b
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Design of a Deep Borehole Tiltmeter
J.C. HARRISON, J. LEVINE & C.M. MEERTENS, Boulder, CO
with 2 figures
Harrison, J.C., Levine, J. & Meertens, C.M., 1983: Design of
a Deep Borehole Tiltmeter. Proceedings of the Ninth International Symposium on Earth Tides, pp. 2 7 3 - 2 8 1 .
Abstract: A deep borehole tiltmeter.has been developed which
can be operated below the near surface layers to reduce the
influence o f meteorological effects. This is relatively inexpensive to build and to install. A 15 cm diameter borehole
is cased with steel irrigation pipe and has a stainless steel
instrument compartment 10 cm in internal diameter at the
bottom of the hole. The tiltmeter is contained in a 2 m
stainless capsule held against the sides of the hole with flat
springs. The tilt sensors are mounted on a platform which can
be leveled by means of wtorscontrolled from the surface,
allowing for hole deviations of up to five degrees from the
vertical. A number of different tilt sensors have been used
on such platforms. Simple pendulums and horizontal pendulums
have so far yielded the best results. A depth of 33 m is
normally used, although this is not a critical aspect of the
design, because the electronics are inside the instrument
capsule. The instruments are capable of operating unattended
for long periods of time at tidal sensitivity; results of our
tidal measurements can be found as well in these proceedings.
Keywords: borehole tiltmeter, secular tiit,.tilt tides,
vertical pendulum.
1. Introduction
A t the time of the Eighth International Symposium on Earth
rides in Bonn, it was clear to many of us that there are important
advantages to making earth tide tilt observations with borehole
tiltmeters, and that many applications--such as the investigation
of elastic inhomogeneities in the crust or earthquake prediction-would require deploying a considerable number of these instruments.
We therefore set out to develop a borehole tiltmeter which would
be relatively cheap to build and install; this not only meant
building an inexpensive tiltmeter but it also meant using a small
diameter hole and not going any deeper than necessary.
2 . Borehole design
The holes we use are nominally 15 cm in diameter and 33 m deep.
After the hole is drilled, a steel casing 135 rmn in diameter witha
wall is pressed into the hole. The casing is shipped to the
Publisher: E. Schwekerbart’sche Verlagsbuchhandlung, D-7000 Stuttgart
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s i t e i n l e n g t h s of six m e t e r s , where i t i s w e l d e d i n t o a continuous
w a t e r t i g h t p i p e a s i t i s lowered down t h e h o l e .
The c a s i n g i s
s e a l e d i n p l a c e b y means of cement poured i n t o t h e bottom of t h e
h o l e b e f o r e t h e casing i s i n s e r t e d , and around t h e s i d e s of t h e
casing a f t e r it i s i n place.
The s t e e l c a s i n g t e r m i n a t e s a t t h e bottom i n a s t a i n l e s s s t e e l
s e c t i o n t h a t i s 2.3 meters long and which i s used t o hold t h e t i l t m e t e r c a p s u l e . The bottom s e c t i o n i s 1 1 . 5 Cm i n o u t s i d e d i a m e t e r ,
and i t h a s a w a l l t h i c k n e s s of 6 mm.
The t o p of t h i s s e c t i o n has
a t r a n s i t i o n s e c t i o n t o t h e s t a n d a r d carbon s t e e l p i p e which cons t i t u t e s t h e r e s t of t h e c a s i n g .
The bottom s e c t i o n i s c l o s e d a t i t s lower end by a p l a t e welded
on i n t h e shop. A hemispherical knob i s welded t o t h e i n s i d e of
t h e bottom p l a t e t o s u p p o r t t h e weight o f t h e t i l t m e t e r c a p s u l e .
3. I n s t r u m e n t c a p s u l e
The i n s t r u m e n t c a p s u l e i s a 1 . 8 meter long s t a i n l e s s s t e e l t u b e
which i s c l o s e d a t t h e bottom and has a p a i r of c o n t a c t p o i n t s and
a f l a t s p r i n g welded on n e a r i t s t o p and a second p a i r w i t h a
second f l a t s p r i n g n e a r i t s bottom. The t o p o f t h e c a p s u l e i s
s e a l e d w i t h a cap a t t a c h e d by screws a n d t h a t c o n t a i n s an O-ring.
