GEOPHYSICAL AND GEOCHEMICAL MARINE SURVEY OF CORNWALL
By Nigel C. Kelland
Research workers: W. N. Li, J. Wheildon, P. Ong.
Diving team: N.C. Kelland (diving leader) D, Durnley, Miss L. J. Dunn, E. Sigurdson, R. Wiley, M. Weeks, F. Davey, D. Welch, R. Warton, B. White, R. Pullin. A. Kingwell, J. McKie, P. Jenkins, R. Powell, J. Love.
Ever since the advent of civilisation man has had many uses for the mineral wealth to be found beneath the surface of the ground. In by-gone ages the search and exploitation of these various resources was generally ill-planned and inefficient. It would often rely on either discovering surface evidence of mineralization, or on intuition and intelligent guess work by the miner.
The present century has seen the introduction of the various methods of geophysical and geochemical exploration in the search for new mineral resources. These methods are unable to indicate the exact location, quality and quantity of any given ore, but they can be used to indicate the most likely areas of mineralization. Further, a proper analysis of the results can, sometimes, give the depth and extent of an ore body. Thus it is possible to plan the optimum location for trial drillings, and eventually the main mine shafts.
This type of work is being carried out successfully today in many land areas of the earth. However, such exploration is confined to the land areas of the earth’s surface, an area covering a mere 29 per cent, of the total surface. Consequently it is of interest 10 extend the various exploration methods to continental shelf areas. Unfortunately, the techniques of geological, geochemical and geophysical prospecting which are applied to sub-aerial geology are unsuited to the underwater world, and it is necessary to devise new techniques.
Within the last four years the Geophysics Department of imperial College has been engaged in various research projects related to developing exploration techniques for marine areas. Their investigations have involved the Sparker method, various electrical methods, and the use of a towed magnetometer. This work has been carried out, in the main, off the coast of Cornwall. This summer the work was continued, and the Geochemistry Department also decided to launch a research project related to marine exploration.
Geophysical measurements, however, are insufficient in themselves due to the ambiguity inherent in their interpretation, and must be correlated with geological sampling. Geochemical exploration depends on the laboratory analysis of samples taken in the field, whether rock or sand, and also relies on some form of sea-bed sampling. It is in this very important problem of sampling techniques that, in waters down to 200 fee, the aqua-lung diver comes into his own. Also, further techniques of both sciences rely, for their efficient usage, upon some form of monitoring, and it is again in this connection that valuable work can be carried out by the aqua- lung diver.
In previous work done by the Geophysics Department in Cornwall. all the sampling work required could be performed by relatively few divers. This year the introduction of a geochemical research programme, together with the geophysical work, called for a much larger team. Also, the work continued from the end of April until the beginning of August. To fit in with the requirements it was necessary for the divers to work on a rota basis. From the end of April to the end of June two divers were available. From the end of June until the middle of July eight divers were in Cornwall, and from the middle of July the divers steadily decreased such that the final work was carried out by one diver. In all, 14 divers from the Imperial College Sub-Aqua Club were involved throughout the period.
Accommodation for the research team and two divers was provided in Lelant for the whole of the summer. The main diving team was accommodated in St. Ives from the end of June onwards. Transportation of equipment and personnel was provided by the Geophysics Department Landrover, and work would commence at about 7.45 a.m. when the Landrover arrived in St. Ives to collect the divers.
The first diving work was in connection with Sparker observations in St. Ives Bay. This geophysical technique involves the production of a shock wave by a high voltage spark discharge below the sea surface. The acoustic energy is reflected by the sea-bed and any sub-bottom layers present and detected by a towed hydrophone. It is important to have an exact knowledge of the depth of floatation of both towed devices. The work of a diver entailed being in the water and gauging the floatation depth of the towing cable and the device, relative to a marker line.
Other work at this stage was mainly devoted to establishing an efficient method of carrying out sampling and temperature measurements. This work was performed, in the main, from a boat called ‘Shamrock’. She was ideally suited for diving purposes, being a large (70 ft.) barge with a shallow draught and flat bottom, giving a large deck and cabin space and a low free board. An extra useful facility was a 30 ft. boom, which proved to be indispensable in the loading of various heavy pieces of diving equipment.
