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Loch Ness Bathymetric and Seismic Survey


Reproduced with the permission of the Scottish Naturalist
Copyright: May be used for private research. All other rights reserved


December 1991

By IAN YOUNG
Bentech Subsea A/S

and ADRIAN J. SHINE

Loch Ness and Morar Project

Introduction
The status of Loch Ness as Britain's greatest body of fresh-water was established by Murray and Pullar (1903-08 and 1910) during their
Bathymetrical Survey of the Scottish Fresh-water Lochs.  A Pullar wire sounding machine (Figure 1, 9K) was used in April 1903, and 1,700 soundings were taken from a rowing boat in a series of across-loch transects.  This revealed a steep-sided, flat-bottomed trench-like profile resulting from the Great Glen's tectonic origins. Thus, despite being second to Loch Morar (310 m) in maximum depth, Loch Ness (maximum depth 230 m) has the greatest mean depth (132 m) and hence the greatest volume (7.452 billion cu/m) of all British lakes.

A controversy about the maximum depth occurred in 1969 during the trials of the Vickers Pisces submersible.  Reports claimed that the Pisces had descended to 820 feet (250 m) and that a sonar depth of 970 feet (295 m) had been obtained at a position a quarter of a mile south of Urquhart Castle (Eastaugh, 1970).

An attempt to resolve this was made in 1979 by the Loch Ness and Morar Project (L.N.M.P.), using a Kelvin Hughes MS48 single beam echo-sounder.  A search pattern failed to register depths in excess of 225 m (Shine and Martin, 1988).

The introduction of 'swathe' sounding systems (a fan of multiple beams) in the 1980s greatly reduced survey time, and the first use at Loch Ness was in a survey of Urquhart Bay using a Simrad EM100 in 1987 (Shine and Martin, 1988).

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p25



The 1991 survey was designed to resolve any remaining controversy over the maximum depth of Loch Ness, and in particular to prepare for the L.N.M.P.'s deep-coring programme (the 'Rosetta' project, see Note 1).

Methods

Precision echo-sounding relies on an accurate knowledge of the sound velocity profile, and this was a major concern for the survey.  The complex internal seiches known to exist in Loch Ness during the summer create unpredictable and complex velocity profiles and refractions which are subject to rapid change and cycling (Shine and Martin, 1988).

Conversely, the loch is largely isothermal in the winter, so this was chosen as the optimum time for the 1991 bathymetric survey.  The velocity profiles taken during the survey confirmed this, and clearly show the uniform and stable profile
(Figure 2, 12K).

The hydrographic system used was the Bentech TOPAS, i.e. parametric topography and seismic profiling sonar.  The PS40 parasource had a 24-stave transducer with a primary frequency of 40 kHz and a secondary frequency of 1.0-10 kHz.  Electronic sequential scanning enabled high speed collection of echo-soundings across a selected swathe of 40o perpendicular to the survey vessel's line of travel (Figure 3, 15K).

Information was monitored on surface displays for bathymetry and seismics.  Navigational and sonar measurements were integrated through an RS232 data link.  Data were stored on a Bentech D.S.004 EXABYTE unit (2.5 Giga Byte).

For best results the survey was planned from a knowledge of the general topography of the loch, with swathes made roughly perpendicular to the lines of contour.  Further knowledge of approximate depths allowed planning of survey lines to maximise the coverage of the loch bed.  The deep basins were of primary interest, so survey lines were calculated to provide blanket coverage for all depths below 50 m and a 50% coverage of the rest of the loch.

Because of the very steep side-walls, this in effect represented almost the complete loch.  The survey lines were programmed into the computer to provide a display of the survey position and overview online data via a helmsman's 'head up' display (Figure 4, 18K).

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p29

The 35-ft survey vessel Highland Commander II, supplied by Caley Cruisers of Inverness, demonstrated the capability of the TOPAS to be deployed from a vessel of opportunity, and permitted operation in shallow water.  The transducer was fitted to a rigid pole mounted on the side of the vessel, deployed to a depth of 2.0 m below the waterline to avoid cavitation effects and to give the transducer a clear line of sight to the loch bed below the keel.