The cap h a s a w a t e r - t i g h t opening f o r t h e e l e c t r i c a l c a b l e t o pass
t h r o u g h , hooks f o r a t t a c h i n g t h e l i f t i n g c a b l e , and a p o s t f o r
a t t a c h i n g t h e o r i e n t i n g rods.
The c a p s u l e was designed t o minimize t i l t - s t r a i n coupling due t o
c a v i t y e f f e c t s . HARRISON (1976) has shown t h a t t h e r e i s no c a v i t y
e f f e c t i f t h e s i d e o f t h e borehole i s used a s a r e f e r e n c e a x i s f o r
t h e t i l t m e t e r , and o n l y a s m a l l e f f e c t i f t h e c e n t e r o f t h e bottom
o f t h e h o l e i s used a s one r e f e r e n c e p o i n t . The c a p s u l e i s r a i s e d
and lowered by hand i n a few minutes u s i n g a s t a i n l e s s s t e e l l i f t i n g
c a b l e a t t a c h e d t o t h e top cap.
The l e n g t h o f t h e c a p s u l e a l s o improves t h e c o u p l i n g between t h e
t i l t m e t e r and t h e E a r t h by i n c r e a s i n g t h e e f f e c t i v e l e v e r arm from
a few c e n t i m e t e r s ( t h e l e n g t h of t h e t i l t m e t e r ) t o approximately
two m e t e r s ( t h e l e n g t h o f t h e c a p u s l e ) .
F i g u r e 1 shows t h e c a p s u l e i n s t a l l e d a t t h e bottom of t h e borehole.
4 . Capsule o r i e n t a t i o n
To compare the t i l t s recorded by d i f f e r e n t i n s t r u m e n t s o r t o
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compare t h e t i d a l t i l t with theory, i t i s n e c e s s a r y t o know t h e
o r i e n t a t i o n of t h e t i l t m e t e r .
Magnetic sensors i n s i d e t h e c a p s u l e
which s e n s e t h e d i r e c t i o n o f t h e E a r t h ' s magnetic f i e l d cannot b e
used i n t h i s i n s t a n c e s i n c e t h e c a s i n g i s magnetic. The c a p s u l e
i t s e l f i s n o t v i s i b l e from t h e s u r f a c e , so t h a t w e cannot determine
t h e o r i e n t a t i o n by s i g h t i n g on t h e c a p s u l e from t h e t o p of t h e h o l e .
STEEL
CASING
I4 cm 0.0.
3 0 m LONG
F i g . 1. A schematic diagram of t h e t i l t m e t e r c a p s u l e i n s t a l l e d
a t t h e bottom of a borehole 33 meters deep.
We have developed a system i n v o l v i n g a s e r i e s of l i g h t r o d s t o
d e t e r m i n e t h e o r i e n t a t i o n of the c a p s u l e a t t h e bottom of t h e h o l e .
A p o s t i s welded t o t h e t o p cap of t h e t i l t m e t e r c a p s u l e w i t h a
f l a t s i d e o r i e n t e d s o t h a t t h e normal t o t h e f l a t s i d e i s a l o n g t h e
a x i s o f one o f t h e t i l t m e t e r s . A d d i t i o n a l s e c t i o n s o f rod are
a t t a c h e d as t h e c a p s u l e i s lowered.
The f i r s t s e c t i o n a t t a c h e s t o
- 276 the post on the capsule using a trapped ball and a detent to
provide an easily removable coupling; subsequent sections bolt
together with small screws. Each section is notched so that it
can only be attached in one orientation. After the orientation of
the top notch is determined using a transit and a compass, the
entire series of rods is removed from the capsule by simply lifting
gently to disconnect the bottom rod from the tiltmeter capsule.
This method has been used to determine the azimuth of instruments
at the bottom of holes 33meters deep with an uncertainty of about
1 O . It is difficult to extend this technique to holes much deeper
than 33 meters since the weight of the rods becomes appreciable,
and the capsule becomes very difficult to handle.
