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TUNNEL LOCATION BY MAGNETOMETER, ACTIVE SEISMIC, AND RADON DECAY METHODS
ALEXANDRIA LABORATORIES REPORT NO.: AL-75-1
W. C. Dean
18 June 1975
Teledyne Geotech
314 Montgomery Street
Alexandria, Virginia 22314
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1 REPORT NUMBER 12. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER
AL-75-1 4. TITLE (and Subtitle) s TYPE OF REPORT & PERIOD COVERED
TUNNEL LOCATION BY MAGNETOMETER, ACTIVE SEISMIC, Technical AND RADON DECAY METHODS
6 PERFORMING ORG. REPORT NUMBER
7. AU THOR(s) 8 CONTRACT OR GRANT NUMBERfs)
w. c. Dean F08606-74-C-0013
9 PERFORMING ORGANIZATION NAME AND ADDRESS 10 PROGRAM ELEMENT. PROJECT TASK
Teledyne Geotech AREA & WORK UNIT NUMBER';
314 Hontgomery Street Alexandria, Virginia 22314
11 CONTROLLING OFFICE NAME AND ADDRESS 12 REPORT DATE
Defense Advanced Research Projects Agency 18 June 1975 Nuclear Monitoring Research Office 13 NUMBE~l/- PAr.<'S
1400 Wilson Blvd.- Arlington, Virginia 22209 ~ 1 4 MONITORING AGENCY NAME B: ADDRES$(1/ different from ControllinR Office) 15. SECURITY CLASS. fof this re(lr·rt;
VELA Seismological Center Unclassified 312 Montgomery Street
Alexandria, Virginia 22314 fts;;_· DECLASSIFICATION DOWN GflADI N-c;--SCHEDULE
16. DISTRIBUTION STATEMENT (of this Report)
APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIHITED. '
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17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, If different from Report) . ' ' '·. \[\
' . ' ,_ \'\.
,J ./ ' ... ~~ ' ' ,>
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18 SUPPLEMENTARY NOTES < • "' ~·· \ \ \ ~ --\"\ \\\\/'"
\ \ \ \
\) \.'
19 KEY WORDS (Continue on reverse ,•dde if necessary and Identify by block number)
Tunnel location Proton Magnetometer Seismic location Radon detection
20 ABSTRACT (Continue on reverse side If nectlssary and identify by block number)
This project investigated three methods of detecting mines or tunnels in granite: magnetic, seismic, and radioactive decay of radon.
The magnetic, using a proton precession magnetometer, aims to detect tunnels from the magnetic materials (rails, roof bolts, wiring) these mines are expected to contain. This method works for both surface and borehole surveys when the sensor is less than 20 feet from the magnetic source, For deeper rails a surface survey will probably not be successful since the rail
DO FORM 1 JAN 73 1473 EDITION OF 1 NOV 65 IS OBSOLETE
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anomalies are not of one sign but oscillate above and below the mean background level with wavelengths on the order of two rail lengths. Such anomalies cannot be distinguished from weak geologic anomalies.
The active seismic methods aims to detect tunnels by reflection or refraction of P-waves from the tunnel or by reverberations of the tunnel walls. These experiments were unsuccessful due to insufficient high frequency energy being coupled into the rock containing the tunnel. Some evidence of cavity wall reverberations appeared over a coal mine, but the tunnel was shallow (15 feet) and the medium (coal) had a lower seismic velocity than granite.
The radon decay method aims to detect tunnelling activity by the extra release of radon gas freed into the atmosphere. For a tunnelling rate of 16 cubic meters per day, this method will fail due to a poor signal-to-noise ratio relative to all of the other sources of radon in the atmosphere.
• • Unclassified /I SECURITY CLASSIFICATION OF THIS PAGE(When Dntn 1-:ntete{t:
ABSTRACT
This project investigated three methods of detecting mines or tunnels in
granite: magnetic, seismic, and radioactive decay of radon.
The magnetic, using a proton precession m: -... tometer, aims to detect
tunnels from the magnetic materials (rails, rr lts, wiring) these mines
are expected to contain. This method works th surface and borehole
surveys when the sensor is less than 20 feet from the magnetic source. For
deeper rails a surface survey will probably not be successful since the rail
anomalies are not of one sign but oscillate above and below the mean background
level with wavelengths on the order of two rail lengths. Such anomalies cannot
be distinguished from weak geologic anomalies.
The active seismic methods aims to detect tunnels by reflection or
refraction of P-waves from the tunnel or by reverberations of the tunnel walls.
These experiments were unsuccessful due to insufficient high frequency energy
being coupled into the rock containing the tunnel. Some evidence of cavity
wall reverberations appeared over a coal mine, but the tunnel was shallow
(15 feet) and the medium (coal) had a lower seismic velocity than granite.
The radon decay method aims to detect tunnelling activity by the extra
release of radon gas freed into the atmosphere. For a tunnelling rate of 16
cubic meters per day, this method will fail due to a poor signal-to-noise
ratio relative to all of the other sources of radon in the atmosphere.
-ii-
ABSTRACT
EXECUTIVE SUMMARY
INTRODUCTION
MAGNETIC METHODS
TABLE OF CONTENTS
Conclusions on the Magnetic Experiment
ACTIVE SEISMIC METHODS
Conclusions on the Seismic Experiments
ESTIMATE OF THE DETECTABILITY OF MINING ACTIVITIES MONITORING AIRBORNE Rn222
REFERENCES
ACKNOWLEDGEMENT
APPENDIX A
APPENDIX B
-iii-
Page
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BY 56
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Figure No.
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LIST OI' FIGURES
Title
Nomogram for estimating magnetic anomalies.
Magnetic survey over rail spur at Ft. Belvoi1, Virginia.
Map of magnetic survey, Alexandria, Virginia.
Magnetic survey over rail spur, Alexandria, Virginia.
Magnetic profiles parallel to the rails, Alexandria, Virginia.
Operator with portable magnetometer over east portal, Carothers Tunnel, B&O Railroad.
Map of magnetic survey over Carothers Tunnel, B&O Railroad.
Magnetic anomalies over Carothers Tunnel, B&O Railroad.
Magnetic survey, upper profiles, over Carothers Tunnel, B&O Railroad.
West portal of Tunnel 19, DRGW Railroad.
Map of magnetic survey over Tunnel 19, DRGW Railroad.
Cross section of Tunnel 19 and magnetic profiles, DRGW Railroad.
Operator with portable magnetometer of upper profiles over Tunnel 19, DRGW Railroad.
Trackside magnetic. survey near Tunnel 19, DRGW Railroad.
9
10
ll
12
13
14
15
17
18
19
')' ·- .l
Magnetic survey over Tunnel 19, DRGW Railroad. 22
Smoothed magnetic profiles over Tunnel 19, DRGH 24 Railroad.
Expected anomalies from rails over Tunnel 19, 25 DRGW Railroad.
-iv-
LIST OF FIGURES (Continued)
Figure No. Title
18 Actual versus theoretical anomaly, profile M, Alexandria survey.
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Theoretical gradient for 21 profiles over Tunnel 19, DRGW Railroad.
Actual gradient for 21 profiles over Tunnel 19, DRGW Railroad.
Magnetic anomalies from freight train passing through Carothers Tunnel, B&O Railroad.
Special sensor for borehole magnetometer.
Drilling rig over Colorado School of Mines experimental mine.
Cross sectional view of borehole and CSM experimental mine.
Magnetic survey over mine rails at CSM experimental mine.
