Two clusters of instruments were emplaced in fractured bedrock in eastern Massachusetts in 1970 and 1975. The intrasite agreement at tidal periods was about two percent, but there is no agreement at longer periods. A strong temperature-induced annual component ranging from 3 to 15 urads was present on instruments installed at depths of 15-20m; it was not apparent on those at 100-120m. One instrument, in continuous operation for three years at 100m, showed a net drift of 0.3 urads down to the SW, with a maximum departure of 2.0 urads from the trend. Pore pressure variations, material corrosion and creep, and local movements are apparently the limiting factors to long-term measurements

Cabaniss, G. H.

English

NASA Technical Reports Server

The Measurement of Long Period and Secular Detormauun
Deep Borehole Tiltmeters
Gerry H. Cabaniss
Air Force Geophysics Laboratory
Hanscom Air Force Base, Massachusetts 01731
21484
Abstract. Biaxial borehole tiltmeters have
been or soon will be deployed in deep boreholes
by groups in W. Germany, England, the United
States, and Canada. Their purpose is to mea-
sure tidal loading and premonitory earthquake
phenomena as well as long-period and secular
deformations. Two clusters of instruments
were emplaced in fractured bedrock in eastern
Massachusetts in 1970 and 1975. The intra-
site agreement at tidal periods was about two
percent, but there is no agreement at longer
periods. A strong temperature-induced annual
component ranging from 3 to 15 urads was pre-
sent on instruments installed at depths of 15-20m;
it was not apparent on those at 100-120m. One
instrument, in continuous operation for three
years at 100m, showed a net drift of 0. 3 urads
down to the SW, with a maximum departure of
2.0 urads from the trend. Pore pressure vari-
ations, material corrosion and creep, and local
movements are apparently the limiting factors
to long-term measurements.
Deep Borehole Tiltmeter Experiments
Borehole instrumentation for the measure-
ment of tilt (as well as strain and stress) has
proved attractive for geophysical measurements
because of the isolation from surface noise, such
as that produced by meteorological and cultural
sources, as well as the logistical advantage pro-
vided by the option of drilling at sites of interest
as opposed to dependence upon mines and caves.
Of even greater importance is. the fact that bore-
hole measurements should, in principle, be vir-
tually immune to cavity effects [Harrison, 1976],
which dominate the results from short-base tilt
measurements made in niches carved in the walls
of mines [Melchior, 1978]. Additionally, it is
comparatively simple to deploy clusters of in-
struments to mitigate local perturbations.
Disadvantages include short baselength, cost,
the lack of standardized instrumentation, and
installation problems. Measurements from in-
struments installed at shallow depth (about 3m)
in soils [Mortensen & Johnston, 1975], Lewco-
wicz & McConnell, 1977] have proved to be hea-
vily contaminated by temperature and rainfall
effects [Wood and King, 1977]. To avoid these
the instruments must be placed at substantially
greater depth. There we are confronted with
groundwater problems, pore pressure changes,
casing corrosion, etc. The short baselength
means that very small vertical displacements
and local tilting, which is not representative of
the region under study, can dominate the mea-
surements. Material creep, stress corrosion,
stress release, and mineral weathering of the
order of several atomic diameters, can produce
significant spurious tilts. A number of long
Proc. of the 9th CHOP Conference. An Inlemalional Symposium on the Appliraliuns of
Grades? to Gtodynamics, October 2-5.1978. Depl. of Geodetic Science Rept. No. 280 The
Ohio State Univ., Columbus, Ohio 43210.
baseline instruments, which in essence measure
the variations of height between two reservoirs
and are free from effects of local tilt, have been
developed (e.g. , Beavan & Bilham [1977]). De-
finitive results at non-tidal periods have not yet
been published.