5 . Leveling platform
The leveling platform mounts inside the tiltmeter capsule and is
used to support and level the tilt sensors. It is supported on a
three-point kinematic mount. Two of the supports are screws connected to small motors. These motors are driven from the surface
and are used to re-zero the instrument. The leveling platform has
a range of approximately five degrees in any direction, so that
the instrument must be vertical to within five degrees to start
with. This limited re-zeroing capability imposes some constraint
on the drillers, but the required tolerance can be met in 33 meter
deep holes.
Figure 2 shows the tiltmeters and the leveling platform.
6 . Tilt sensors
All of our instruments use mechanical tilt sensors. The sensors
are pendulums mounted on fine wire suspensions, manufactured by
Larry Burris of Instech, Inc. We have experimented with three types
of sensors: simple pendulums 5 cm in length, small horizontal pendulums with a one-second free period, and a rather complex "straight
line level" with a free period of three seconds. Our $est results
t o datehave come from the horizontal pendulums, and we expect to
use this type of sensor in our future instruments.
In all of the sensors the pendulum is placed between two fixed
plates separated by about 1 um to form two arms o f a capacitance
bridge. The other two arms are formed by two fixed impedances,
traditionally the center-tapped secondary o f a carefully balanced
transformer. Two precision resistors are used to complete the
- 277 If t h e two o u t e r p l a t e s a r e d r i v e n out-of-phase by a 2 1 kHz
bridge.
a c s i g n a l , t h e s i g n a l a t t h e c e n t e r p l a t e has a magnitude proport i o n a l t o t h e d e f l e c t i o n o f t h e center p l a t e from t h e e l e c t r i c a l
midpoint of t h e system ( i . e . from t h e p o s t i o n a t which t h e
p o t e n t i a l i n t h e gap i s t h e same as t h e mean of t h e outer p l a t e
d r i v i n g v o l t a g e s ) , and a phase ( r e l a t i v e t o t h e d r i v i n g s i g n a l )
g i v i n g t h e d i r e c t i o n of t h e d e v i a t i o n .
SUPWRT ROD
FOR ELECTRONICS
.. _ .
SCALE:
0
t
F i g . 2.
platform
t h e tilt
shown i s
INCHES
I
2
1
S c a l e view o f t h e t i l t s e n s o r s mounted on t h e l e v e l i n g
i n s i d e o f t h e c a p s u l e . The e l e c t r o n i c s package i s above
sensors i n s i d e t h e c a p s u l e and i s n o t shown. A l s o n o t
t h e thermal i n s u l a t i o n .
7 . Tiltmeter electronics
- 278 Since both t h e amplitude and phase of t h e s i g n a l a t t h e c e n t e r
p l a t e carry information, w e use a phase-sensitive d e t e c t o r t o
d e r i v e a v o l t a g e p r o p o r t i o n a l t o t h e t i l t . The r e f e r e n c e f o r t h e
phase d e t e c t o r i s obtained from t h e d r i v e s i g n a l t o t h e o u t e r
The s i g n a l i n p u t t o t h e p h a s e - s e n s i t i v e d e t e c t o r i s d r i v e n
plates.
by a p r e a m p l i f i e r having a g a i n of s e v e r a l thousand. The a c t u a l
a test
t a b l e and a d j u s t i n g t h e g a i n t o y i e l d an o v e r a l l s e n s i t i v i t y a t t h e
o u t p u t of nominally 2 v o l t s / m i c r o - r a d i a n .
gain of the instrument is set by running t h e tilt s e n s o r on
The electronkcboards a r e designed and b u i l t by J e r r y Larson o f
Maryland I n s t r u m e n t a t i o n .