Borehole magnetometer survey at CSM experimental mine.
Seismic recording truck of Colorado School of Mines.
Dinoseis truck.
Recording units of the Bison hammer seismograph.
Seismic profile over Tunnel 19, DRGW Railroad.
Center set of Dinoseis records taken over Tunnel 19, DRGW Railroad.
Summation record over Tunnel 19 with static and dynamic corrections applied.
Center set of Dinoseis records taken over Tunnel 19, DRGW Railroad.
Conventional stacking of Tunnel 17 data corrected for two-way travel times to tunnel.
Three stacked records over Tunnel 17, assuming a tunnel at the south end, center, and north end respectively.
-v-
Page
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LIST OF FIGURES (Continued)
Figure No. Title P~rL
36 Seismic cross section over Tunnel 17, DRGW Railroad. SJ
37 Typical Bison records over coal mine. 53
38 Map over coal seam with contours showing region of 54 reverberation seismograms.
-vi-
EXECUTIVE SUMMARY
This project investigated three geophysical methods of detecting
underground mines or tunnels. The three methods are magnetic, seismic, and
radioactive decay (of radon).
The magnetic method aims to detect mines from the magnetic anomalies
generated by the rails these mines are all expected to contain. Other
magnetic material in mines might include electrical wiring and structural
material such as roof bolts and steel reinforcement bars in concrete slabs.
We conducted magnetic surveys with a proton precession magnetometer
directly over railroad rails (sensor height eight feet), and over railroad
tunnels where the rails were buried 50 to 100 feet below the magnetometer.
In addition we drilled a borehole which passed within 12 feet of a mine and
ran a magnetic survey with a borehole sensor.
The results show that the rails do exhibit strong magnetic anomalies
(up to 1500 gamma for mainline railroad rails). However the magnetic anomalies
are not constant as we had expected but oscillate above and below the back
ground magnetic field of the earth with wavelengths comparable to the length
of the rail sections. This oscillation further obscures what was already
expected to be a small anomaly amongst the much larger geologic anomalies
which are ever present. Moreover in the U.S.A. the railroad tunnels invariably
are accompanied by fences and phone lines running over the tunnel parallel
with the rails. These create surface anomalies, which are readily detected
by the magnetometer, but further obscure the weak rail anomalies. Even
without the confusion of parallel fences and phone lines, detection of rails
in a mine by a surface magnetometer survey is doubtful even for mine (rail)
depths of 15 to 50 feet.
In contrast the magnetometer shows strong anomalies, and oftentimes quite
erratic behavior on repeated readings, when the sensor is close (a few inches
to a few feet) to magnetic materials. The borehole survey showed just this
behavior as the sensor passed the roof bolts and reinforcements in the
experimental mine of the Colorado School of Mines. The erratic behavior
-1-
is especially pronounced when there are loops of magnetic Ti>Clt-<:o:: i ,J 1 ,;;-:;r,,-,
(such as wire mesh or barbwire ::ooped around a fencepost). Tile rc:ason is
that the pulsing of the magnetometer creates an electromagnet out of the
magnetic wire loop where the residual effect varies from reading to reading.
Thus borehole magnetic surveys do show some promise of detecting mines
from the magnetic material contained in them although, when successful, the
strongest magnetic anomalies may not be coming from the rails.
The seismic method aims to detect tunnels by reflection or refraction
of P-waves from the tunnel. For this method to work effectively, the seismic
wavelengths in rock should be comparable or shorter than the cross-section
dimensions of the target. Even for railroad tunnels (15 feet by 25 feet)
the wavelengths of seismic energy used in geophysical exploration are much
longer than the tunnel dimensions.
Our field experiments show very little high frequency energy (100 to
500 Hz) coupled into the rocks resulting in no reflection evidence of the
tunnels. This was true when we used Dinoseis, or dynamite caps, or a sledge
hammer (used with the Bison hammer seismograph). The problem at the sites
over the two railroad tunnels in Colorado was due to a low velocity layer on
the surface trapping almost all of the energy preventing any significant
penetration into the granite containing the tunnel.
Watkins (1967) reported on reverberations from underground cavity walls
with a resonant frequency close to that predicted by Biot (1952). We did
not see such reverberations from the railroad tunnels again due to the lack
of energy penetrating the granite. However we did see such reverberations
over a mine in coal (15 feet deep) on a Bison seismic profile. Although the
coal tunnel is smaller (2x2 meters) than railroad tunnels, it is also shallow
(5 meters) and exists in a uniform half space (coal) with a seismic velocity
much less than that of granite.
For any subsequent experiments the field practice should make use of a
multiple geophone system rather than a single sensor and hammer source such
as the Bison. The correlation between traces is better when several traces
are looking at seismic returns from the same source rather than one source per
trace as in the Bison hammer seismograph.
-2-
The radon method aims to detect on-going tunnelling activity by detecting
the release of radon gas freed into the atmosphere. All granite and crystaline
rocks contain some uranium. Radon (Rn222
) is a daughter product of u238 and
will be released to the atmosphere as the crushed rock from the mine is
excavated.
Our study shows that for the amount of tunnelling expected (4 n1eters
per day with a cross section of 2 x 2 meters), it is highly unlikely this
method could detect radon from the tunnelling activity as opposed to all
other sources of radon in the atmosphere, even under the most optimistic
assumptions. With more realistic assumptions there is no hope.
-3-
INTRODUCTION
The objective of this project is to detect underground mines or tunnels
by geophysical methods. The mines are expected to be approximately 2 x 2
meters in cross section, located in crystaline rocks such as granite,
gneisses, or shists in hilly terrain, and with depths of burial of a few
feet to as much as 200 feet or more.
This project has limited its attention to only three methods: detection
of magnetic materials in the mine, reflection or refraction evidence of a
tunnel by seismic exploration techniques, and detection of radon released to
the atmosphere by the excavation activity. Other methods or techniques are
considered outside the scope of this study.
-4-
MAGNETIC METHODS
The tunnels of interest are all expected to have rails. In addition they
probably have electric wiring which may or may not be in use. In some loca
tions, perhaps where the mines are shallow or in unconsolidated material, they
may have roof bolts or iron reinforcements in concrete walls or ceilings.
The strength of magnetic dipoles decrease as the inverse cube of the -3 distance (r ). However, rails are good approximations to infinitely long
magnetic sources and so would be expected to decrease as the inverse square -2 of the distance (r ). Figure 1 from the Geometries Applications Manual for
Portable Magnetometers (Breiner, 1973) is a nomogram showing the expected
magnetic field strength from various types of objects versus the distance
from the magnetometer. Note that the field strength from most objects vary
as the inverse cube of the distance but that from pipelines vary as the
inverse square.
In Figure 1 Breiner used the cross sectional area, A, of a pipeline as:
A = nDt
where Dis the diameter of pipeline (6") and tis the thickness (1/4"). Such
a pipeline has a volume of 0.1 cubic feet per yard or approximately 50 pounds
per yard. By comparison mainline rails are frequently 135 pounds per yard
and mine rails are approximately 30 pounds per yard.
If we can detect infinitely long magnetic anomalies of 10 gamma, then
we might expect to detect a 6-inch pipeline or a pair of mine rails buried
at a depth of 40 feet. Other expected characteristics of our mine rail
anomalies are that their expected direction is known and that their route
will be roughly horizontal so the anomaly strength, will decrease with the
square of the ground elevation (proportional to h-2).
The equipment we used were proton precession magnetometers manufactured
by Geometries. These instruments can measure the earth's magnetic field
accurately to within one or two gammas. We used both a base station with a
recorder (Model G-826) and a portable unit (Model G-816).
