Pipeline Route Investigation Using Geophysical Techniques
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.
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
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
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.
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
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.
|Ground probing radar
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
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.
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.
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).
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
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.
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.
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
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.
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.
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
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.
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