An e q u a l l y d i f f i c u l t problem i s t h e s e n s i t i v i t y o f t h e system
t o f l u c t u a t i o n s i n the p a r a s i t i c capacitances i n t h e c i r c u i t
( e s p e c i a l l y i n t h e components t h a t d r i v e t h e o u t e r p l a t e s ) and
between t h e connecting w i r e s and ground. The c a p a c i t a n c e between
t h e c e n t e r p l a t e and e i t h e r end p l a t e i s of o r d e r 3 p f , and a f u l l s c a l e t i l t , of o r d e r 5 m i c r o - r a d i a n s , r e s u l t s i n a change of only
about one p a r t p e r m i l l i o n i n t h i s c a p a c i t a n c e . Thus t h e e n t i r e
system must have s t r a y c a p a c i t a n c e s whose v a l u e s change by l e s s
than a small f r a c t i o n o f a p i c o f a r a d , a n d i n p r a c t i c e , t h i s r e q u i r e ment i s f a r more s t r i n g e n t than t h e need f o r high e l e c t r i c a l g a i n .
We have addressed t h i s requirement by keeping t h e system a s s m a l l
as p o s s i b l e and by p l a c i n g t h e e n t i r e e l e c t r o n i c s package i n t h e
t e m p e r a t u r e - s t a b i l i z e d environment a t t h e bottom o f t h e h o l e .
The d r i v e v o l t a g e f o r t h e o u t e r p l a t e s i s o b t a i n e d from a small
step-up transformer.
The secondary of t h e t r a n s f o r m e r d r i v e s a
pair- of matched r e s i s t o r s t o ground; i n t h e c i r c u i t , t h e b r i d g e
i s formed between t h e two c a p a c i t a n c e s o f t h e t i l t s e n s o r and t h e
two p r e c i s i o n r e s i s t o r s , and t h e balance may be trimmed by i n s e r t i n g a small p o t e n t i o m e t e r i f d e s i r e d . Although we have n o t
found t h i s t o be d e s i r a b l e , i t has been n e c e s s a r y t o a d j u s t t h e
r e l a t i v e phase of t h e two o u t p u t s s o t h a t t h e two p l a t e s a r e d r i v e n
e x a c t l y out o f phase.
Unless t h i s i s done q u i t e c a r e f u l l y , t h e
s i g n a l a t t h e i n p u t s t a t e does n o t go t o zero even a t t h e e l e c t r i c a l c e n t e r due t o t h e a p p r e c i a b l e q u a d r a t u r e v o l t a g e . The
quadrature v o l t a g e may be l a r g e enough t o s a t u r a t e t h e i n p u t s t a g e ,
if t h e system i s s u f f i c i e n t l y o u t of b a l a n c e , thereby l i m i t i n g t h e
s e n s i t i v i t y of t h e system. The trimming c a p a c i t o r s r e q u i r e d t o
cancel t h e quadrature v o l t a g e a r e on t h e o r d e r of a few p i c o f a r a d s
S O t h a t l e a d d r e s s i s c r i t i c a l . The e n t i r e c i r c u i t i s enclosed i n a
s h i e l d e d box t o minimize changes i n t h e s t r a y c a p a c i t a n c e s . Never-
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t h e l e s s , i t i s p o s s i b l e t h a t some o f o u r long-term t i l t s and s o m e
o f t h e r e s i d u a l s e n s i t i v i t y t o temperature changes a r e caused by
changes i n t h e v a l u e s of some of t h e s e components w i t h a r e s u l t i n g
change i n t h e b a l a n c e p o i n t o f t h e c a p a c i t a n c e b r i d g e .
8 . D i g i t i z i n g systems
Every h o l e has i t s own d i g i t i z e r .
The most important advantage
of t h e d i g i t i z e r i s t h a t i t ' e l i m i n a t e s t h e need € o r t r a n s m i t t i n g
a n a l o g t i l t i n f o r m a t i o n on t h e s u r f a c e where i t i s s u b j e c t t o n o i s e
pickup.
It a l s o i n c r e a s e s t h e i s o l a t i o n between h o l e s and p r e v e n t s
l a r g e ground l o o p s between widely-spaced i n s t r u m e n t s . The d i g j t i z e r
i n each h o l e i s made up o f an analog m u l t i p l e x o r , an a n a l o g - t o d i g i t i z e r c o n v e r t e r , and a c o n t r o l s e c t i o n which i s made up of
approximately 25 i n t e g r a t e d c i r c u i t s .