-5-
500
400
300
200
100
50
40
10
~ ~ q.
.<)
5 -:<,
4
y ~
(:'
3
2
FEET 2 4 6 10 20 40 60 80 100 150 200
CENTIMETERS 100 200 400 GOO 800 1000 2000 3000 4000
DISTANCE FROM MAGNETOMETER
Figure 1. Nomogram for estimating magnetic anomalies.
-6-
Magnetometer surveys over rail lines do show magnetic anomalies of 1500
gammas relative to the background field when measured by the portable sensor
eight feet above the surface of the ground. Figure 2 shows two parallel
profiles normal to an east-west rail spur in Fort Belvoir, Virginia.
We ran a more elaborate survey of eight profiles over a rail spur in
Alexandria, Virginia (see map on Figure 3). The anomalies over these profiles
are shown on Figure 4. The interesting feature of these profiles normal to
the rails is that they are not all of the same sign. Profiles run parallel
to the rails as shown on Figure 5 demonstrate the oscillatory behavior of
these anomalies. The half-wavelength of these oscillations correspond roughly
to the length of the rails (33 feet). These anomalies are reproducible even
when measured on different days as indicated by the x's on the center profile.
It is as though one end of the rail is a north pole and the other a south
pole.
The first magnetometer survey over a tunnel we conducted at the Carothers
Tunnel on the main line of the B&O Railroad near Paw Paw, West Virginia.
Figure 6 shows a photograph taken over the East Portal of the Carothers Tunnel.
An operator is shown with the portable magnetometer and the sensor at the end
of the 8 foot aluminum pole. Also shown are the telephone wires running over
the tunnel. In many places these wires are less than 8 feet off the ground.
Figure 7 shows a plan view of the railroad (which is double tracked), the
east tunnel portal, the phone line, and several of the magnetometer profiles
run over the tunnel. Figure 8 shows the magnetic field variations for two
profiles over the rails at a scale of 1000 gammas per inch. The lowest
magnetometer profiles over the tunnel are also plotted on the same figure with
the same vertical scale.
Figure 9 shows the magnetometer profiles at the higher levels with a
vertical scale of 20 gammas per centimeter. The position of the rails under
each profile and the location of the phone line along each profile are clearly
marked.
There is a distinct magnetic anomaly due to the phone line on each
profile. There are other magnetic variations over the rail but it is difficult
-7-
PROFILE A
A - 56000y
PROFILE 8 8 - 56000y
A - 55000y
8 - 55000y
PROFILES 15 FEET APART
-120 -100 -80 -60 -40 -20 0 20
DISTANCE FROM RR CENTER IN FEET
s N
Figure 2. Magnetic survey over rail spur at Ft. Belvoir, Virginia.
-8-
X X X X
PA BUS X
RKING ~ X X X
XX XXX XXX
,1' FE NCE
J K L
I
PROFILES M N
I
0 p Q
~N
RR LINE
j.-20 FT~
- .,......- SENSOR 0: r BASE STATION
THE RR LINE IS ONE BLOCK EAST OF N. FAIRFAX ST., ALEXANDRIA, VA. PROFILE M IS ON THE NORTH EDGE OF WYTHE ST., ALEXANDRIA . PROFILE SEPARATION IS 15 FEET. READINGS EVERY 1 FOOT FROM 0 TO 6 FEET FROM RR,
" 2 FEET " 6 TO 20 " " " 5 FEET 10 TO 100"
10 FEET " 100 TO 130 "
Figure 3. Map of magnetic survey, Alexandria, Virginia.
-9-
-N 56000 y
-M 56000y
-L 56000y
-K 56000y
-J 56000y
+40 +30
-0 56000y
-P 56000y
-0 56000 y
+20 -+10 0 -10
DISTANCE FROM RR IN FEET
T 1000
GAMMAS
1 -20
Figure 4. Hagnetic survey over rail spur, Alexandria, Virginia.
-10-
0
p
0
N
M
L
K
J
-30
WB
W4
W-RAIL
RR CENTER
E-RAIL
E3
E5
E10
E16
0
PROFILE LETTER INDICATES EAST OR WEST OF RR CENTER; NUMBER INDICATES DISTANCE IN FEET FROM RR CENTER. NOTE: x'S ARE SAME PROFILE SIX DAYS EARLIER.
20 40 60 80 100 120 140 DISTANCE FROM E- W PROFILE M IN FEET
160
X
l 1000
GAMMAS
T
Figure 5. Magnetic profiles parallel to the rails, Alexandria, Virginia.
-11-
Figure 6. Operator with portable magnetometer over east portal, Carothers Tunnel, B&O Railroad.
-12-
I f-' w I
:::; 300 .... .... z .... z .... 200 .... z Cl : a.
:IE ~ 100 .... .... y z < len Cl 0
s
~
EAST PORTAL ~ .. ~ RR
~ PHONE LINE
0
IX
IV VII VIII VI
v Ill
r--•t'"' I'• I '-. ~ ~ """'"
100 200 300 400 500
DISTANCE AlONG TUNNEL EAST-TO-WEST IN FEET
...,
600
TUNNEL HILL CREST
+
700
~w
Figure 7. Map of magnetic survey over Carothers Tunnel, B&O Railroad.
.....__
'EST PORTAL
11/lml
I !--' ~ I
IV .. ~·~···•""'"-"" , ...• --.,..,_ ·-~-·----·-~·---,---~ ....._ ·-~' "'""-
IV - 56000y·
Ill ·· ..... ----·,· ,-,·--··--·~---· ______ , _____ , . .,., .. , ___ ._, ... "'"
Ill - 56000y
A
A - 56000y 8
B - 55000y
II ~ ---------i:i6000y
--·,~--------
-50 j_
-40 -30
I - 56000y
:boo.. N
-20 -10 0 10 20 30 DISTANCE FROM RR CENTER (FEET)
T 1000
GAMMAS
1
40
F i. 1;ure :::l. ~1agnetL: ;momalies over Carothers Tunnel, B&ll Ra ilrodd.
50
D=305'
D=225'
D=115'
D=125'
D=95'
D=75'
Ill D=55'
H=120'
H=114'
H=109'
H=102'
H=98'
IX - 56340y 1 1
1 l
1.1 .ll.
RAIL POSITION UNDER PROFILE
l. l.
VI- 56340y
v - 56340y
IV - 56340y
! Ill - 56340y
t POSITION OF PHONE LINE
H=DEPTH TO RAILS IN FEET l D = DISTANCE FROM EAST PORTAL IN FEET
-100 -80 -60 -40 -20
DISTANCE FROM PHONE LINE IN FEET s ,.....,
0
20 GAMMAS
T 20
Figure 9. Magnetic survey, upper profiles, over Carothers Tunnel, B&O Railroad.
-15-
IX
VIII
VI
v
IV
Ill
t.o attribute these variati.ons directly to the railc:,, ~speclt1ly i:c Jight of
the strange anomalies measured on the two parallel profi]es directly over the
rails (profiles I and II on Figure 8).
A more thorough magnetometer survey was run over Tunnel 19 of the Denver
and Rio Grande Western Railroad near Eldorado Springs, Colorado. This tunnel
(the west portal of which is shown in Figure 10) is in granite, has a single
track, and has much easier access over the tunnel than the one in West
Virginia. Figure 11 shows a map of the railroad, east: portal of the tunnel,
fences, phone lines, and the 21 profiles run. Figure 12 shows a cross
section of the tunnel and the profile elevations along the phone line.