Three types of sampling can be effected underwater: sand or mud, seaweed and rock sampling, each with its own peculiar problems.
In this case problems arise due to the continual sea water movement, and a consequent unknown factor in the degree of sand migration. Would a sand sample taken in any one area be accurately representative of that area? To investigate this problem it was determined to collect samples from the surface using a shovel and polythene bag, and to compare the analysis (grain size and composition) with the samples taken using a grab sampler and a geochemical auger. The first device was controlled from the surface, whereas the second required the use of a diver who would collect a sample about one and a half feet below the sea bed.
Using a shovel proved very easy and initial work was performed by divers equipped with air cylinders. The grab sampler was operated from the surface, and in the beginning its performance was checked by a diver. The auguring proved a far more difficult task and required about 10-15 minutes of hard physical work to twist it to the required depth. The analysis of the sand samples showed that the diver or grab collected sample was sufficient.
Analysis of in situ rock samples is more diagnostic of the presence of any anomalous distribution, Three types of rock samples were collected; Killas or slates, a harder metamorphic diabase called blue elvin, and a very hard granite. The initial collecting work was carried out using a geological hammer. This proved suitable for sampling the softer rocks, but it was impossible to obtain any granite. For this purpose we were fortunate to have the use of a low pressure percussion hammer, on loan from Holman Brothers, Ltd., of Camborne. This hammer was tested in depths up to 110 ft. and an efficient sampling system was worked out.
Other work involved the development of a thermal technique for detecting changes in rock type, or the presence of sulphide mineralisation. The early work led to the development of a weighted device which automatically forced the temperature measuring heads about six inches into the sediments. However, analysis of results showed that a penetration exceeding one and a half feet was required. This could only be achieved by divers who could knock the probes the required distance into the sediments.
Throughout the summer divers were further employed in monitoring the underwater performance of such devices as dredges and corers. In fact this latter work, although unsuccessful, proved to be the most exciting, and even hazardous, of the whole summer.
Initially all work was carried out by divers equipped with air cylinders. However, air cylinders seriously limit the duration of any dive, especially in deeper waters. Further, they are awkward and heavy, and must be refilled regularly. This last factor was especially annoying at the beginning when the only supply of high pressure air was provided by a bank of British Oxygen cylinders. Later, the Club high pressure compressor was mounted on board ‘Shamrock’. This was used to refill the bottles immediately after the completion of a dive, and greatly added to the efficiency and ease of the survey.
However, much of the rock sampling did not require the mobility provided by the diver wearing air cylinders. Consequently, a diving system was introduced utilising a low pressure compressor. This could supply unlimited quantities of air, via an air line, to a diver down to depths of 120 feet. As a safety precaution the diver carried a small reserve high pressure cylinder on his back. Every diver was trained in its use by the simple expedient of stopping the compressor when he was about 40 feet below the surface. Such a system was seen to allow the most efficient usage of the diver, with the minimum effort, as well as being more economic.
However, an important limiting factor in diving is that of decompression. When a diver is working in deep water his system accumulates nitrogen, and should he remain under water for any excessive length of time he must stop, whilst ascending, and decompress. The decompression time increases with the time of dive and the working depth. Also, excessive physical exertion under water increases the respiration rate, and compensation must be allowed for this. Thus, although the air line allowed the diver us much time as he desired, his physical and physiological limits now dictated how long he might dive. Such considerations as these, coupled with the very real limitation of a low water temperature, made the early divers look forward to the arrival of the main team.
The low pressure air compressor was also used to provide the air required for the percussion hammer. One test with this instrument demonstrated its working limit to be 115 feet, this being 95 per cent, of its calculated maximum. A second carefully controlled experiment demonstrated that, provided a large reservoir was in the air circuit, it was possible to provide sufficient air for the percussion hammer and the diver simultaneously. As a result of these experiments, subject to the proper working of the percussion hammer, this system was used in obtaining nearly all the rock samples. In fact it was obvious that without such a system, the required rock sampling work could not have been successfully carried out. However, considerable maintenance difficulties arose due to the severe corrosive action of the sea water on the percussion hammer, and necessitated it being thoroughly cleaned every few hours. (Other difficulties were those associated with the high noise level, although this could be overcome with practice.)
