The Eskdalemuir Seismological Station
J. R. Truscott
(Received 1963 December 23)
Summary
This station set up by the United Kingdom Atomic Energy Authority
as part of its seismological research programme has now completed
more than a year of operation. The paper describes the site, the layout
of the array of seismometers installed there, and the electronic recording
equipment and facilities.
This is followed by a summary of the methods used to determine
the crustal structure at the site of the station, of typical seismological
background noise recorded during initial operation, and of the capabilities of the array.
1. Introduction
In 1959, the United Kingdom Atomic Energy Authority began a programme of
seismological research bearing on the problem of the detection and identification
of underground nuclear explosions. An increasingly useful technique in this work
was the use of arrays of seismometers, which now form the basis of the U.K.A.E.A.
programme together with various computing methods. These techniques improve
the sensitivity of a recording station and enable detailed analyses of the character
of the seismograms to be made.
Initial experience with an array was gained from a small pilot installation on
Salisbury Plain during early 1961. This was followed at the end of that year by a
larger array installed at Pole Mountain, in Wyoming, U.S.A., by arrangement with
the Advanced Research Project Agency. This array was set up by the U.K.A.E.A.
to record the GNOME underground nuclear shot in Nevada.
Early work on Salisbury Plain was sufficiently promising to support the establishment of a larger, more fully instrumented array in the United Kingdom. The
site finally chosen was at Eskdalemuir, and work commenced on the construction
of the station in the Autumn of 1961. The station became operational in 1962 May.
This paper gives a description of the installation, a general account of the
instrumentation, and a reference to some of the initial results from data already
recorded.
2. Choice and location of
The first requirement
and reasonably isolated
industrial establishment,
site
was to find an area situated on well consolidated rock,
from so-called “cultural” noise (road traffic, railways,
cities, etc.). In broad terms Dartmoor and Southern
59
60
J.
R. Truseott
Scotland were considered. Dartmoor was eliminated owing to difficulties in
obtaining wayleaves and Southern Scotland was therefore examined in the light
of the following site parameters :
1. The periphery of the site to be not closer than:
(a) 10 miles to the sea
(b) 5 miles to any railway line
(c) 1 mile to any “A” class, or well-used “B” class roads.
2. Site to be free from large numbers of trees or woods; scattered individual
trees being acceptable. Rivers and streams must not be near possible pit
locations.
3. Contours to permit siting of pits within +200 ft of a mean level.
4. Cables to be buried, but not necessarily directly routed.
5. Service facilities of electrical power, tetephone and water to be available.
6. Good access by road to recording laboratory.
The above requirements together with the known spread of the array were
fairly restrictive, and only two suitable sites were found. Seismic background
measurements were carried out, using portable equipment, at each site. On the
basis of these tests and in consideration of various logistical factors, Eskdalemuir
was finally selected.
3. Site description
The array is sited on Llandovery rocks of Silurian age, which form part of a
geosyncline extending from Western Ireland to the South of Scotland. They are
isoclinally folded and compacted, but no metamorphism is evident on the surface.
Typical rock types are grits, shales, mudstones, greywackes and conglomerates.
The total thickness of the L. Palaeozoic succession is unknown, but is unlikely to
be less than 20 0oO to 25 OOO ft.
The ground surface is largely open rolling moorland, which is in many places
peat covered, and used as sheep pasture. There are few trees. The altitude of the
seismometer pits varies between 900ft and 14OOft. The mean annual rainfall is
82 in.; and the moorland is crossed by a close irregular network of drainage
ditches.
4. Layout of array, and pit construction
The complete installation comprises an array of seismometers, recording laboratory and seismological vault. The array consists of two straight lines of specially
constructed instrument pits which intersect at right angles (Figure 1). The lines
contain ten or eleven pits, which are spaced 980 yds horizontal distance apart.
The maximum difference in altitude between one pit and the next is 300ft in one
line, and 275 ft in the other.
The pits (Figure 2) were excavated through topsoil of peat and boulder clay,
and through the underlying weathered shale until reasonably well consolidated
rock was exposed. The bottom of each excavation was levelled off with concrete
about 6 inches thick. When this had set hard a sectional cylindrical steel shell was
lowered to stand upright.