Profile A is 50 feet and profile U is 90 feet above the rails. Figure 13
shows an operator with a portable magnetometer at the upper profiles over
Tunnel 19.
We conducted a track level survey as well. The railroad crosses a high
fill immediately east of Tunnel 19 which provides only limited access to the
north and south of the rails. Figure 14 shows the magnetic anomalies over
the north rail and 10 feet to the north, and over the south rail and 10 feet
to the south. Again the oscillatory character of the magnetic anomalies is
evident and again the oscillation half-wavelength corresponds roughly to the
rail length of 39 feet.
The magnetic data for the 21 profiles over the tunnel are shown in
Figure 15. As was consistent with most of our surveys magnetometer readings
were taken every 5 feet along the north-south profiles. The profile spacing,
east to west, was 8.5 feet. A base station magnetometer was recording at the
top of the mountain 140 feet above the raih:. A constant base station correc
tion was applied to each half profile (i.e. to those readings taken within
10 minutes of each other).
The magnetic profiles show strong fence and phone line anomalies. Since
they are parallel to the rails, their strong anomalies tend to obscure the
weaker ones which might be expected from the rails.
-16-
Figure 10. West portal of Tunnel 19, DRGW Railroad.
-17-
~ 0:, I Ill
1\1
J r I 1-
H 1-----
G ~ I= L___ ' I
E 1--D ~C 1-----B 1----· -A ~-----
RR
FENCE /! f I I
/..,___PHONE LINE
U f- · · ····-·--- ----- _/ / I U T ~ -- { J ! IT
s ~---------- - .J.. / I S
QRl:___ i . f! II~ t~ --- --f- l- ! 1ci
N ---------- i -----· j- : I N M t· - - - .. . - - - . 7 I ~ M
-~~==-==~-~===~=-· ~~ ! I? 1---- -----F / II
--~-~ f I H . --r- . (
I F f E
\
D c
B
SCALE: 1" = 50" - --- \"\ I A
\ ~ EAST '
PORTAL \ !TUNNEL 191\
I::::::::- N
Fi~~ure J 1. ~-l~lp dt magnctiL· sur\·'t'\' l)\'L'l I'ttllilL'l 1'-l, l)l\~,,~., i\,li rl'.~1d.
I 1--'
"' I
50'
PROFILES p
N 0 • • M • L • K • J •
R s T
u • •
0 • • •
G E F
D 8
c • •
A •
• •
H • • •
PROFILE ELEVATIONS ESTIMATED AT PHONE LINE • •
------------TUN-;;E~TOP-;r----- ------- __ _ ------
SCALE: 1" = 20'
EAST PORTAL
RR TRACKS
Figure 12. Cross section of Tunnel 19 and magnetic profiles, DRGW Railroad.
90'
Figure 13. Operator with portable magnetometer of upper profiles over Tunnel 19, DRGW Railroad.
-20-
N + 10'- 56000y
\.
N -56000y
N RAIL (SHOWING JOINT POINT POSITIONS)
S - RAIL
s- 56000y
0 100 200 300
0 !STANCE ALONG RR IN FEET
Figure 14. Trackside magnetic survey near Tunnel 19, DRGW Railroad.
-21-
T 100 GAMMAS
_l_
PROFILE;;S SOUTH FENCE ."' I ~
;;'~/ U \ I
'\\: /
:0\~/ R ~/~~·~ .. /
n ://\~ '-/ : ~/''~. :~~~~ I r~j\~j
I~J• K I \
~\\~~0 J r ~\~
H~~~· G ~\ F ______;-J~ \ E _______J I
D -----J/ c ------J ~Y\-~/" v (\ \. . B ~ I ·vf\\
I~ \ A~ I
READINGS ALONG PROFI~ES EVERY 5 FEET \. PROFILES SEPARATED BY 8.5 FEET
I
-200 -100 0 100 200 DISTANCE FROM SOUTH FENCE IN FEET
Figure 15. Magnetic survey over Tunnel 19, DRGW Railroad.
-22-
We applied several kinds of smoothing. First we smoothed out the fence
and phone line anomalies by hand. We also made a computer interpolation for
each profile as follows:
Profile A +I sin(x-k~x)rr/~x k=-Nak (x-k~x)rr/~x
Trends were removed first to avoid the Gibbs effect at the ends which would
otherwise be discontinuities. These smoothed data are plotted in Figure 16.
We can estimate the rail anomalies at each of the over-tunnel profiles
from a theoretical argument. If the anomalies at rail level vary sinusoidally,
and vary inversely as the square of the rail-magnetometer separation, then
to a first order approximation, the rail magnetic anomalies should be:
~(x,y) M
0 cos ~ L
(1)
where h is the height of the sensor above the rails, x is the distance along
the profile in a north-south direction from the rail center, and y is the
distance in the east-west direction from Profile A. The half-wavelength L,
is equal to the rail length. Figure 17 shows the expected anomalies from
the rails at each of the profiles over the tunnel plotted to the same scale
of the measured data on Figures 15 and 16. Since the rail anomalies are
small, broad and oscillatory, they are readily obscurred by the larger,
sharper, and more continuous geologic anomalies and especially the larger
and sharper phone line and fence anomalies running parallel to the rails.
Since it is the oscillatory behavior of the rail anomalies which most
effectively hides them in the geologic anomalies, we can ask whether some
other measure of the magnetic field would give an effect more constant in
value along the axis of the rails. One possiblity is the gradient. The
magnetometer measures the vector sum of the earth's field and that from the
rails. Thus the effect from the rails could be a rotating vector rather
than one that goes to zero. If the form of the rail anomaly is given by
equation (1), then the gradient will be:
-23-
PROFILES
~ /+ u +
T + /(;\, /"/~ /- + \ ~-----"
s + ;r~\/ ~//, R + J/\\~J +
0 + ~\_ +
p + ;; ~~v~~~/ +
0 + /~/\¢~~ +
N + ~·'-/-"'\ ~I +
M + I J"\~::; +
L + y\~~~ +
K + +
J + +
I + +
H + +
G + ~ +
____r~---\fi~\ F + +
E + Yr~ \ ~/"-·~
+
0 + +
c + ~~-,~ +
T B + ~ \ +
100 GAMMAS ~~'v· --~~j\\ _1_ A + +
+ \ +
-200 -100 0 100 200 DISTANCE FROM SOUTH FENCE IN FEET
Figure 16. Smoothed magnetic profiles over Tunnel 19, DRGW Railroad.
-24-
T 100 GAMMAS
_L
PROFILES u T
s
R
Q
p
0
N
M
L
K
J
H
G
F
E
0
c
8
A
-200 ~
-100 0 100
DISTANCE FROM RAILROAD CENTER IN FEET S ,..... ,..._ N
Figure 17. Expected anomalies from rails over Tunnel 19, DRGW Railroad.
-25-
'Vcp(x,y)
The absolute value
lv¢1 h2
In this form when
or when x +.!::.+ n-
M 0
-i h2 + 2 h2 X
of the gradient
M 2x 0
2 h2 + + X X
L 2 2 - h
1/2
'TT
(2)
2x ~. 'TT sin ~ cos L -J -
2 L L + X
will be
2 2.2!Z
2 . 2~
1/2 'TT
cos L + - Sln 1 2 L
the coefficients of the sine and cosine terms are equal and the sinusodial
variation of the absolute value of the gradient, <lv¢1), disappears.
