Pipeline Route Investigation Using Geophysical Techniques
by P.J. Fenning & S. Hansan
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
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
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
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
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
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
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
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
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 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
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
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.
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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