Navigation equipment supplied by Marconi Udi (Note 2) used the U.S. Department of Defence NAVSTAR global positioning satellites (G.P.S.), the overall accuracy of the system being further enhanced by Marconi Udi's STARTRAK differential G.P.S. system.  STARTRAK (Note 3) applies established survey principles of differential correction to the G.P.S. data ( 3.0 m with 1.0 second updates).

These corrections are calculated at a surveyed reference station in Aberdeen, and are transmitted by dual terrestrial private circuit lines to Goonhilly for sending through the Inmarsat standard 'A' communication link to the survey vessel.  Inmarsat was chosen as the data link because of its high integrity and rapid data transfer speed.

For vessels of opportunity not fitted with Inmarsat, as in the case of this survey, Marconi Udi provide a Local Extension Service (L.E.S.) from a remote station set up within U.H.F. telemetry link contact with the survey vessel.  This receives Inmarsat data and relays it to the survey vessel. 

There was also a risk that the Inmarsat transmissions might not be picked up when running survey lines close to the shore, because of the surrounding hills.  Prior to the survey, Marconi Udi established workable locations for the remote stations on the loch side.

Survey Operation

The survey was completed in five days between 2nd and 6th December 1991, with the first day being devoted to installation and calibration.  The calibration of TOPAS was restricted to alignment of the transducer only, the sonar automatically providing roll compensation.

Days 2-5 were the survey days.  Figures 5a, 5b, 5c, 5d (8K each) give details of the lines surveyed on each day.  The total survey time over the four days was 31 hours, and a total of 288 km of survey was completed at an average survey speed of 5.5 knots.  A total of over 200,000 bathymetric soundings and 0.8 Giga Bytes of seismic profile data were recorded.

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p35

Five across-loch seismic profiles were completed; four in the North Basin and one in the South Basin.  Along axis data were also recorded over the rise off the River Foyers.  Seismic data apparently in excess of 30 m below the loch bed were recorded (Figures 6a, (15Kphoto) 6b, (13Kchart) 6c, (17Kchart) and further processing of this data is planned for the future.

Post-Processing
Post-processing of the data was undertaken by Britsurvey, a division of Svitzer.  The data set recorded contained a considerable wealth of information, which included the vessel position and approximately 4,000 amplitude sub-samples of each trace.  The first task was to extract the loch bed data from the sub-samples.  A programme routine was written, to calculate the spatial location of the loch bed index of each ping, and was adjusted for the speed of sound and transducer depth.

The loch level was recorded on each day of the survey, and for comparison purposes the datum was transposed back to the level used by Murray and Pullar (16 m).  The resulting output was a concise XYZ data set.  To assist with the interpolation of the depths between the shore-line and the limits of the survey, the perimeter of the loch was digitised and an XYZ data set generated with a Z value of zero.

 The data set was imported into Britsurvey's CPS-3 mapping package running on a SunSparc work-station.  The information was computed onto a 100 m grid using a convergent gridding algorithm.  The Z values were then converted to imperial units and contoured at 100 feet intervals.

 The resulting chart was transferred to Britsurvey's Autocad system, and plotted at the imperial scale of 1:21,120 (3.0 inches to 1.0 mile) to allow comparisons with the earlier imperial charts produced by Messrs Bartholomew from the Murray and Pullar survey of 1903.  Metric charts were also produced with 25 m contour intervals (Figure 7, 16K).

Results
The results show a positive similarity between the 1991 survey and the 1903 survey of Murray and Pullar.  The general regularity of the loch basin was confirmed.  Some of the areas of rocky side-wall caused difficulties acoustically due to local irregular rock facets.

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p41

A particular feature was investigated, on the suggestion of Mr. George Edwards (pers. comm.), that an area of increased depth lay at the base of the 'wall' off Urquhart Bay.  The presence of a gully approximately 3.0 m deep was confirmed.  Subsequent coring by the Loch Ness and Morar Project suggests that this may have resulted from turbidity currents caused by a major flood (Miller, 1993; Bennett, 1993; Bennett and Shine, 1993).

The seismic data are of considerable interest and show deep strata, thus confirming the need for equipment capable of extracting cores in excess of 10 m in length.  The maximum depth of the loch was of particular interest, because of previous claims in excess of the depth originally charted by Murray and Pullar.