To test whether empirical data yields reasonable values for h and L,
we consider profile M from the Alexandria survey shown in Figure 18. The
theoretical curve gives the fits the data approximately when h = 6.5 feet,
a fairly close match to the sensor height of 8 feet, and L = 21 feet (found
by averaging distances between zero crossings), which is the same order of
the rail length (33 feet).
A theoretical gradient computed for the 21 profiles over Tunnel 19 of
the DRG-RR is shown in Figure 19. Here the height, h, is the sensor height
over the rails and rail length (L) is 39 feet.
The actual gradient of the data over Tunnel 19 with the phone line and
fence anomalies smoothed out is shown on Figure 20. As shown on Figure 19
the expected gradient is still oscillatory as was the original field.
Moreover, the gradient of the actual data does not show an anomaly trend
parallel with the rails which can be readily attributed to the rails.
-26-
I N -....J I
W,.......... DISTANCE IN FEET (y) t::==..... E -40 -30 -20 -10 0 10 20 30
-CENTER OF TRACKS IS AT y = 0.
THEORETICAL
-- I "'· j_./
DATA
T 1400y
Figure 18. Actual versus theoretical anomaly, profile M, Alexandria survey.
PROFILES ---u +
T +
s +
R +
Q +
p +
0 +
N +
M +
L +
K + +
J +
I +
H +
G +
F +
E +
0+
C+
T a+ 1 GAMMA/FOOT
A~l-===~~=---~?~----=~==--~1 _i_
-200 -100 0 100 200 DISTANCE FROM CENTER OF RAILS IN FEET
Figure 19. Theoretical gradient for 21 profiles over Tunnel 19, DRGW Railroad,
-28-
SOUTH FENCE
PROFILES 1 PHONE LINE
' + u +
+ T +
+ s +
+ R +
+ Q +
+ p +
+ 0 +
+ N +
+ M +
+ L +
+ K +
+ J +
+ + + H +
+ G +
+ F +
T+ E +
10 GAMMAS/FOOT + D +
l_ c + +
I I
+ B I +
I A +
-200 -100 0 100 200 -- -- -DISTANCE FROM SOUTH FENCE AT PROFILE A IN FEET
Figure 20. Actual gradient for 21 profiles over Tunnel 19, DRGW Railroad.
-29-
One type of magnetic anomalies is easy to detect. When a train is
moving through the tunnel, erratic readings of several hundred gamma are
easily detected on a magnetometer. Figure 21 shows a time series of such
readings taken 10 seconds apart. This behavior is always apparent even when
the sensor is 150 feet above the rails. Similar changes (20 to 100 gamma)
in the background magnetic field were noticed when busses were parked or
removed from a lot 100 feet from the base station during the Alexandria
survey. Heavy equipment moving through a mine might be expected to have a
similar effect.
The final test was a borehole magnetic survey conducted at the experi
mental mine of the Colorado School of Mines near Idaho Springs, Colorado.
Geometries built a special sensor for borehole use. It has a 2.5 inch
outside diameter and is approximately two feet in length. It is hermetically
sealed to a 600 foot cable. Figure 22 shows a photograph of the special
sensor. Because of the extra length of the cable the base station magneto
meter with a higher current capacity than the portable unit, must be used
to drive the sensor.
Geometries reports that this special sensor is not as good in cancelling
stray electric fields as their standard sensor (4.5 inch outside diameter).
However, we noticed no difference in the response of the sensor \vhen the
mine lights were on or off during our borehole tests.
We drilled a 34 foot borehole some 30 feet from the entrance of the CSM
experimental mine. Figure 23 shows a photograph of the entrance to the
experimental mine for the drilling operation underway over the mine. A map
and cross sectional view is shown on Figure 24. The borehole was in granite
and passed within 12 feet of the edge of the mine. The borehole was 15 feet
from the center of the rails at its closest point.
We ran a survey over the rails outside the CS~1 Mine with the portable
magnetometer. Figure 25 shows the magnetic profile along the rails. These
mine rails weigh 28 pounds per yard. The rail anomalies measure 1000 gamma
above the background field.
-30-
I w f-' I
- 57000y
APPROACHING
+
t ENGINE AT PORTAL
- 56000y t t CABOOSE LEAVES TUNNEL
TRAIN IN TUNNEL
I I I I I I I I I I I I I I I I 0 60 120 180 240 300
TIME IN SECONDS (ONE READING EVERY 10 SECONDS)
Figure 21. Magnetic anomalies from freight train passing through Carothers Tunnel, B&O Railroad.
Figure 22. Special sensor for borehole magnetomeLer,
·-32-
Figure 23. Drilling rig over Colorado School of Mines experimental mine.
-33-
MINE
RAILS
PROOF BOLTS AND
WIRE MESH )
0'
- -5'
- -10'
29'
- -25'
- -34'
Figure 24. Cross sectional view of borehole and CSM Experimental mine.
-34-
r 58000y
4-- ANOMALY DUE TO CAR DUMPER
Cl ...... ~ ....
I c.J w - 56000y V1 .... I 1.1,1
z c.:l C(
:IE
54000y
I I I I I I I I I I I 0 20 40 60 80 100 120 140 160 180 200
DISTANCE ALONG MINE RAILS IN FEET
Figure 25. Magnetic survey over mine rails at CSM Experimental mine.
The main anomaly (3000 gammas) is due to a mine car dumper, a 2 foot high
by 6 foot long iron hump along the inside rail designed to dump the cars over
the edge of the slag pile.
The borehole survey is shown in Figure 26. Repeated surveys all show
the same results. The main anomaly is not due to the rails. Rather it occurs
at the mine roof level and is due to the proximity of roof bolts, wire mesh,
and steel reinforcements. The readings at this level are always erratic
which is indicative of iron wire loops. The probable mechanism is that the
impulse field from the magnetometer sensor magnetizes these iron loops and
the loop residual magnetism varies from reading to reading.
Conclusions on the Magnetic Experiment
As a result of our experiments with the proton magnetometers we arrive
at the following conclusions:
1. The rail magnetic anomalies are the size expected but they oscillate.
2. The oscillation half-wavelength seems to correlate with the length
of the rail sections.
3. These oscillations further obscure already weak anomalies from the
rails in surveys over the railroad tunnels.
4. We tried computing gradients of the magnetic field in an effort to
enhance an anomaly .of one sign parallel to the rails. However the gradient
does not help to detect the rails appreciably.
5. Moving trains cause erratic readings of 1000 gammas or more and
are easy to detect on magnetometers even when they are placed 150 feet above
the rails.
6. Vehicles (buses, trucks) moved into or out of the vicinity of the
magnetometers, 100 feet or more away, give similar (20 gammas to 100 gammas
or more) changes to the background field. Such changes in the background
(vehicle removed from vicinity within 10 to 30 seconds) are easy to detect.
7. Nearby wires and loops of magnetic material show up easily (a few 100
to 1000 gammas or more) often by erratic behavior or repeated readings.
8. Borehole surveys show promise, but surface magnetometer surveys do
not, especially when the sensor to rail depth of burial is greater than 20
to 30 feet.
-36-
1-... ... I.&.
z a: Q Cl)
z ... Cl)
I.&.
Q
:::c: 1-g, ... Q
0 SURFACE
• X
"' 5 •X
J ~
\ 10 4(
/ K
I 15 )(
I •X (
20 .c
/ X •
25
30
35
55000
...............
)(
( ,. \ k
\
55200
X
55400 55600
•
• SENSOR DESCENDING
X SENSOR ASCENDING
READINGS 30 SECONDS APART
MINE LIGHTS OFF
55800 56000
MAGNETIC FIELD IN GAMMAS
56200
Figure 26. Borehole magnetometer survey at CSM Experimental mine.
