The Eskdalemuir Seismic Array

The Eskdalemuir Seismological Recording Station is located in southern Scotland and has been in operation since 1962, making it the longest-operating steerable seismic array in the world. It is situated on the eastern side of the Langholm-Innerleithen road (B709) about 18 miles north of Langholm and two miles north of the Eskdalemuir meteorological observatory. 

It comprises a recording laboratory, a seismological vault and an array of seismometers installed in pits spaced over an area 10 km square.

 The seismological vault is about a quarter of a mile east south east of the laboratory, and the array lies to the east in the form of a cross, its centre, about 1½ miles from the laboratory.  The latitude of the point of intersection of the two lines of the array is 55^o20' north and the longitude is 03^o09½' west.

Geography and geology

The array is situated across the watershed between tributary headstreams of the Teviot and Tweed flowing to the north-east, and headstreams of the Esk which generally flow to the south-west.  The ground surface is largely open rolling moorland and forest plantations, which in is in many places peat covered.  The altitude of the seismic pits varies from 900ft to 1400 ft.  The isolated location ensures that microseismic interference is kept to a minimum.  There is very little traffic on the Langholm-Innerleithen road.

Location of the Eskdalemeur SRS. (Image produced from the Ordnance Survey Get-a-map service and reproduced with kind permission of Ordnance Survey and Ordnance Survey of Northern Ireland.)

Average annual rainfall is heavy, and the area is crossed by a close, irregular, network of drainage ditches.

The seismic pits have been excavated through an overburden of superficial soil (peat in some instances) or thickness from 0 in to 4 ft into shales of the Llandovery series (Silurian age).  These were folded during late Silurian times, and as a result of the lateral pressures exerted are highly cleaved.

Layout

The array consists of two straight lines of instrument pits intersecting at right angles.  Each line has eleven pits (of which only ten on each line are used) approximately 1000 yards apart.  Each line intersects the other off centre, forming a cross whose four arms are unequal.  The lines run roughly from SSW to NNE (Blue line) and from WNW to ESE (Red line).  The overall length of each line is approximately 5½ miles.

Buried recording cables connect each instrument pit to the recording laboratory.

Layout of the Eskdalemuir facility.

Construction

A typical instrument pit is illustrated in the diagram.

Construction of a seismic pit.

The pits are excavated through topsoil and underlying weathered shale to a depth where the shale becomes reasonably consolidated.  The depth varies, depending on location, but on average is about 7 ft 6 in. below surface level.  The bottom of each pit is levelled off with concrete to a depth of about 6 inches.  When this concrete had set hard a sectional cylindrical steel shell, or liner was lowered to stand upright on the concrete base, and the bottom of the steel cylinder was sealed with a bitumen compound.  While still "green" the surface of the concrete was levelled by trowelling on cement mortar.  The bottom outside of the steel liner was then buried in concrete back-fill to a depth of 18 inches.  Excavated material was then backfilled to within 12 inches of the top of the cylinder, on which is fitted a removable domed lid.  The lid edge is bolted to the top flange of the cylinder.  A neoprene gasket is interposed between the flange and the lid.  Provision is made for cable entry via a tube at the top of the cylinder; this entry is sealed when the cable is inserted, thus making the whole of the interior waterproof.

Instrumentation

The instrumentation at the array dates back in part to 1962, and is subject to upgrade within the next 12 months.

The array

Each pit on the array contains one vertical Willmore MK2 short period seismometer.

The photograph below shows a typical short period pit construction with sensor placed at the bottom of the pit, the pit electronics secured to the side of the pit housing and the transient suppressor.

A pit with installed seismometer.

 The signals from the seismometers are transmitted via buried cables back to the recording laboratory using a transmission technique called FM-DC.  An FM carrier (sender unit) transmits the data from each seismometer back over the power cables supplying power to each seismometer pit.

Design of the station system.

At the recording laboratory the FM-DC demodulator converts this FM signal back to a DC voltage where it is then digitised using 3 separate CMG-DM16-R8 digitisers.  A central SCREAM acquisition system then records this data.

Counts per meter for complete system including the FIR filter esponse for the CMG-DM16-R8 @40 sps.

Poles and zeros (displacement)

The vault

The vault at Eskdalemuir contains four seismometer plinths. Currently a 120s to 50Hz CMG-3TD is installed in the vault. Data is transmitted to the recording station using a leased line modem.

Inside the vault.

Instrument details:

A typical noise plot from 4sps data output

Recording station

Data from the Array consisting of 20 short period Willmore MK2 instruments are digitised by 3 Guralp CMG-DM16R8 digitisers, this with the CMG-3TD data from the vault is recorded by a central Acquisition computer running SCREAM!.  Data from this acquisition computer is then transmitted on two separate networks in TCP/IP. From the local network the data is transmitted via leased line to Blacknest and a second computer records the data locally onto a tape backup system.

(Data about Eskdalemuir is taken in part from a publication on Eskdalemuir Seismological recording station published by Blacknest. Diagrams of the station are reproduced from this booklet.)