On the shelf: geology beneath the waves

From Earthwise
Jump to navigation Jump to search
From: Wilson, H.E. Down to earth - one hundred and fifty years of the British Geological Survey. Edinburgh:Scottish Academic Press, 1985.
The text is derived from an 'orphan' work. BGS are committed to respecting the intellectual property rights of others. After extensive effort we are unable to trace the copyright holder of this work. Despite this, we would like to make this very important work on the history of BGS, written by a former member of staff of the Survey, available for researchers. If you are a rights holder and are concerned that you have found this work for which you have not granted permission please contact us with proof you are the rights holder.
XVI On the shelf:geology beneath the waves. Chapter-head sketch by Robery Geary.
Gravimeter observations and observors. M V Whitethorn in her role as sampling ship. The cabins mounted on the forward deck were used as temporary laboratories.

XVI On the shelf:geology beneath the waves[edit]

It is salutary in retrospect to recall how smug the average British geologist was, in the nineteen fifties, about the geology of the Continental Shelf around the British Isles. If we thought about it at all, which was rarely, we assumed that the geological structure of the shelf was a continuation of that seen on-shore. One of the two greatest changes in geological attitudes in the last thirty years has been the realisation that around much of the European coast the on-shore geology stops at low-water mark!

This enlightenment has come in the wake of the hydrocarbon exploration boom of the sixties and seventies, which pushed forward the search for, firstly, natural gas following the discovery of the Groningen gas field in the Netherlands and later the development of the North Sea oilfields when the full extent of the North Sea Basin became apparent.

Compared with the unlimited resources of the oil companies it must be admitted that the role of the Geological Survey has been peripheral but as a repository of information and as the collator of data from diverse sources the Survey has become increasingly vital.

The first off-shore activities of the Survey were incidental in that the airborne magnetometer lines flown to cover the land areas of the United Kingdom inevitably included some sea. In 1966 the Geophysical Department undertook some follow up work on anomalies revealed by this in Loch Ewe in western Scotland and in the Moray Firth.

As well as magnetometer and seismic profiling in these areas the geophysicists were joined by two geologists from the Edinburgh office, R A Eden and J D Peacock, both enthusiastic scuba divers, who attempted some sea-bottom sampling and coring with self-contained diving equipment. Of greater long-term significance was the start in 1966 of a programme of curation and correlation of records and samples from commercial hydrocarbon exploration boreholes in the North Sea, on behalf of the Ministry of Fuel and Power, initiated by two members of the Yorkshire and East Midlands Field Unit.

The Director's Report for 1967 records the formation of a Continental Shelf Unit in Leeds, to study the southern North Sea and the Irish Sea. At the end of that year the nucleus of a second Unit was deployed in Edinburgh. Continental Shelf Unit I at Leeds started work on a bottom-sampling programme in the Irish Sea and in the following year, with the Geophysical Division, took part in a sedimentological investigation of the Humber estuary, planned by the Humber Estuary Research Committee, which included representatives of the Humber Conservancy Board, British Transport Docks Board and Hydraulics Research Station.

In 1968 a section of the Geophysics Division was designated the Marine Geophysics Unit, with responsibility for geophysical work in the offshore programme. Continental Shelf Unit II in Edinburgh was, at first, administratively part of the South Lowlands Field Unit but in 1969 it was established as a separate unit and was at once involved in work in the Firths of Clyde and Forth and in the Sea of the Hebrides. From its inception this unit was based at Granton, appropriately near the sea but 4 miles from the Grange Terrace office, where it remained till it moved to Murchison House in 1976.

When the Director visited the Belfast office in 1969 he referred to the development of off-shore work in an interview on local radio which was picked up by an astute shipping agent. A few days later its owners offered a ship for charter to the Geological Survey of Northern Ireland and this offer, passed on to London, was the start of a long association with the Liverpool shipowners responsible, though the ship concerned, M V Whitethorn, was chartered in the first instance by the drilling contractors, Messrs Wimpey. For the next five years the two Continental Shelf Units continued their systematic sampling programme on the sea floor around the UK, but in 1974 they were transferred from the Field Divisions to the Geophysical Division and all work off-shore, geological and geophysical, came under the control of the Chief Geophysicist.

The new Continental Shelf and Geophysical Division was probably the largest the Institute is ever likely to have, with nearly 200 staff. Apart from the Continental Shelf Units and Marine Geophysics Unit, the Applied Geophysics Unit and Engineering Geology Unit were also involved in off-shore work — the former in the compilation and interpretation of commercial seismic data and the latter in the study of pock marks and the engineering properties of sea floor sediments. Pock marks are shallow circular depressions on the sea floor usually several metres in diameter, and probably formed by the escape of gas from muddy sea bed sediments.Their presence obviously causes problems in the siting of production platforms and pipe-lines.

In 1976 the two Continental Shelf Units and the Marine Geophysics Unit were reformed as the Continental Shelf Division under John Wright. The increasing demands for and financing of, off-shore work by the Department of Energy made it logical that their work be concentrated under one Assistant Director and this arrangement proved effective over the next seven years, during which a close and fruitful partnership was developed. In the 1980s the Head of this division, because of his unique relationship with the Department of Energy, also assumed responsibility for those activities of the Deep Geology Unit funded by the Department, though it was in a different division.

