Elsevier Encyclopedia of Geology

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method or methods being proposed. For example,

in the case of electromagnetic and magnetic

methods, signal degradation or geophysical ‘noise’

may be introduced by the presence of surface me-

tallic structures and overhead power lines. For

microgravity or seismic surveys, noise may result

from traffic movements or wind and waves. Where

the level of noise exceeds the amplitude of the

anomaly due to the target and where this cannot

be successfully removed, the target will not be de-

tectable. The best way to assess the likely influence

of site conditions is to visit the site at the design

stage and/or carry out a trial survey.

The design of a survey can be aided by the use of

powerful two-dimensional and three-dimensional

forward-modelling geophysical software. With in-

formation on the expected target size, depth, and com-

position, a geophysicist can evaluate the feasibility

of a particular method, including likely error bars

on modelled size or depth, and can determine opti-

mum survey design parameters such as the sampling

interval and the optimum configuration for the

method (e.g. transmission frequency for ground-

penetrating radar and electromagnetic surveys or the

geophone spacing in a seismic survey).

Figures 6 and 7 illustrate the results of a feasibility

study for a tunnelling project in London, UK. Figures

6A and 7A are the client’s conceptual model of a

Victorian well thought to be associated with a risk

of subsidence caused by tunnelling at 20 m depth. The

feasibility analysis facilitated an early appreciation of

Figure 3 Examples of different display formats. (A) Plot of residual magnetics data showing ferrous targets, draped on an

aerial photograph of the 80 ha brownfield site. (B) Modelled seismic velocity cross-section for proposed pipeline route. (C) Three-

dimensional visualization of the geometry of the base of a landfill cell (green) and the top of leachate (blue mesh) derived from a

combined two-dimensional resistivity imaging and time-domain electromagnetic survey.

486 ENGINEERING GEOLOGY/Geophysics

the limitations of the selected methodology and en-

abled a go/no-go decision to be made on carrying out

a full-scale geophysical survey.

Working closely with the engineer, a geophysical

advisor can clarify the likely value of using geophysics

in a site investigation. Assuming that geophysics is

viable for solving a particular problem, then the next

step is to help the client decide what combination of

geophysical screening and direct sampling would give

the optimum information on the subsurface for the

budget available (Figure 8).

Applications

Common applications of engineering geophysics can

be broadly separated into seven main classes, each of

which encompasses a range of different target types:

1. transport infrastructure – the application of geo-

physics in the design and monitoring of structures

such as roads, railways, bridges, tunnels, and

canals;

2. foundation design – the measurement of the engin-

eering properties of soils and bedrock, for example

in determining the depth to a pile-bearing layer;

3. pipeline route evaluation – the selection of appro-

priate route and design criteria, for example meas-

uring soil corrosivity for cathodic protection;

4. hazards identification – the identification and

monitoring of natural hazards such as cavities

and landslides and man-made structures including

mineshafts and unexploded ordnance;

5. non-destructive testing of structures;

6. containment structures – integrity testing and

monitoring of structures such as dams and land-

fills; and

7. buried assets – mapping the location of utilities

and underground storage tanks.

Targets tend to be either isolated structures – such

as a mineshaft, tunnel, foundations, or underground

tank – or laterally extensive – such as bedrock layers,

overburden, or groundwater. Table 3 presents some

examples of the use of geophysics to solve common

engineering problems.

Transport Infrastructure

Geophysical methods have a valuable role to play

in both the design and monitoring of linear en-

gineering structures, such as roads, railways, and

canals, and related structures, such as bridges and

tunnels.

The near-continuous lateral coverage afforded by a

technique such as continuous-wave electromagnetic

profiling is particularly useful during route selection,

as it provides a rapid and cost-effective means of

assessing near-surface ground conditions. Reconnais-

sance surveys of this type may be followed up by more

detailed studies using quantitative techniques such

as seismic refraction profiling and two-dimensional

resistivity imaging within areas identified as being

of particular risk or that display anomalous ground

conditions (Figure 9).

Figure 4 Approximate minimum radius of cavity that will produce a 15 mgal microgravity anomaly in host rock of density 2.3 Mg/m

ENGINEERING GEOLOGY/Geophysics 487

The main application of seismic refraction profiling

is in the determination of bedrock depth and depth

to groundwater (Figure 10). However, analysis of the

P-wave velocity values obtained during such surveys

can also be employed to map the variability in the

strength of the bedrock along a route. The evaluation

of so-called bedrock rippability is particularly useful

in areas where excess material may need to be re-

moved, such as in cuttings and the formation of em-

bankments, in helping to determine the type of plant

that will be required.