The 1991 survey found that the maximum depth lies in the North Basin, at a depth of 745 feet (226.96 m) at 413640E 6353598N U.T.M. (Note 4) or Latitude 57o 18' 57.1" N and Longitude 04o 26' 0l.6" W.  This depth is a little less than the Murray and Pullar maximum of 754 feet (230 m) and was recorded 1.0 km further N.E. at a position approximately 1.0 km south of Urquhart Castle (Figure 7, 16K).

The maximum depth found in the South Basin was 726 feet (221 m), at 407538E 6345533N (U.T.M.) or Latitude 57o 14' 32.0" N and Longitude 0.4o 31' 55.3" W.  Again, this is a little less than Murray and Pullar's figure of 739 feet (225 m).

Summary
In December 1991 the Loch Ness and Morar Project, in collaboration with Bentech Subsea, Marconi Udi, and Britsurvey, carried out a hydrographic survey of Loch Ness to map the contours, in particular those of the two deep-water North and South Basins, and to carry out seismic profiles.

The survey combined the use of a Bentech TOPAS topographic mapping and swathe profiling parametric sonar system, the Marconi Udi STARTRAK differential G.P.S. navigational system, and the post-processing and map production capabilities of Britsurvey.

Useful seismic data was collected and the maximum depth found was 745 feet (226.96 m), which is comparable to that found by Sir John Murray and Laurence Pullar's bathymetric survey of 1903.

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p42

Acknowledgements
Contributions to this paper were made by a number of colleagues and associates.  In particular, the authors would like to thank Mr. John Minshull and members of the Loch Ness and Morar Project, Mr. Ian Padgham and Mr. David Whitcombe of Marconi Udi, Mr. Andrew Grey of Oceanscan, Mr. John Dybedal of Bentech Subsea, Mr. Keith Burgess of Svitzer Ltd, and Mr. David Siviter of Subtechnique who co-ordinated the operation.

The survey vessel was generously provided by Mr. Jim Hogan of Caley Cruisers, Inverness.

Special thanks are also due to Mr. and Mrs R.A. Bremner of the Official Loch Ness Exhibition Centre for their hospitality in accommodating the survey team.

Notes

1.  The ROSETTA project (Recovery of Sediments Enabling Translations to Acoustics).  The L.N.M.P.'s deep-coring programme is designed to complement the European Community REBECCA project (Reflection from Bottom, Echo Classification and Characterisation of Acoustic Propagation).  This is a seismic programme for which the British participant is Dr. Bryan Woodward of Loughborough University of Technology.

2.  Now Fugro Udi Ltd.

3.  Now STARFIX II.

4.  U.T.M. (Universal Terrain Measurement).  International Spheroid (ED50) U.T.M. Projection, Zone 30, Central Meridian 3 W.

 

References

Bennett, S.  (1993).  Patterns and Processes of Sedimentation in Loch Ness.  B.Sc. Dissertation, University of Staffordshire.

Bennett, S. and Shine, A.J.  (1993).  Review of current work on Loch Ness sediment cores.  Scottish Naturalist, 105: 55-63.

Easthaugh, R.W.  (1970).  Report of freshwater trials of the submarine Pisces.  Loch Ness Investigation, Annual Report, 1969: 7-8.

Vol.105, The Scottish Naturalist: Loch Ness Bathymetric and Seismic Survey p43

Miller, K.C.  (1993).  A Study of Sedimentary Markers within the Lacustrine Environment.  B.Sc. Dissertation, University of Edinburgh.

Murray, J. and Pullar, L.  (Eds.) (1903-08).  Bathymetrical survey of the fresh-water lochs of Scotland.  Geographical Journal, Vols. 22-31.  A series of papers by various contributors, preliminary to the six-volume publication of 1910.

Murray, J. and Pullar, L.  (Eds.) (1910).  Bathymetrical Survey of the Scottish Fresh-Water Lochs.  Vols. 1-6.  Edinburgh:  Challenger Office.

Shine A.J. and Martin, D.S.  (1988).  Loch Ness habitats observed by sonar and underwater television.  Scottish Naturalist, 100: 111-199.

Received July 1993

 

Mr. Ian Young, Bentech Subsea A/S,

Perry Tritech, Units B1 and B2, Kirkhill Place,

Kirkhill Industrial Estate, Dyce, ABERDEEN AB2 0ES.

Mr. Adrian J. Shine, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.

 

 

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Loch Ness Seismic and Bathymetric Survey