The shell is of 40-in. internal diameter and has sections either 40 in. or 20 in.
high, which were bolted together as necessary. The bottom of the cylinder was
sealed with bitumen compound and a second 6-in. layer of concrete placed inside
The Eskdalemuir Seismological Station
To Haw icl
FIG. I.-The
U.K.A.E.A. Eskdalemuir Seismological Recording Station.
FIG.2.-Typical seismic pit (not to scale).
61
62
J. R. Truscott
the cylinder, with its surface levelled by trowelling on cement mortar. The bottom
outside of the shell was then buried with concrete backfill to a depth of 18in.
Excavated material was then backfilled to within 12in. of the top of the cylinder.
A domed lid was fitted, this shape being chosen to give rigidity and prevent
“drumming”. The edge of the lid was secured by means of captive wing nuts and
bolts and fitted with a neoprene gasket seal. Provision was made for cable entry
through a tube at the top of the cylinder which was sealed after insertion of the
cables. Brushwood fascines were laid around the cylinder top and drainage ditches
dug where necessary, to remove surface water from the top.
5. Field cables
The cables have 26 cores and are plastic sheathed, with moulded-on waterproof
connectors; they are supplied in 255 yard lengths. The instrumentation system
requires six cores (two of them screened) to each pit, and specially moulded T-pieces
are used to break into the main cable run and feed off six cores when needed for
a pit. In addition, the T-piece carries two cores which are common throughout
a run, and are used for telephone communication either from pit to pit, or from
pit to laboratory.
Thus one run of the main cable feeds four pits with six cores each plus a common
telephone pair. There are six such main cable runs through the array, giving a
total of twenty four cable channels, one to each of the twenty two pits plus one
extra to the central pit in each line (Blue 6 and Red 6). The cables were buried
throughout to a depth of approximately 18 in., using a “mole” plough, and in
general they follow the lines of the array. Where several main runs lie together
the standard lengths have been arranged such that joints in the several cables
occur together, and they are marked on the surface by a 10-in. square concrete
slab.
6. Recording laboratory
This is a single storey brick building which contains the main laboratory,
battery-room, store-room, office etc. Electrical power, telephone and water are
all available, and it is proposed to install a d.c./a.c. static inverter supplied in
turn from storage batteries on float charge from the mains. This would give a
reserve of power for a limited time in case of mains failure and give control over
frequency and voltage variations.
For protracted power cuts a transportable motor-generator set can be readily
connected to feed the station. The laboratory is used both to house the electronic
recording equipment and also to provide working space for maintenance of the
equipment. Field cables enter through an underground duct and terminate on a
large distribution panel surmounting the duct and concealed within a cupboard.
Floor ducts around the laboratory permit the routing of cables between equipment
racks etc.
7. Seismological vault
This is intended mainly to house long-period seismometers and experimental
seismometers. Essential requirements for long period instruments are a stable
platform with minimum tilts and a thermally stable environment. To secure these
objectives, the vault is completely buried in unweathered rock with an overburden
The Eskdalemuir Seismological Station
63
of some 4 ft of soil. An elaborate form of construction has been used, with a number
of sandwiched layers in the walls to improve thermal insulation and also to reduce
the ingress of moisture. An air lock is provided for entry and this space contains
a dehumidifier. Within the vault, four concrete instrument plinths are provided,
each measuring 7 ft 3 in. by 6 ft 3 in. and 2 ft high above the gangway. The latter is
supported on cross joists which rest only at their ends on the foundations, so
that tilts due to the presence of an operator are minimized. Cables run from the
vault through buried ducts back to the laboratory.
8. Field instrumentation
The system used for the array enables one or two short period seismometers
to be operated in each pit over one six-core cable. This permits duplication at pits
where access is difficult or simultaneous operation of a vertical and horizontal
seismometer in one pit. Each seismometer is provided in the pit with a low consumption battery operated head amplifier (battery life, 6 months approximately)
to give an improved signal/noise level down the line.