There have been three types of deep borehole
tiltmeters deployed: The Askania mechanical
pendulum [Rosenbach & Jakoby, 1969], the
Arthur D. Little diamagnetic suspension [Simon
et al, 1968] and the servoed bubble flat [Hansen,
1968]. Two groups in West Germany, the Claus-
thal Technical University [Herbst, 1976] and
Geophysical Institute at Kiel [J. Zschau, per-
sonal communication, 1978] have deployed a num-
ber of the Askania instruments in profiles across
various geological structures. In England, the
Institute for Coastal Oceanography and Tides has
evaluated the Askania in a vault and is planning
to deploy a Hughes sensor in a deep borehole
[Baker, personal communication, 1977]. In
North America, the Air Force Geophysics Labor-
atory (AFGL) operates two clusters of instru-
ments in E. Massachusetts; details are presented
in the next section. The University of Colorado
plans to install about ten instruments in Colorado,
Wyoming, and Montana [J. C. Harrison, personal
communication, 1977]. The sensors are simple
mechanical pendulums developed by Larry Burris
of Instech, Inc. .
The AFGL Tiltmeter Arrays
AFGL operates two deep borehole tiltmeter
clusters (small arrays), as well as several
shallow instruments, at two sites in eastern
Massachusetts. The Bedford site is located on
the north side of Hanscom (Air) Field in the town
of Bedford, about 14 km WNW of Boston. Ano-
ther 17 km to the WSW is the Maynard array,
located on the Sudbury Annex of the Army Natick
Laboratory in the town of Maynard.
Bedford Site
The Bedford array has been described in de-
tail by Simon [1971] and Cabaniss [1974], so a
brief summary will suffice. Three holes 20 cm
in diameter and lOO.mapart were drilled about
18m into a foliated granitic gneiss beneath 0-lm
of overburden. The holes were cased to a depth
of about 6 m; so water in the fracture system
was free to enter. Each instrument was em-
placed at the bottom of a hole, resting either on
its own 5 cm flat base or on a 10 -cm blunt spike
screwed into the base. The tiltmeter was
wedged into position with heavy bronze weights.
Alignment was made to a known direction using
a set of rods rigidly coupled to the top of the
instrument but which could be removed after-
wards. Several analyses for the M- tidal com-
ponent showed that the three instruments agreed
among themselves to within 2% in amplitude and
2° in azimuth and phase, although the discrep-
ancy between the observed and calculated tides
165
https://ntrs.nasa.gov/search.jsp?R=19790013313 2020-03-22T00:00:31+00:00Z
C?
A.
'-bO.OO -45.CO -40.CC -35.00
SH
Fig. 1. Long-term tilt from Bedford Inst. 1.
1971-72 (left), 1972-73 (middle) and 1973-74
(right). Each set begins at the left in October,
proceeds clockwise, and terminates in June.
No connection between years is implied. Tilt
is positive down to the SE and SW.
was considerably larger [McConnell, 1971;
McConnell et al, 1973].
At periods greater than that of the diurnal
tides, the coherence among the data from the
three instruments approached zero. Although
an annual term was apparent on each, its orien-
tation and amplitude showed no apparent rela-
tionship to neighboring instruments. As an
example of the annual behavior spot readings
from an instrument oriented SE-SW (positive
down), covering about three-quarters of three
successive years are shown in Fig. 1. The
1971-72 and 1973-74 data show annual ellipses
oriented in an E-W direction which can be ex-
plained by unequal temperature effects in the
two orientations in response to the 0. 1° C an-
nual temperature variation at this depth. The
data were interrupted in July of each year be-
cause of lightening damage; the instrument was
removed and re-emplaced after repair by Octo-
ber. Thus any connection between the sets
would be highly speculative. The differences
from year to year were ascribed to slight
changes in installation details [Cabaniss, 1974].
"Secular" drifthas shown even greater disagree-
ment and far exceeds the.4-15 nanoradians
(down to the north) per year, as estimated from
shoreline, geodetic re-leveling, and tide gage
data [Cabaniss, 1974]. Those discrepancies
are undoubtedly related to the problems outlined
above.