The d i g i t i z e r communicates w i t h t h e r e c o r d i n g system v i a two
o p t i c a l l y i s o l a t e d coaxial cables using a b i t - s e r i a l transmission
system. The t r a n s m i s s i o n system i s t h e same a s o r d i n a r y RS-232C
e x c e p t t h a t t h e v o l t a g e l e v e l s a r e s t a n d a r d TTL l e v e l s r a t h e r t h a n
t h e b i - p o l a r l e v e l s used i n t h e RS-232C p r o t o c o l .
By o p t i c a l l y
i s o l a t i n g t h e c a b l e s , t h e i n s t r u m e n t i s decoupled from t h e ground
a t t h e r e c o r d i n g system which may be l o c a t e d s e v e r a l hundred m e t e r s
away.
The Boulder s i t e i s connected by a d e d i c a t e d telephone l i n e t o a
PDP 1 1 / 3 4 computer on t h e U n i v e r s i t y o f Colorado campus. T h i s
computer i n t e r r o g a t e s t h e d a t a l o g g e r s e v e r y 6 minutes and s t o r e s
t h e v a l u e s on d i s k .
A t t h e E r i e s i t e (24 km from Boulder), t h e
d a t a l o g g e r s a r e i n t e r r o g a t e d by a l o c a l microprocessor which s t o r e s
up t o s i x hours of d a t a . The d a t a a r e t r a n s m i t t e d t o Boulder over
a r a d i o l i n k once p e r hour.
9 . Power supply
The power consumption of t h e e l e c t r o n i c s package d e s c r i b e d above
t o g e t h e r with t h e d i g i t i z i n g system i s approximately 7 w a t t s . This
i s somewhat more 'than can be c o n v e n i e n t l y s u p p l i e d by b a t t e r i e s .
All o f o u r e x i s t i n g s i t e s have commercial power nearby. The power
supply w i l l be a r e s t r i c t i o n on s i t e s e l e c t i o n f o r t h e f o r e s e e a b l e
future.
The d a t a l o g g e r boards and t h e DC-DC c o n v e r t e r t h a t s u p p l i e s
power
t o t h e t i l t m e t e r s suspend i n a p l a s t i c c a p s u l e about 5 m
i n t o t h e b o r e h o l e from t h e s u r f a c e .
The DC-DC c o n v e r t e r allows us
t o d e f i n e a completely independent ground system f o r t h e t i l t m e t e r s ,
t h u s i s o l a t i n g them from s u r f a c e e l e c t r i c a l n o i s e .
1 0 . Summary
We have developed a borehole t i l t m e t e r t h a t i s capable o f opera t i n g for long periods o f time a t t i d a l s e n s i t i v i t y .
In addition
t o t h e t i l t m e t e r and i t s a s s o c i a t e d e l e c t r o n i c s , we have c o n s t r u c t e d
a p p r o p r i a t e a n c i l l a r y hardware t o a c q u i r e and analyze d a t a from
many widely s e p a r a t e d instruments.
W e c u r r e n t l y have s i x of t h e s e i n s t r u m e n t s i n o p e r a t i o n . R e s u l t s
t o d a t e are d e s c r i b e d i n a companion paper i n t h i s volume.
ACKNOWLEDGEMENTS
This work i s supported i n p a r t by t h e A i r Force Geophysics
Laboratory and by t h e National Bureau o f Standards.
REFERENCES
H a r r i s o n , J . C . , 1976. Cavity and topographic e f f e c t s i n tilt and
s t r a i n , J . Geophys. Res., 81: 319-328.
H a r r i s o n , J . C . & Meertens, C.M.
C o o p e r a t i v e I n s t i t u t e f o r Research i n Environmental S c i e n c e s
Campus Box 449
U n i v e r s i t y o f Colorado
Boulder, CO 80309
Levine , J.
J o i n t I n s t i t u t e f o r Laboratory Astrophysics
Campus Box 440
U n i v e r s i t y o f Colorado
Boulder, CO 80309