In 1980 the demands of Department of Energy work were such that the Division was re-organised, with the two Continental Shelf Units amalgamating into a new Marine Geology Unit — though still located in Edinburgh and Leeds — and a new Hydrocarbons Unit was formed in Edinburgh. This new grouping could deal more effectively with the increasing amount of work on the assessment of hydrocarbon prospectivity on the British areas of the continental shelf, which was being commissioned by the Department of Energy. Prospectivity' is a jargon term meaning the potential for the existence of commercially viable hydrocarbon reservoirs. These studies, using commercial seismic and well data, had been carried out by specialists in the three units formerly making up the Division, but the vast accumulation of data — some 1800 off-shore exploration and development wells — and the increasing demands of the Department made concentration of this work essential and the new unit, growing rapidly till in 1982 it was equal largest in the Institute, took over the whole of the Grange Terrace building.

A concurrent problem was the storing of the very large number of rock cuttings and core samples from the wells. In the 1960s a large hangar-like building, a war-time food store, at Kippax near Leeds had been obtained as a specimen store and at that time looked as if it would serve for a century. The advent of the oil industry samples from 1966, a few at first and then an ever-increasing torrent, filled most of the available space by the end of the 1970s, in spite of ever higher stacks and mechanised handling. In Edinburgh some material was stored in buildings at Newbattle near Dalkeith but space was a problem there too and a new specimen store for the whole offshore commercial sample collection has been purchased at Gilmerton, near Edinburgh, by the Department of Energy. It is operated for the Department by the Hydrocarbons Unit.

The work of the Continental Shelf Division now falls into two distinct parts — the geological survey of the Shelf and the assessment of the hydrocarbon potential of the UK off-shore area. The latter involves the consideration of a great deal of confidential commercial geophysical and geological information — over 2 million km of seismic reflector traverses and 2000 well records — to provide the Department of Energy with structural contour maps of selected stratigraphical horizons and to use these to assess the 'prospectivity' of the UK shelf.

With all the intensely competitive oil companies required to provide details of their exploration results to the Department — and hence to IGS — the data bank of exploration results is, in the highest degree, confidential and though the results are released after five years there is a great deal of very sensitive information held in the files of the Hydrocarbon Unit.

This has two obvious results — the work, some of great scientific interest, done by the members of the Unit (and the Deep Geology Unit which covers on-shore hydrocarbon work) cannot be published and there is therefore a degree of academic frustration among staff. Secondly, the staff involved, trained by IGS in such arcane arts as seismic interpretation, are commercially hot property, not only for their skills but for the background information they possess and which, notwithstanding the Official Secrets Act, they will inevitably use if they succumb to the lure of higher commercial salaries. The result has been a steady loss of trained people and the suggestion of higher salary scales for staff working in this field, to counter the fleshpots of the oil companies, has produced some acrimony in staff relations. In the Department of Energy itself hydrocarbon specialists have for several years been offered special rates.

The primary task of the Marine Geophysics Unit and the Marine Geology Unit remains, however, the production of geophysical and geological maps of the off-shore areas of the UK. It is clearly much more difficult to survey geologically an area where there is not only drift cover — the bane of the on-shore field man — but a considerable depth of salt water too. After early attempts to use aqualung divers, useful in obtaining samples from rock outcrops but ineffective in the open sea, numerous methods of submarine sampling are used— grab samples which give specimens of the sea floor, gravity corers and vibracorers which give drift samples down to a few metres, and ship-mounted shell and auger and rotary drill rigs which can penetrate sea floor drift deposits to produce rock cores to a depth of several tens or hundred of metres. To make sure that these relatively expensive operations give the maximum possible information extensive use is made of geophysical and other methods to fix their sites. Apart from the use of commercial seismic information, shallow seismic surveys are conducted with the Institute's own equipment and special surveys are commissioned from commercial operators. Extensive use is also made of sideways-looking sonar equipment to get an accurate picture of the sea floor topography.

As all this work has to be carried out from moderately sized vessels (500-2000 tons), it is, relative to most land-based geological operations, very expensive. From the early stages, most ships used have been chartered commercial vessels, suitably modified for the special requirements of geophysical surveys, seabed sampling, coring and shallow drilling (down to about 300m below seabed). Use of NERC ships has been limited to a few seasons work. Some surveys have also been conducted from Hydrographic Survey Ships in collaboration with the Hydrographic Department of the Navy and from French survey vessels during a period of Anglo-French collaboration in the survey of the English Channel (La Manche). IGS staff, who spend several months at sea each year, have varying opinions about the different life-styles which exist aboard these vessels, but on one point most agree — the cuisine aboard the French vessels created considerable problems for the weight-watchers.

The results of this work over the last fifteen years have given us a fairly detailed knowledge of a large part of the UK continental shelf and the production of 1:250 000 geophysical and geological maps, which include the land areas in many cases, is now well advanced. This advance of scientific knowledge, financed by the revenues from North Sea oil and gas, is perhaps the only way in which successive governments have put these revenues to any long-term use. The information obtained from these surveys is widely used in support of off-shore engineering ventures such as the routing of oil and gas pipelines.