by P.J. Fenning & S. Hansan

Recent increased expenditure on water supply infrastructure has involved the construction of many underground pipelines in a variety of geological situations. Variations in ground conditions not revealed by site investigation boreholes have sometimes led to major cost implications.

It is suggested that geophysical surveys along planned pipeline routes before construction can assist in highlighting potential problem areas and lead to the cost effective location of site investigation boreholes.

Background

In many instances the route of a new pipeline is constrained by significant factors such as land access, topographic variations along the route and local planning conditions.

The geological conditions along the route are often relegated to a minor consideration to be determined by a few shallow boreholes after the route has been almost decided and land access provisionally agreed. These boreholes or trial pits are often conveniently sited at regular intervals along a pipeline route. Unfortunately, the geological conditions are often variable and carry with them cost implications. Typical examples are when a sandstone which can be easily ripped by a machine changes laterally into a more durable lithology which requires blasting, or when the depth of soil cover overlying the bedrock decrease sharply, requiring a change in the type of excavator used.

In the selection of any pipeline route an initial desk study is carried out by engineers and planners, who collate all the available relevant information. This usually involves a geological appraisal in which local geological maps, photographs, old Ordnance Survey maps, the proceedings of local geological and archaeological societies and local archives (newspapers, museums) are collected and examined. Unfortunately, as reported by Howland (1991), there is no legal requirement in the UK for details of shallow site investigation boreholes to be filed with the National Data Bank of the British Geological Survey (BGS) at Keyworth in Nottinghamshire. Existing legislation requires that details of boreholes over 100 ft in depth for mineral exploration and over 50 ft in depth for water must be notified to the BGS. The most valuable information is lost to a pipeline route planner, who checks with the BGS for available borehole information, only to find that what is needed has not been recorded in the data bank.

The planner usually decides to use a small number of sample boreholes and trial pits at regular intervals along the pipeline route and concentrates all or some of them at known problem locations, such as former mine workings and valley crossings. The difficulty in this approach is that boreholes or trial pits located at regular intervals often do not encounter the problem areas. This is generally known as Murphy's law and is well illustrated in a classic example from the Love Canal area of Buffalo, USA, described by Benson et al. (1983), in which six boreholes or wells were drilled to investigate a concealed pollution plume, but did not make contact. A subsequent geophysical survey of inductive conductivity outlined the concealed pollution plume. Figure 1 vividly demonstrates the need to target boreholes.

A report from the Institution of Civil Engineers (Littlejohn 1992) commented 'Much money can be wasted by covering sites with regular grids of boreholes and extensive programs of routine tests rather than targeting the investigation towards areas whom information is required and by using more appropriate investigation methods'.

Thus the route planner needs assistance in targeting the boreholes and trail pits in the areas of potential subsurface problems. These problems have previously been encountered in the routing of hydrocarbon product supply lines for the petroleum industry and have often been solved, both on land and offshore, with the use of non-invasive surface geophysical surveys. White (1986) refers to the experience of the water industry in utilizing geophysical techniques to locate boreholes for water supply world-wide over many years.

Geophysical surveys

With the advent of modem electronics and computer-assisted geophysical interpretation methods, surface geophysical surveys offer cost effective assistance in the early identification of ground condition problems along a pipeline route. They assist in the targeting of anomalous areas where boreholes should be located. A number of geophysical techniques are available, including: inductive electromagnetic conductivity; electromagnetic ground probing radar; electrical resistivity; seismic refraction and reflection; magnetics; and gravity.

These geophysical techniques are based on the difference in physical properties between various geological strata and soils. In selecting a technique to investigate a specific location, it is rewarding to carry out a laboratory examination of hand specimens and borehole cores to determine the differences in the physical properties of the strata along a pipeline route during the desk study phase.

In some instances reference to the relevant BGS geological sheet memoirs of a specific location gives details of laboratory measurements of seismic velocity, magnetic susceptibility and electrical resistivity for representative lithologies in that area. A typical example (Fenning 1968) is in the memoir for the geology of the Elgin district (sheet 95). Unfortunately, this listing of physical properties of rocks geological memoirs, which started in the mid-1960s, now appears to have been discontinued.

The first three techniques relate to variations in the electrical properties of materials, whereas seismic refraction and reflection relate to the elastic properties. Magnetic surveys are related to variations in the magnetic mineral content and gravity surveys to the density variation of materials. A comprehensive account of most of these techniques can be found in Telford et al. (1990) and Griffiths & King (1981).

These techniques vary widely with respect to applicability and progress over the route, i.e. km/day and financial cost. In terms of applicability to specific problems, Table 1 relates the geophysical techniques to six parameters typically required in assessing a route, namely; depth to bedrock, rippability indication; corrosivity index; depth to water-table; lateral variations in lithology, including presence of faulted underground services.