-37-
ACTIVE SEISHIC METHODS
An active seismic experiment is one which uses dynamite or some other
source of compressional waves with a geophone and recording system listening
to the response of the medium. Such a system is in contrast to a passive
seismic system which detects and locates whatever is going on, be it earth
quakes, quarry blasting, or tunneling activity. This study is concerned
only with the detection of tunnels by an active seismic system. The objective
is to test whether seismic exploration techniques can detect an underground
tunnel by reflection or refraction evidence.
Cook (1965) shows that underground cavities can be detected by seismic
reflections when the cavity dimensions (diameter of the tunnel) are the same
order of magnitude or larger than the wavelength of the reflected seismic
energy. Thus for tunnels in granite with P-wave velocities of 16000 feet/second
wavelengths of the order of the tunnel diameter (25 feet) would require
appreciable seismic energy at 640 Hz.
f v A
16000 ft/sec 25 ft/sec
640 Hz.
Watkins, et al. (1967) demonstrated another method of seismic detection
of near-surface cavities. This method, which detects oscillations of the
cavity walls on the geophones directly over the cavity, is based upon a
theoretical development by Biot (1952). According to Biot 1 s group velocity
curves, we have for a cylindrical hole in an infinite solid:
f = v
s 1.55D
where V is the shear-wave velocity, D is the diameter of the cylinder, and s
f the resonant frequency of the cavity wall oscillations. Our field experi-
ments were designed to test both methods for detecting tunnels.
We used a standard exploration seismic system owned by the Colorado
School of ~1ines and built by SIE. Figure 27 shows a photograph of the instru
ment recording truck and the geophone implacements along the road. We used
-38-
Figure 27. Seismic recording truck of Colorado School of Mines.
-39-
both 6 geophones per channel and a single geophone per channel, 24 channels,
digital and analog tape recording, geophones of the Hall-Sears Junior style.
(See Appendix A for their characteristics.)
Sources used included dynamite caps and the Dinoseis, a truck-mounted
cylinder filled with propane and oxygen and exploded while in contact with
the ground. Figure 28 shows a photograph of the Dinoseis truck.
Other equipment and sources included the Bison hammer seismograph,
(Bison Manufacturing Company, Minneapolis, Minnesota). A sledge hammer blow
on the ground is recorded by a single geophone onto an oscilloscope with a
digital memory. Subsequent blows can be added (stacked) to the original
permitting shot-array procedures. In addition to the oscilloscope a paper
recorder can provide a permanent record of any desired seismogram showing on
the oscilloscope. Figure 29 shows a photograph of the Bison oscilloscope
and recording units.
The primary sites chosen were two tunnels of the DRGW Railroad near
Eldorado Springs, Colorado. They are single track, in granite, and possess
fairly easy equipment access over the tunnel. The road is 250 feet above
the Tunnel-17 and 120 feet above Tunnel 19. Figure 30 shows the seismic
profile over Tunnel 19; the setup over Tunnel 17 is similar. Railroad
tunnels are larger than the mines we are seeking. However, we expected con
siderable difficulty in seeing the tunnels seismically because the seismic
wavelengths are so long compared to the cross section dimensions of any
tunnel. If the seismic method will not work on a railroad tunnel, we would
not expect it to work on a mine. Other advantages of the railroad tunnels
are that they exist singly and are well mapped. In contrast most of the mines
in Colorado exist in multiple shafts and drifts and are poorly mapped making a
controlled experiment difficult.
There are four Dinoseis records taken at each of the 24 geophone stations
plus six more beyond each end of the geophone spread. The four common shot
point records were added. Figure 31 shows a few of these seismograms where
the Dinoseis sauce is near the center of the spread. The complete set of
these plots are reproduced by the Phoenix computer of the Seismograph Service
Corporation in Denver.
-40-
Figure 28. Dinoseis truck.
-41-
Figure 29. Recording units of the Bison hammer seismograph.
-42-
I ..,. w I
24 23
s ..e:::1.
11 10 9 8 7
120' GEOPHONE SPACING 20'
25' ~N
__L
Figure 30. Seismic profile over Tunnel 19, DRGW Railroad.
t::I'Tl :::0 1-'· CJOQ ::;:;c .., ::oro Pl 1-'·W 1-'1-' .., . 0 Pl P..CJ • (1)
::J rt (1) ..,
~ .;.i rt •
rt Pl ?' (1)
::J
0 < (1) .., >-3 c ::J ::J (1)
1-'
1-' -o
~~
t
. t·· . -~ ~ ttl ~ 11 f ~l~
··••••t! ·• Jtllt ....
..n1l!···'*+H~
1 1 t rlt l
.. ...
.. w
.. "'
.. 0
-l II l II t II
HI· 1l)!JlJ 1Jll_-~-_'ll-!1]·1llUIU .. J~-~--- -j.•··j·tti1·11l1!_
111 · jl···· ·•.1 ·r1r.~~ 0 • ),/ · I i ' (\ ', loJol\ '!I I )\ ... • · 't/.( 1 1 1 1
'"' )rp ' f/,' .lh tr."f.; ~ -:JII).
~ I-' 1 J ~ ij-\' , , l (, ' {-~~....-.)<) ( ( < , •rB. -~-. 1-~~ 1')\ . <.. !"" . ._.,,tJ ! ,)/ .... ~~,., o o t\ -· tr~:- )-:,;., ..• .l;tJ oil +- ({Hr+ .· -.·-·--- ~\(tt_t-1·-tfh·~-l····--- ..... ·f"~+h:- r i:i )-·1·-t_- -~. .- . ~-. \~_~, --,\u","ir~v_---. - ~~-----2~. U .~~~·~;•:;l:~i·~~:·(~ !{\(·qJ·J ~;~\~:~~~ ,'\}'}; .t~,-''jtl 1 !.~r~ )'\1) (l·:.~~~Hb )'"n;<:.~ ·. J}; g;o~ rl )i_hk(rt·~:=:)\.11U~~ \1.1,~)) '_.~~~~\\~~~(· .. , <:(dtL~~?,..-!t~\\\~: HJi{\( ;,~-~~~!{/ r.))(\(J-' · _ ·:~ t-'• 0. I-' • .., 0 Pl (") o.ro . ~
rt ro .., rn
I ro ~ rt v 0 I H1
t:1 t-'• ~ 0
~ ... . .... T ll r .l I lrl I' I I ' rl I lj II n I 'I ' ~ -. . ..... l.. . .. .. .. r ... ···)j . ... .. ' ). ·I) ........ ··r . .. . . \1111 . ... . 1-- --
; ·· ·••· · •· . •• ·· ·· ·• · ·· ···· · · ·· ·· ···• •· ·· · ·· ·· · .. ·· · ·· ··. •••· •• .. •• .. ·· 1 , ·· · ·· ·· 1 .... · lr~~~ ri rl 1111111111111111111 111 1111111111 J 111111111111 llllllllllllllllllllll 11111111111111111111111 mil rrmnn1nrnrr1
The Dinoseis uses two time breaks, one from the recording truck triggering
a second from the Dinoseis truch which actually fires the shot. The programs
in the Phoenix computer respond to the first time break whereas the first
arrivals are consistant with the second one. Since there can be 10 to 20
milliseconds variation between the two, static time corrections had to be
entered into the Phoenix computer to align the four common shot point records.