Maynard Site
The Maynard tiltmeters were installed in
cased holes (16 cm diameter) with a horizontal
separation of about 100 m drilled 100 m into
granitic gneiss beneath 20 m of overburden.
The instruments were the Hughes servoed bub-
bles packaged by Earth Sciences Research, Inc.
They were empiaced 5-15 m above the hole bot-
toms, each resting on a holelock which forces
wedges against the.casing wall with a.heavy
spring [McConnell, 1978]. They were aligned
by sighting on incandescent lamps with a transit
device; .Preliminary, results from tidal analy-
ses' showed inter-:instrument and monthly varia-
tions reaching 10% in amplitude [McConnell,
1978]. These have been tentatively ascribed to
local variations in scale factor caused by bubble
flat topography [McConnell, personal communi-
cation, 1978; Cabaniss, in preparation].
The long-term tilt variations are presented
as a series of spot readings spanning a period
of three years (Figure 2 and Figure 3). The
two instrument components have been computa-
tionally rotated to North/East coordinates, and
relevels have been removed, except those that
occurred during the first eight months for Inst.
1 and two months for Inst. 3. Tides have not
been removed and reach an amplitude of 0. 25
urads (p-p) in the East component and 0. 07 urads
in the North. As at Bedford, there is little
agreement among the data from the three instru-
ments. No. 1 drifted back and forth, often in a
well-defined direction; No. 2, after a stabiliza-
tion period of about a year, moved in a very
linear fashion until Day 740, when both instru-
ment components drifted rapidly about the same
amount but in opposite directions for about 45
days, after which the previous rate resumed.
The direction of the rapid excursion was asso-
ciated with that of one of the holelock wedges;
so it has been surmised that casing corrosion
might have been the cause of the event. Inst.
3 displayed remarkable stability over the same
period. The first year was marked by decay-
ing exponentials on both channels but the max-
imum rate was on the East component. ,The
second year was characterized by roughly
equal motion on both channels at the beginning
but finished with accelerated movement on the
North component. The last 18 months of the
record are shown vectorially in Figure 4.
There has been little motion during that period
except for a counter-clockwise elliptical move-
ment, equally partitioned between both instru-
ment axes, which may have an annual periodi-
city. The net drift over three years on Inst. 3
was 0. 3 urads, with a maximum excursion of
2. 0 urads from the trend.
Discussion
Our results show that the annual components
generally decrease in amplitude with depth but
are highly dependent on the installation epoch.
Herbst [1976], however, reported annual ellipse
amplitude (semi-major axes) of 0. 9 and 0. 1
urads at depths of 15 and 30 m, respectively,
which can be explained by therm^elastic effects
caused by the coupling of the annual temperature
wave with the topography.
The long-term records from the Bedford and
Maynard arrays show drifts that vary widely in
both rate and direction. If it is assumed that a
low-drift record is "best", Maynard #3 shows a
mean rate of 0. 1 urads per year to the SW, com-
pared to the tide gage and geodetic re-leveling-
estimates of about 0. 01 urads per year down to
the north. The disturbing result so far from
166
MRYNRRI> NORTH
JULY 7S-JUNE 7B
I 0.0 110.0 in.O 3)0.0 »0.0 «30.0 . W.O sio.o san.o •10.0 MO.O •30.0 MO.O IIH.O 11)0.0
Fig. 2. North component of the long-term tilt from the Maynard array from
readings taken at intervals of about 10 days. Data have been rotated from the
azimuths of the instrument axes (#1, 294° - 24°; #2, 35° - 125°; #3, 2° -92 ).
Tilt is positive up to the nor it and east. The time axis is in days since 0 Jan 75.
these experiments is the wide disagreement
among instruments spaced less than 100 m a-
part. Herbst [1976], for example, reported
annual drift rates of 1. 8 and 0. 75 urads for.in-
struments at depths of 15 and 30 m, respectively,
spaced several meters apart. The drift direc-
tions varied by 15°. The question is whether
this type of measurement is limited by the
length scale of the phenomena, by the installa-
tion techniques, or by the inherent stability of :
the instruments. Parenthetically, no "events",
including tilt steps associated with magnitude
7.7 teleseisms, have been detected on all in-
struments within a cluster.
