow versatility in
the analysis of the seismic signals. The quality and reproducibility of
the signals can be monitored on time delay oscilloscopes. Either individ
ual traces or composite traces can be viewed and photographed on an oscil
loscope screen by using intensity modulation or the standard vertical
deflection of the oscilloscope scanning spot . Thus, seismic signals can
be examined by either intensity contrast on the oscilloscope screen or as
FlIP·FLOP ONE-SHOT
5.61(
561(
""'\I"
0-1 MA
2.21(
3.31(
82
5.61(
GATE
82
3.3K
·This capacitor controls time interval
~3 '~~ ...
TRANSISTORS - 2N2925DIODES - lN198OSCILLATOR - MODEL JC 15-22Z
ACCUTRONICS INC.BOTAVIA, ILL.300 cps! 0.0251 at 25- C.
FIGURE 10. SEISMIC CONTROL TIMERSHOT TONEOUT
TUNING FORKOSCILLATOR
.....\0
No
VOICE DATA
.'AMPLIFIER
ITAPE DECK
TIMER
~E!S~~_~Qt:J~h-_J STRIKING PLATE
GEOPHONE
QJ)
Figure 11. Schematic Diagram of Thumper and Recording Equipment.
21
FIGURE 12. ACOUSTICAL SI;ISMIC SIGNALS CREATED BY A "THUMPER" WEIGHTDROPPING SOURCE WERE RECORDED ON MAGNETIC TAPE USING AMODIFIED AMPEX 860 TAPE TRANSPORT. THIS THUMPER IS USEFULIN INHABITED AREAS NOT PERMITTING THE USE OF EXPLOSIVES.
22
wiggle "u" lines. By photographing repeated seismic signals derived from
the same set of field conditions and location, signal averaging occurs by
the weakening of noise signals due to their erratic nature, while the re
petitive seismic energy signals are reinforced.
A Tektronix Type 32lA oscilloscope is being used for monitoring
signals in the field, and a Type 453 oscilloscope is used for laboratory
evaluation and photography of seismic data (Figure 7).
23
TRAINING AND EDUCATION
Considerable outside interest was activated by the utilization of
seismic instrumentation for determining geologic groundwater conditions.
Besides Water Resources Research Center personnel, other research per
sonnel and graduate students at the Hawaii Institute of Geophysics, and
city and state ground water agencies followed the development of the
instruments and techniques closely. Considerable volunteer field assis
tance was obtained in almost all field operations. For these reasons, the
training and education of personnel extended far beyond those directly
employed in the project. A later study, exploring four techniques for
geophysical exploration for ground water was a direct result of interest
developed during execution of this preliminary phase.
24
SUMMARY AND CONCLUSIONS
Initial generation and recording of seismic signals were carried
out with conventional explosives and photographic recording. Develop
ment of magnetic tape seismic data recording has been accomplished with
accessory filters and a timer adapted to an inexpensive magnetic tape
transport. Inexpensive and rapid generation of seismic signals was
developed with a weight-dropper.
Versatility in handling and analysis of seismic data is provided
by both analog and digital input of magnetic tape seismic-recording. High
ly reproducible results have been obtained.
This project has been primarily involved in procuring, assembling,
and debugging equipment, training student helpers and technicians on field
procedures, and determining laboratory analysis techniques. No final an
swer has been obtained, however, concerning the feasibility of seismic
exploration for structure relevant to groundwater control and movement.
Interpretation of the seismic data does not indicate that seismology
would not be an appropriate tool for studying geological structures. There
fore, funding has been obtained for ~nother project to continue this work.
Success in that effort is assured due to the extensive ground work in in
strumentation and the personnel training afforded by this initial project.
A significant accomplishment of this study is the development of
equipment for seismic exploration using inexpensive and readily available
components which produce good quality records and of the versatility in
analysis techniques. The reasonable cost and availability of the equip
ment enables it to be readily duplicated for groundwater exploration in
areas where availability and costs might otherwise prevent seismic studies.
2S
ACKNOWLEDGEMENTS
Assistance in design and construction of instrumentation by the
personnel of the Hawaii Institute of Geophysics greatly aided in achiev-
ing the objectives of this project. Although many individuals assisted,
special appreciation is due Noel Thompson, Griffith Woodruff, and per-
sonnel of the electronics and machine shops.
Graduate students who assisted gratuitously in field seismic tests,
include Loren Kroenke, Frisby Campbell, Don Hussong, Mel Caskey and Don
Walker. The assistance of Ron Tracy as graduate research assistant on
the project is appreciated. Professor Augustine Furumoto has been gen-
erous in giving guidance and help.
26
BIBLIOGRAPHY
Swartz, J .H. 1937. Resistivity studies of some salt-water boundariesin the Hawaiian Islands. American Geophysical Union Transactions 3
pt. II, 387-393.
Swartz, J.H. 1939. Geophysical investigations in the Hawaiian Islands.American Geophysical Union Transactions 3 v. 20, pt. 1, 292-298.
Swartz, J.H. 1940a. Resistivity survey of Schofield plateau, p. 5659. In H.T. Stearns, Supplement to the Geology and GroundwaterResources of the Island of Oahu3 Hawaii. Hawaii Division, Hydrological Bulletin, v. 5.
Swartz, J .H. 1940b. Geophysical investigation on Lanai, p. 97-115. I nH.T. Stearns, Geol ogy and Gr oundwater Resources of the Island ofLanai and Kahoolawe3 Hawaii. Hawaii Division, Hydrological Bulletin,v. 6.
Watkins, J.S., DeBremaecker, J.e., Loney, R.A., Whitcomb, J.H., andGodson, R.H. 1966. Investigations of in situ physical propertiesof surface and subsurface site materials by engineering geophysicaltechniques. Annual report, fiscal year 1965. U.S. Geol. Surv.