As a result of this adjustment the Phoenix computer has aligned the second
time break at the 0.1 second mark which becomes the time origin for these
plots.
In addition a time correction for weathering and the one-way travel time
to the top of the tunnel was applied to each of the 23 different shot-point
records along the spread. Then these 23 records with both static and dynamic
corrections were summed together yielding the single record playout shown on
Figure 32.
The objective of summing the 23 corrected records was to suppress coherent
noise and enhance any diffraction patterns emanating from the tunnel. If the
diffraction effect from the tunnel is present, it is not readily apparent.
Similar computations were applied to the data from the Tunnel 17 profile.
Figure 33 shows the center records with no corrections applied. The remaining
seismograms are shown in the Appendix. Again no reflection evidence of the
tunnel is apparent.
Two kinds of stacking procedures were applied to the Tunnel 17 data.
The first shown on Figure 34 is conventional where corrections for two-way
travel time are applied. The second on Figure 35 shows three records with
the static and dynamic corrections applied as for Tunnel 19. The center
record has been corrected to a one-way travel time where the tunnel is. The
other two records have been corrected to the top of a tunnel under one or the
other end of the profile where, of course, there is no tunnel. If this
stacking operation were helping, the center seismogram would look appreciably
different than the other two. It does not.
A major problem arises from the presence of a low velocity layer.
Figure 36 shows the geophone spread and two layer cross section over Tunnel 17.
-46-
HlD TF= 1
.... I);
1)··.
a .
.. ... . t1;
..... I);
.. 1) •
.... a.
. . r··· () .
Figure 32. Summation record over Tunnel 19 with static and dynamic corrections applied.
-47-
(1) ;:l rt (1)
" 'JJ ro rt
u H1
t::! f-'· ;:l 0 CJl
1(1) .p. f-'· OJUJ
I " (1) (l 0
" 0.. [f)
rt Ill ;>;" (1) ;:l
0 < (1)
" >-] c ;:l ;:l Q I-'
I-' \.0
"IlL
• ! 1;
.. I. • I . ~ ·, ~
.. ,· ..... ' ... i .. ! .
,::· ·---·······---
••• 1 •• L
--- •. ·-----1
., ...• ··: ··:··
- ··1---,-··i····'--··---·--·-·-j-··'--·--
:···i : ••• 1 ••• ,. , . r·
-1·-
, !-..... ,·
' ,: ' ' ;, ~ ~ ~
a · ·~ ':1, '~~· b; ·,~~ .:~ ~~ · ·'·b~ .:,,
Figure 34. Conventional stacking of Tunnel 17 data corrected for two-way travel times to tunnel. -49-
. :_,
-. ,·
.0
.6
'I
,;3 ........... +····~
.9
-t. ,..,t, -::rr i'l I
S4..,-/J uu;~J:. ""'-f"'o -1~~-ee.-di{fe ,.._,..-r r•-1! 1 dd;i: C ~,,.. .. ·c
l ' .
Figure 35. Three stacked records over Tunnel 17, assuming a tunnel at the south end, center, and north end respectively.
-50-
j
. ' ~ ( r ! 'It
1
r' /)! . ·' ( ••
I\ I I
. ·...:
·,r)
.... '7
8
g
7600
...I
~ 7500 1.1.1 ...I
cc 1.1.1 en 1.1.1
> 0 a:l cc z 7400 0
Icc > 1.1.1 ...I 1.1.1
3 5 7 9 11 13 15 17 19 21 23
............... - v1 = 3350 ft./sec ·~ ..
-200
..._. ............. .
-100 0
TUNNEL 17 DRGW-RR
100
DISTANCE FROM PROFILE CENTER (FEET)
v2 = 15500 ft./sec
200
Figure 36. Seismic cross section over Tunnel 17, DRGW Railroad.
-51-
300
An analysis of first breaks shows that the upper layer has a velocity of
3350 feet/second whereas the lower medium with the tunnel has a velocity of
15,500 feet/second. Since the sine of the critical angle is v1;v
2, all
energy more than 12.5° from the vertical will be totally reflected. Thus
less than 14% (ratio of 12.5°/90°) of the energy from the source will pene
trate the lower layer. The seismograms are showing mainly the energy trapped
in the upper layer.
Other negative evidence comes from Biot's (1952) relation for the
resonance of the cavity walls. With a shear velocity of 3800 meters/second
in granite and a tunnel diameter of 8 meters, the expected resonnant frequency,
f, of the tunnel walls would be:
f v
s 1. 55D
3800 m/sec 12 .. 4 m
307 Hz
which is significantly higher than the 50 Hz energy on the records.
Better evidence of tunnel reverberations showed up with the Bison equip
ment over a coal mine. A coal seam at Walden, Colorado was being strip
mined by the Kerr Coal Company when one piece of equipment fell into an old
coal mine which was not known to be there. Bison records show the differences
over solid coal and over cavities (see Figure 37). The response over solid
coal is impulsive; the response over the stope reverberates for several cycles
longer.
We contoured the map in t11VO levels, cross hatching the area where
reverberation records were recorded and leaving blank the area where impulsive
records were recorded (see Figure 38). We postulate that the reverberations
lie over the route followed by the old coal mine. As of this writing the coal
strip mining has not proceeded far enough to verify the precise route of the
mine.
Conclusions on the Seismic Experiments
As a result of our seismic experiments we concluded:
1. No reflection evidence of the railroad tunnels showed up on our
records, whether the sources were Dinoseis, dynamite caps, or Bison hammer.
2. The stacking of two dozen seismograms to enhance diffraction patterns
from the top of the tunnel was not successful.
-52-
IMPULSIVE. OVER SOLID COAL
RINGING. OVER CAVITY
0
Figure 37. Typical Bison records over coal mine.
-53-
I IMPULSIVE\
SCALE: It---~ 10'
Figure 38. Map over coal seam with contours showing region of reverberation
seismograms.
-54-
3. No evidence of wall reverberations as reported by Watkins (1967)
appeared on the seismograms over the railroad tunnels.
4. The primary problem was a low velocity layer over the granite which
trapped most of the seismic energy. Hence very little energy penetrated the
granite layer which contained the tunnel.
5. Some evidence of tunnel wall reverberations appeared over a coal
mine. At this site although the tunnel is smaller, there was no low velocity
layer (coal outcropped and was being strip mined) and the P-wave velocity is
much lower than that for granite.
6. Seismic field experiments to detect tunnels by reverberations of the
walls should be conducted using multichannel (24) seismic systems rather than
single channel systems like the Bison. Neighboring traces from single channel
systems are often difficult to correlate.
-55-
ESTIMATION OF THE DETECTABILITY OF MINING ACTIVITIES BY
MONITORING AIRBORNE Rn222
The detection of a source of radioactive gas for given conditions of
atmospheric stabiilty depends primarily on the source strength (Q), the wind
velocity (u), and the limit of detection (x).
For a m1n1ng operation which advances 4 meters per day with a cross
section of 4m2
, Q is calculated as follows for granite based on the parameters
in Table 1. Metamorphic rock is assumed to be the same composition as granite
and basalt is omitted because it averages five times less uranium than granite.
Volume of rock mined, v
Uranium content for granite, u g
TABLE
16
25
4
1
3 m /day
X 10-6
X 10-6
6 or 16 x 10 cc
g U/g rock (upper range)
g U/g rock (average)
The specific activity of U, U s o.332 ~ci/g u238
~Ci 2.2 x 10 6 disintegrations per minute (dpm)
The density of granite, ~ 2.8.