Provision is made for remote calibration of the instruments. This is done in
two parts, (a) the seismometer may be calibrated and checked for free movement,
by removing the damping and causing the mass to oscillate by means of a step
impulse of d.c. current. (b) A known voltage pulse, or sine-wave signal, may be
injected from the laboratory to the input of the head amplifier, so providing an
overall calibration of the electronic system. A selector switch at the laboratory
gives the option of calibrating all the pits in the array individually or in groups.
All seismometers in use are the Mark I1 type of Willmore seismometer working
with a natural period of 1 s and damping factor 0.6.
9. Laboratory instrumentation
Incoming signals are amplified and filtered (low pass, 15 C.P.S. cut-off) and
frequency modulated for recording on magnetic tape. Each seismometer output
is recorded on a separate track of a 24 channel tape deck, using 1-in. wide tape
on 14-in. reels, carrying 7 200 ft of tape. With the recording standards now in use,
the tape speed is 0.3 in./s giving a recording time per reel of over 3 days. The
carrier frequency is 270c.p.s. with a peak deviation of 335 per cent. Of the 24
tracks available, 21 are used for signal purposes, one for timing and two for “wow”
and “flutter” compensation. Playback heads on the deck with associated demodulators and an eight-channel strip chart pen, enable any eight of the 21 signals to
be selected for replay monitoring. A drum recorder, giving a single channel helical
trace on one sheet of paper, with a duration of 24hr, is used as a continuous
monitor on any selected channel. A second drum recorder charts thc additive output (without phasing) from cight selected channels.
A further facility now installed, is an on-line analyser giving the summed output
of eight channels with automatic gain correction, and also a cross-correlation
output, i.e. the integrated product of two groups of summed seismometer outputs.
These are displayed together with timing on a 4 channel strip chart which is triggered from an amplitude discriminator on the correlation output.
The overall frequency response of the system, including seismometer magnetic
tape recording, and tape replay on to a paper recorder, is given in Figure 3, where
it is plotted for a constant velocity input. The maximum dynamic range is 52db
on the magnetic tape, and average system noise referred to the head amplifier
64
J. R. Truscotl
input is equivalent to a peak ground velocity of lo-' cm/s, within the pass band of
the system.
Timing is obtained from a crystal-controlled chronometer with a stability of
five parts in ten million. This is checked twice daily against the time standard
broadcasts from station MSF Rugby. with a maximum error of +10ms. The
chronometer feeds a time coding unit. whose output can be reset with a maximum
ci-ror o f
10 ins. thus giving a maxirnuin ovctall i-esctti!ig error of rfr 20 ins. Thc
absolute time accuracy is thus +45 ins with a short term stability of five parts in
ten million.
7
I~III.1ICIC
FIG. 3.-Eskdalemuir-
4
h
I I-c'lllcnc>. c
X I 0
I
10
1
I
40
/
1
1
l
hO HI) IOU
\
overall frequency response for constant velocity
input.
The output from the crystal chronometer is then encoded (Figure 4) in a form
which gives 1 s, 10 s and 1 min markers, together with a code once per minute
which describes the absolute time in hours and minutes. This encoded time sequence
is supplied to tape decks and all visual monitors.
Instrumentation for three long period seismometers has been installed, one of
which records earth motions in a vertical axis, and the others earth motions in
two horizontal orthogonal axes. They have a natural period of 1 5 s , and carry a
velocity transducer which, with integration and filtering, yields a system characteristic having a substantially flat velocity response from 20-75 s. On an experimental basis, these signals are telemetered over a telegraph link to the U.K.A.E.A.
seismological laboratories at Blacknest (Brimpton, Berkshire) and there recorded
on tape. A visual monitor of the long period signals is available at Eskdalemuir
using a narrow-band system working at a higher gain.
10. Handling of data
Initial searching of the single channel 24-hr monitor is carried out at Eskdalemuir, where events noted are checked against epicentre cards from the U.S.C.G.S.
The Eskdalemuir Seismological Station
65
and other sources. The tapes are returned to Blacknest and filed, to await detailed
analysis of selected events. Phasing, normalization, filtering, summation and crosscorrelation are some of the processes applied to the tape records during analysis.