An exceedingly important aspect of any diving operation is the level of safety precautions. During this past summer circumstances showed that the precautions taken were adequate and satisfactory. At all times a safety diver was “kitted up” on deck ready for any emergency. The actual working diver was connected to the surface via a safety line and a telephone line. Further, when working off the low pressure compressor, he had an additional air line connection with the surface. This last connection provided a convenient safety precaution as it was possible to “hear” the diver breathing from the variation in the compressor revolutions.
The most efficient and satisfactory safety link was that of the telephone. A surface operator wore a conventional head set and a transistorized amplifier strapped to his belt, and could listen to the diver breathing at all times. The diver wore a bone conduction transceiver against his head. Communication from-surface to diver was very simple, but, due to the impediment of the mouth piece, it required considerable practice for the surface operator to fully understand the diver. The advantages of a telephone were demonstrated during the experiment determining the feasibility of supplying both the diver and the percussion hammer simultaneously from the low pressure compressor. It was possible for the surface operator to hear the diver and to inform the diver continuously of the outlet pressure gauge readings as he descended. In the instance of a telephone failure conventional diving signals were used.
Manner of work
When working from ‘Shamrock’ one diver was dressed by tenders in the low pressure apparatus, and then completed two or three dives—the exact number depending on his total dive time as recorded by a surface control. If he reported sand on the telephone he would proceed to fill a polythene bag with sand using a shovel tied to his wrist. He would note such facts as the presence and characteristics of ripple marks. If rock was reported the chipping hammer was lowered gently on top of the diver's bubbles, and he would proceed to obtain a sample. Sometimes it was necessary to collect three or four samples along a traverse line planned to intersect, say, granite-killas contact. In such an instance, using the telephone and ship's compass and noting the position of the diver’s bubbles, the surface control could guide the diver, who also wore a compass, in the desired direction. This technique radically increased the diver’s efficiency as a rock sampler because it removed the necessity of his returning to the surface after each sample, in order to retake his bearings.
Preliminary geochemical analysis of sand samples taken on a half mile grid covering practically the whole of Mounts Bay as far as Porthleven, revealed two areas as locations of possible mineralisation, and it was decided to implement more detailed bulk sampling in both areas. In this work a weighted line, approximately 1,500 feet long, was laid along the sea bed from the ship's gig, and anchored at both ends.
A diver, wearing air cylinders, swam down one anchor line and filled a polythene bag tied to the anchor, with sand. He then swam along the line 100 feet to where a second polythene bag was tied, and proceeded to fill it with sand. This was repeated at intervals of 100 feet along the whole length of the line. The diver was connected by a telephone and life line to the gig, and reported to the surface control his depth, the state of the bottom, and his position along the line. Thus it was an easy matter for the surface control to maintain the gig exactly over the diver.
When a line was finished it was pulled in, the sand samples transferred to proper sampling bags, and the complete procedure repeated for a second traverse. Meanwhile, the air cylinders, which were now empty, were refilled aboard ‘Shamrock’ from the high pressure compressor.
During the final two weeks the divers worked with the temperature probe method. This was performed from a very sea-worthy fishing vessel in St, Ives Bay. Such a beat was not really suited to diving, and a day's work was preceded by a prayer for fine weather and a calm sea.
A diver, wearing air cylinders, swam to the bottom carrying a long probe, which he would knock about one and a half feet into the sea bed using a mallet. After each dive, when the probe had come to equilibrium, the temperature was measured. The diver then re-entered the water and attempted to knock the probe a further one and a half feet into the sand. The divers found the effort required so great that it took up to twenty minutes to knock in the last six inches.
Thereafter, measurements were taken at different Decca points in the bay. Great difficulty was experienced in this work due to the severe ground swell of St. Ives Bay which swept the diver away from the probe such that only one in three hammer strokes connected.
The total diving time logged throughout the summer was 69 hours and five minutes. In this time 879 samples were collected, 79 dives made in connection with temperature measurements and 20 monitoring dives. The working depth varied from 0 to 155 feet, although the majority of work was in water not exceeding 100 feet.
Nigel in the red coat.