Additionally, an attempt has been made in Table 1 to place an indiction on a cost per kilometer index. The lower the index number, the lower the financial cost. Naturally, any such assessment must be very generalized, but does indicate the general geophysical approach for a pipeline route assessment.

Table 1

Electromagnetic Inductive Conductivity

This parameter is the reciprocal of earth resistivity. It is a rapid reconnaissance technique which involves an operator carrying a 4 m long horizontal boom along a survey route. With modern data logging there is no need for the operator to stop and take point readings. Variations in the ground conductivity to a depth of 5.6 m are measured on a continuous basis, with pauses only to key in fiducial navigation/distance points.

Figure 2 show an EM-31 inductive conductivity meter in use. Typically one operator with such a meter can measure the electrical conductivity variations continuously over an 8-10 km route in a day. At the end of the day the data logger is connected to a computer allowing survey data to be listed and plotted in a very short time. Should information be required to greater depths, then two-man operated units such as the EM-34(3) are used, again with similar data logging. However, in this instance progress of the survey is slowed.

Figure 2

To relate the electrical conductivity variations measured to geological variations, it is necessary to tie in the data to the exposed surface geology and to any existing trial pits or boreholes.

Ground Probing Radar

This technique is generally considered to be fairly mobile. High frequency (50-500 MHz) radar pulses are transmitted into the subsurface and the corresponding reflection from the underlying strata are recorded. If the ground conditions are favorable to this technique, then the radar transducer may be pulled behind a survey vehicle at rates of 5-10 km/h. Radar profile, particularly in areas of electrically resistive rock, often give excellent results. Figure 3 shows an example of a radar plot over a subcropping limestone bed. However, in the extensive clay areas of many parts of the UK, radar penetration of clays, particularly if wet, is severely limited and often reduced to less than 1m.

Research by manufacturers of ground probing radar equipment has shown that the use of a much lower radar frequency (25-50 MHz) and slower survey progress, similar to seismic reflection surveys, will give more satisfactory results; progress of up to 2 km/day is still feasible.

Figure 3

Electrical Resistivity

These determinations are made by introducing an electrical current into the ground via electrodes, or metal rods, and measuring the resulting voltage distribution. Two survey modes are possible. In the first an electrode array is moved horizontally to detect lateral variations: the so-called electrical 'profiling' or 'trenching' method. In the second method the inter-electrode spacing is expanded about a fixed center and the variations in resistivity with depth are measured. This is termed vertical electrical sounding (VES). The electrical profiling method in which an array of metal electrodes is moved along a survey route by a field crew of two or three people has generally been superseded by the more cost effective inductive conductivity profiling methods. However, research by the University of Birmingham (see Griffiths et al. 1990), in which a large number of electrodes are inserted into the ground and a computer based system scans the whole array, effectively investigates a series of depth ranges and results in a resistivity 'pseudosection'. The VES method is effective for determining the variation of resistivity layering with depth at a given location. A realistic interpretation of the results will indicate the nature of the subsurface geological layering, the depth of overburden and the water-table. It is a technique by which two or three people could achieve 15-20 VES locations each day as a normal production rate. Again, correlation with the known geology, trial pits and boreholes gives a realistic interpretation. The VESs are often used to calibrate the conductivity variation detected by the rapid inductive conductivity method.

One bonus to the pipeline engineer of carrying out VES is that the likelihood of underground corrosion occurring on a buried metal structure, i.e. the future pipeline, is also determined. The procedure for carrying out this in situ corrosivity test is well documented in B.S. 1377 (Anon 1990) and should be specified in any general VES investigation along a pipeline route. Generally, the higher the apparent resistivity of the soil, the lower the risk of corrosion. Additional information is available in CP1021 (Anon 1973).

Seismic Refraction

This technique measures the velocity of a seismic wave through subsurface soils and is a function of the soil and rock density and elasticity. Additionally, seismic refraction surveys provide the depth to and the thickness of the underlying strata. Seismic refraction surveys involve the introduction of a seismic pulse, such as a hammer blow or small explosion, into the ground. A layout of sensitive vibration detector termed geophones, detects this seismic pulse transmitted through the subsurface strata. By measurements of the time taken for the seismic pulse to reach successive geophones, the characteristic velocity and thickness of the underlying layers can be measured.

This type of survey can be carried out at intervals along a pipeline route or, if funding is available, a continuous refraction profile may be carried out along the pipeline route. However, unlike inductive conductivity, progress is slow and involves a two to three person crew, which achieves 0.3-0.5 km of route each day.