11. Station calibration
To make fullest use of the records from such a station as Eskdalemuir it is
necessary to acquire as much detailed knowledge as possible of the crustal structure
beneath.
To this end an exercise was carried out with the cooperation of the Royal Navy
in 1962 July. This involved the dropping of 320-lb depth charges in the North
Sea and Irish Sea in a controlled pattern with accurate timing of the shots.
Additional information covering those distances and azimuths which are inland
is being provided by the accurate timing at source of selected quarry shots etc.
The construction of timedistance curves for the main seismic phases and the
determination of a crustal model for the area on the basis of results from these
measurements are described by Agger & Carpenter (1964).
12. Results
Although the scope of this paper does not include a detailed report on results
obtained from the station, some information on background noise level is appropriate. Since a prime objective of the station is the recording of relatively small
teleseismic events, background noise levels must be low even when using the added
sophistication of separately recorded seismometers in array configuration.
Every precaution has been taken in the choice of location and in form of construction in order to achieve this condition, although any site in the British Isles
must nevertheless be more noisy than a well-chosen, inland continental site. The
noise background at Eskdalemuir under typical conditions is given in Figure 5,
which shows a peak particle velocity varying from 3.5 x
to 2 x
cm/s in
66
J. R. Truscott
the band 3 to 3 C.P.S. It was found necessary after construction to move the site
of one pit which proved excessively noisy, having been originally sited on boulder
clay rather than true bedrock.
A comparison of Figures 6 and 7 gives some idea of the need for initial selection
of data, showing the relative numbers of local events recorded per day, as against
teleseismic events noted by the U.S.C.G.S.
Whilst not specifically directed towards the analysis of close events, they are
of interest, and a topical example is given in Figure 8, which is the recording made
from a tremor experienced near the south coast of this country in the early hours of
1963 October 25.
The figure shows a direct replay from the tape of individual seismometer
channels with no processing. This demonstrates the quality of the record and
although the P and S phases are readily seen, the capability of the station is not
fully realized until the individual channels are suitably processed. For this purpose
delays are inserted between channels, appropriate to a given azimuth and phase
100
P d i'eloclt! A
FIG. 6.-Number
(I0
" ciii
300
hcc)
of local events ( <200 km) recorded at Eskdalemuir
16 August to 15 September 1962.
I000
The Eskdalemuir Seismological Station
67
velocity. The channels are then summed into two groups and the two combined
signals thus obtained (already much improved in signal-to-noise ratio) are crosscorrelated by multiplication and integration. These techniques are demonstrated
with reference to the event shown in Figure 8, p. 68 by Key & others (1964).
Data such as this, now being recorded and stored, will provide material for
theoretical and experimental studies well into the future. Already much has been
learned, not only from the seismological point of view but also in the instrumentation and operational techniques necessary to run such a station on a continuous
basis.
Peak VL‘IOCI~)
A (10
FIG.7.-Number
“CIII XC)
of events recorded at Eskdalemuir and listed by U.S.C.G.S.
(June-October 1962 inc.).
13. Acknowledgment
The author wishes to thank the U.K.A.E.A. for permission to publish this
paper, and to acknowledge the efforts of the staff of Blacknest and those associated
with them in the design, installation and operation of arrays. In particular, Mr J.
Whitfield of Seismograph Service Ltd. and Messrs. E. Mullard and J. Milne of the
U.K.A.E.A., who have all been closely concerned with the Eskdalemuir station.
U.K.A.E.A.,
Blacknest,
Brimpton,
Berkshire.
1963 December.
References
Aggei-, H . E. & Carpenter, E. W., 1964. A crustal study in the vicinity of the Esk-
dalemuir Seismological Array Station, Gcwp/i~.v.J., 9, 64.
Key. F. A., Marshall. P. D. & McDowall, A . J., 1964. T w o recent British carthquakes recorded at the U . K . A . E . A . Scismoinetcr Array at Eskdalcniuir,
“ / / ! / / ~ P . 201, 484.
J. R. Truscott
1