The AFGL experiments suggest that improve-
ments might be made by emplacing the instru-
ments in sections of stainless steel casing at
depths greater than 30 m at the bottoms of
sealed holes. Installation in comparatively
fracture-free material and at close spacing
might also prove efficacious.
SB BUYS
MRYNRR[> ER5T
JULY 7S-JUNE 7B
>.o »o.o no. I 330.0 Hj.o " oo.o 410.0 «».» Ho.o"" iso.'i" 600.0 w.i ' :io.o"" iio.o" iio'.o ""i)o.o tiaTa i o j o . o i S T o m o T o m o l SsoTo"
..._ nrE ionvs i
Fig. 3. East component of the long-term tilt from the Maynard Array. (See
Fig. 2 legend,)
167
E2-a
MRYNRRI> #3
NDV 7B-JUNE 7B
STOUT B*
0.25 LJRRD5
.28 9.00 -5.75 -5.50 -5.25 -5.00
ER5T
-4.75 -4.50 -4.C5 •00 -3.76
Fig. 4. Vector plot of a portion of the long-term data from Maynard Inst. 3.
Symbols and numbers correspond to times indicated on Fig. 2 and 3. The last
point is at Day 1250.
References
Beavan, J. , and R. Bilham, Thermally in -
• duced errors in fluid tube tiltmeters, J. Geo-
phys. Res. , 82_, 5699, 1977.
Cabaniss, G. H. , Crustal Tilt in Coastal New
England - An Experimental Study, PhD Thesis,
Boston Univ. , 1974.
Harrison, J. C. , Cavity and topographic effects
in tilt and strain measurements, J. Geophys.
Res. , _81,. 319, 1976.
Herbst, K. , Interpretation of Tilt Measurements
in the Period Range Above that of the Tides.
PhD Thesis, Tech. Univ. of Clausthal, W.
Germany, 1976.
Lewcowicz, J. , and R. K. McConnell, Jr. ,
Preliminary results from a shallow borehole
tilt array, AFGL, TR-77-0168, 1977 (NTIS
A047523).
McConnell, R. K. , Jr. , Experimental and theo-
retical studies of long-period tilt of the earth's
crust! Part H - The&reticftl.;-AFCRL^ 71-0388
(II), 1971 (NTIS AD729917).
McConnell, R. K. , Jr. , Research on the nature
of ground tilts in the period range 103 to 107
seconds, AFGL, TR-78-0105, 1978.
McConnell, R. K. , Jr., P. C. Dunn, B. D. Pea-
tie, Research on crustal tilts in the greater
Boston area, AFCRL. TR-73-0681, 1973
(NTIS AD776058).
Melchior, P. , The Tides of the Solid Earth,
Pergamon Press, New York, 1978.
Mortensen, C. E. , and M. J. S. Johnston, The
nature of surface tilt along 85 km of the San
Andreas fault - Preliminary results from a
14-instrument array, Pure Appl. Geophys. ,
11-3, 237, 1975.
Rosenbach, O., and H. Jakoby, First exper-
iences with the Askania borehole tiltmeter
(earth tide pendulum), in Problems of Recent
Crustal Movements of the Earth, Moscow,
1969.
Simon, I., Experimental and theoretical studies
of long-period tilt of the earth's crust: Part I-
Experlmental, AFCRL, 71-0388 (I), 1971
(NTIS AD 729916).
168
Simon, I. , A. G. Emslie, P. F. Strong, and R.
K. McConnell, Jr. , Sensitive tiltmeter utiliz-
ing a diamagnetic suspension, Rev. Sci. Inst. ,
39, 1666, 1968.
Wood, M. D. , and N. E. King, Relation between
earthquakes, weather, and soil tilt, Science,
197, 154, 1977.
169


The measurement of long period and secular deformation with deep borehole tiltmeters

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