In addition to giving the route planner the thickness of the underlying strata and, via an as assessment of the velocity variations, the likely nature of the subsurface lithology, the characteristic velocity can be correlated to underlying strata by excavating machinery.

Seismic Reflection

Recent advances in instrumentation have led to high resolution seismic reflection surveys becoming an accepted technique. This technique yields variations in the depth and thickness of the underlying rock layer, but no characteristic velocity information which has to be obtained by the refraction method or by a reflection survey located at a known borehole. Again, progress is slow, but high resolution of the subsurface layering thickness and variation is achieved.

Magnetics

In magnetic surveying, the variations in the Earth's total magnetic field due to anomalous underlain magnetic material are measured. Typically, on a pipeline route, anomalous magnetic material is ferrous material such as buried metal pipes and drums. However, old mine workings and shafts often show magnetic anomalies due to the presence of relict metals in the shaft linings or cappings. Mine shafts may be lined in a hard stone facing different in magnetic susceptibility from the surrounding host rock.

In areas where sedimentary rock prevails, magnetic surveys are of little assistance in monitoring variations in subsurface rock types, but in areas of igneous rock where basalt and granite prevail, magnetic surveys can be used to map the boundaries and contacts between various rock types and, on occasion, the thickness of overburden.

Magnetic surveying is a one-person technique requiring only a few seconds spent at each measurement location. A traverse distance of 2-3 km/day along a pipeline route with readings every 5in is achievable.

Gravity

A gravity survey involves the measurement of the variation in the Earth's gravitational field and variations are correlated with the variation in thickness and density of subsurface soils or rocks. Such a survey involves the measurement of the gravity variation at point locations, so the topographic elevation and spatial position of such locations must be know very accurately. Detailed corrections to the measured variation are required and generally make gravity surveys an unattractive proposition. Generally , a three-person crew (a topographic surveying crew and the gravity meter operator) is required and if the locations are measured at 5m intervals, progress is limited to 150 meter readings each day or a profile length of 750 m. Relative gravity variation data provides useful information on overburden thickness variations and lateral variations in bedrock density. Fenning (1968) and Becker et al. (1990) describe the application of the gravity technique in the detection of buried channels.

Additional Information

In carrying out geophysical surveys for geological appraisal there is a potential spin-off in locating manmade or so called 'cultural' features. As mentioned previously, magnetic surveys detect buried metal pipes and particular types of old mine workings and shafts.

A combination of techniques (the EM-31 inductive conductivity and total field magnetic survey) provide substantial information about buried metal pipes and services, often defining the locations where more sophisticated electromagnetic pipe and service location devices should be used to accurately define services before excavation.

Archaeological appraisal is becoming a necessary requirement in route planning and here the inductive conductivity, total field magnetics and ground probing radar are standard techniques in the detection of zones of likely archaeological interest.

Conclusions

The use of modern geophysical techniques can assist in locating boreholes or problem areas along a pipeline route, allowing cost effective targeting. In addition, useful information relating to concealed services and mains, archaeological appraisal, rock rippability and corrosivity will be obtained.

References

Anman, P. A., Cosway. S. W. & Redman, J. D. 1991. Water table detection with ground penetrating radar In: Expanded Abstracts S.E.G. Meeting, Houston, Texas, 494-496.

Anon 1973. Code of Practice for Cathodic Protection, B. S. CP]021. British Standards Institution. London, 56-64.

Decker, S. R., Benjamin, H. R. & Wolfe, P. J. 1990. Delineation of buried valleys using integrated geophysical Techniqucs. In: Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society, Golden, Colorado, 309-323.

Benson, R. C., Glaccum, R. A. & Noel, M. R. 1983. Geophysical Techniques for Surveying Buried Wastes and Waste Investigation. Report 68-03-3050 Environmental Monitoring System Laboratory. Office of Research and Development. US EPA, LAs Vegas.
Caterpillar Company 1988. Handbook of Ripping. 8th ad. Caterpillar Inc., Peoria, IL.

Fenning, P. J. 1968. Geophysical Investigation-the Geology of the Elgin District. Memoir Geological Survey Scotland. HMSO, London, 140-153.

Griffiths, D. H. & King, R. F. 1981. Applied Geophysics for Geologists and Engineers. Pergamon Press, Oxford.

Howland, A. F. 1991. New boreholes for old. Geoscientist. 1(5), 20-21.

Littlejohn, G. S. 1992. Inadequate Site Investigation Report by Ground Board of Institution of Civil Engineers. Telford, London.

Telford, W. M., Geldart, L P. & Sheriff, R. E 1990. Applied Geophysics. 2nd edn. Cambridge University Press, Cambridge.

White, R. 1986. Improved borehole siting success using integrated geophysical techniques. World Water, June, 265-268.