S . R 222 · d h of u238 only d h h. h t · 1nce n 1s a aug ter an t e 1g es concentrat1on
that can exist in the activity in equilibrium with the parent isotope, the
Rn222
activity in a rock will be equal to the u238 activity of 0.332 ~Ci/g U.
Since U = 25 x 10-6 g U/g rock the total Rn222 activity = 8.3 x 10-6 ~Ci/g
g rock if the upper range of U in granite is chosen. The amount of rock mined
6 6 per day is Vp = 16 x 10 cc x 2.8 g/cc = 45 x 10 g. The source strength, Q,
is 8.3 x 10-6 vCi/g rock x 45 x 10 6 g rock/day or 375 ~Ci/day if we assume
that all of the available Rn222
escapes from the rock material.
Although the detection limit of Rn222 is very low in the laboratory, the
practical limit of detectability in the natural environment is limited by the
natural background which is omnipresent and quite variable depending mainly
on wind velocity, atmospheric stability, barometric pressure, and uranium
-56-
content of the terrain. -6 3 3 A concentration of x = 10 ~Ci/m (2.2 dpm/m ) would
probably not be detectable above background, but let us for purposes of
illustration take this level as the minimum detectable level (MDL). The
ground level (2 m) concentration of a-active aerosols (regarded as represen
tative of Rn222
concentration) was found by Kirichenko (1970) to vary from
2 x 10-4 to 6 x 10-4 ~Ci/m3 and a typical value given by Rankama and Sahama
(1949) is 10-4 ~Ci/m3 , so the value of x = 10-6 ~Ci/m3 for background is
probably too low by a factor of 100 even if a signal to noise ratio of 1:1
could be accepted.
The areas over which a given source of radioactive gas is detectable have
been calculated for a variety of combinations of atmospheric stability, wind
velocity, detection limit, and source strength. The relation between detection
limit, source strength and wind velocity reduces to a single parameter
(x/Q) ~which has the units of ~Ci/m2 • For x = 10-6 ~Ci/m3 , Q = 375 ~Ci/d and a wind velocity of 1 mph (.44 m/sec):
X u = Q
10-6 ~Ci/m3 375 ~Ci/d (.44 m/sec) (86,400 sec/day)
The larger this quantity, the smaller will be the area of detection.
Therefore, an increase in wind velocity or detection limit, or a decrease in
source strength will decrease detectability.
The area of detectability calculated for moderately stable conditions - -4 -2 and (x/Q)u = 1 x 10 m gives a maximum range of detection of 1 km for a
stationary sampler directly downwind from the source.
Since we have taken the most optimistic parameters, i.e., moderately
stable atmospheric conditions, wind velocity of 1 mph, a signal to noise ratio 222 of less than 1, total release of all Rn in the rock, and a granite with
the upper range of uranium content, we can conclude that it is highly unlikely
that mining activities of the type contemplated can be detected at useful
d . b . . i b Rn 222 1stances y mon1tor1ng a r orne •
-57-
If the calculations were carried out using what might be termed a
"reasonable" set of parameters rather than the "conservative" values used v,1e
would have
and
u 4 wg/g so Q = 60 wCi/d g
-4 -6 3 x 10 wei rather than 10 wCi/m
u 5 mph rather than 1 mph.
(x/Q)u would be a factor of 3000 larger and the area of detectability would
actually be extremely small.
-58-
REFERENCES
Biot, M. A., 1952, "Propagation of elastic waves in a cylindrical bore
containing a fluid", Jour. Appl. Physics, Vol. 23, pp. 997-1005.
Breiner, s., 1973, "Applications manual for portable magnetometers" printed
by Geometries, Inc., 914 Industrial Ave., Palo Alto Calif. 94303, USA.
Cook, John c., Aug. 1965, "Seismic mapping of underground cavities using
reflection amplitudes"; Geophysics, Vol. 30, No. 4, pp. 527-538.
Godson, R. H. and Watkins, J. S., "Seismic resonance investigation of a near
surface cavity in Anchor Reservoir, Wyoming", Bull. Assoc. of Engr 1 g.
Geologists, Vol. V, No. 1, pp. 27-36.
Kirichenko, L. V., 1970, "Radon Exhalation from Vast Areas according to Vertical
Distribution of its short-lived decay products", Jour. Geophys. Res.,
Vol. 75, No. 18, pp. 3639-3649.
Rankama, K. and Sahama, T. G., Geochemistry, the University of Chicago Press,
Chicago, 1949.
Watkins, J. s., Godson, R. H., and Watson, K., 1967, "Seismic detection of
near-surface cavities'', Professional Paper 559-A, U. S. Geological
Survey, pp. 1-12.
....59-
ACKNOWLEDGEMENT
The author gratefully acknowledges the help received in conducting this
study and writing the report. Dr. Donald Schutz of Teledyne Isotopes
conducted the study and wrote the section on Radon decay. Dr. Phillip Romig
of the Colorado School of Mines (CSM) directed the seismic field efforts and
acted as chief consultant throughout the project on the seismic and magnetic
methods. Mr. Jim Callaway of CSM did the weathering and derived the seismic
cross section as well as conducted the data processing at SSC's Phoenix
Computer. Mr. Steven Peterson of CSM measured the frequency responses of
the Hall-Sears geophones.
-60-
APPENDIX A
A-1
APPENDIX A
Response Curves for Geophones (Hall-Sears-Junior type) on the Shaking Table
Apparatus
1. Audio oscillator and power amplifier (GRC 1308-A)
2. Oscilloscope (Tektronix 561, with plug-in units 63 and 67)
3. Colorado School of Mines shaking table
4. Oscilloscope with transducer and strain gauge preamplifier
(Tektronix 531, with plug-in unit G)
5. One string of geophones (Hall-Sears-junior) with 6 geophones
per string.
Procedure
The shaking table was operated at a constant displacement for frequencies from 20- to 800-Hz using the apparatus described above and portrayed in Figure 2. The movement of the shake table is monitored by the Tektronix 531 oscilloscope. The voltage output from the geophones is measured by the Tektronix 561 oscilloscope. The peak-to-peak voltage output from the string of geophones was recorded at 20-Hz intervals.
Results
The frequency response of this geophone is shown on Figure A-1.
Harmonic distortion is observed for these geophones as follows: 1. From 280- to 340-Hz and from 500- to 600-Hz for a 1/8-micron
displacement.
2. From 500- to 600-Hz for a 1/4-micron displacement.
3. From 360- to 400-Hz for a 1/2-micron displacement.
The logarithmic frequency response is nonlinear above 300-Hz.
Preceding page blank A-2
100~----~--~--~~~~~~----~--~--~~~~~~----~
80
60
40
• • • 20 ••• •• • • •• • • • • • ' •• 10 •• •
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.1------_.--~--~_._.~~~------~----~----~~,._ ______ _ 10 20 40 60 80 100 200 400 600 1000 2000
800 FREQUENCY (Hz)
Figure A-1. Frequency response of the Hall-Sears junior geophone.
A-3
APPENDIX B
B-1
_;\PPENDIX B
Seismograms from north-south profiles over the Denver and Rio Grande
Western Railroad Tunnel 19 and Tunnel 17 near Eldorado Springs, Colorado.
Source: 4 Dinoseis shots at each geophone station
Geophones: Hall-Sears-junior
Geophone Spacing: 20 feet
Seismograph: SIE Digital System owned by Colorado School of Mines
Playout: from Phoenix computer of Seismograph Service Corporation,
Denver, Colorado.
Preceding page blank B-2
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