The heavy weight of traffic on roads and railways

means that monitoring the condition of road pave-

ments and track ballast is essential for proactive

maintenance and planning of renewals. Ground pene-

trating radar (GPR) is particularly suited to this

task. Modern digital GPR acquisition systems enable

detailed mapping of changes in road-pavement con-

struction, layer thicknesses, and ballast thickness

and condition at speeds in excess of 100 km h

1

(Figure 11).

GPR can also play an important role in the evalu-

ation of structures such as bridges and tunnels. Com-

mon applications include the identification of voiding

behind brick or concrete tunnel linings and mapping

the location and condition of bridge-deck reinforcing.

In order to determine layer thicknesses accurately

the velocity of the GPR signal within the materials

Figure 5 Example of the effects of aliasing on the detection of a pipe using magnetic profiling. The profile shown in Figure 5A

illustrates correct sampling of the anomaly, whilst the profile shown in Figure 5B illustrates the smoothing effects of under-sampling.

488 ENGINEERING GEOLOGY/Geophysics

overlying an interface needs to be determined. This

is normally done directly through the use of core

samples but can also be derived indirectly using

geometric techniques such as curve-fitting on hyper-

bolic anomalies and common mid-point analysis. The

effective use of core samples requires that the data be

accurately located.

Foundation Design

Geophysical methods are routinely used to measure

the engineering properties of soils and bedrock as an

input to the design of foundation structures including

piles. These properties include but are not limited

to soil resistivity, shear modulus, Poisson’s ratio, and

percentage moisture content. These properties are

usually derived from measurements taken from

the surface or from boreholes. Methods for deriving

low-strain elastic moduli include surface-wave seis-

mics, seismic refraction, cross-hole seismics, seismic

tomography (Figure 2), and sonic logging.

Wireline logging has been used to correlate

stratigraphy between boreholes and provide a clearer

definition of stratal boundaries and type. In the

Figure 6 (B) The use of two-dimensional forward modelling to determine the feasibility of using the microgravity method to detect

(A) Victorian water wells. The size of the anomaly is 10 mGals, which, in the conditions prevalent on site (busy streets in London), would

have been below the resolution of modern digital microgravity meters.

ENGINEERING GEOLOGY/Geophysics 489

example shown in Figure 12, taken from a log from

the Isle of Dogs in London, UK, the electrical log

demonstrated a difference between the upper and

lower parts of the Thanet Sand that was not obvious

when looking at standard soils data such as

gradings. The electrical log was matched to measure-

ments of limit pressure in pressuremeter tests, and as

a lower bound to SPT (Standard Penetration Test)

data. The geophysical tool proved to be a power-

ful and cost-effective alternative for delineating the

pile-bearing horizon in the Thanet Sand.

Pipeline Investigations

Determination of the most suitable route for a

pipeline and the type of materials or protective

measures required to prevent its corrosion in the

ground can both be aided by the use of geophysical

techniques.

As in the case of other linear structures such as

roads, route evaluation may require the determin-

ation of bedrock depth, mapping of lateral variations

in the nature of overburden materials, and the

Figure 7 (B) The use of two-dimensional forward modelling to determine the feasibility of using the microgravity method to detect

(A) collapse structures associated with Victorian water wells. The size of the anomaly is 25 mGals, which is readily detectable using

modern digital microgravity meters.

490 ENGINEERING GEOLOGY/Geophysics

location of subsurface hazards or anomalous ground

conditions that may affect the construction or long-

term integrity of the structure. Methods commonly

employed include seismic refraction and electrical

resistivity imaging for profiling bedrock depth, to-

gether with resistivity soundings and electromagnetic

profiling for overburden characterization. The linear

nature of pipelines means that they often traverse a

diverse range of terrains, and, consequently, the type

of hazard that may be encountered will be varied and

not always obvious. Figure 13 illustrates data from a

detailed microgravity survey undertaken along the

proposed route of a pipeline in the UK with the ob-

jective of locating buried mineshafts. Forward and

inverse modelling of the data allowed the likely size

and depth of the shaft to be determined.

Figure 8 Example of a cost–benefit analysis to determine the

highest-value spend (i.e. most information derived) for a fixed

site-investigation budget; in this brownfield case 35% of the

budget was allocated for site-wide geophysical screening, to

target trial pits and boreholes that might otherwise miss important

features.

Table 3 Examples of the application of geophysical methods in engineering investigations

Application Example Typical methods

Transport infrastructure

Route evaluation Determination of thickness

of overburden

Seismic refraction, 2D resistivity imaging depending on nature of

expected lithologies and water table

Location of buried channel Seismic refraction or 2D resistivity imaging for large-scale structures

Identification of karstic

structures (swallow

holes, cavities)

Continuous-wave electromagnetic profiling (reconnaissance),

microgravity and seismic refraction for detailed modelling

Pavement/ballast

assessment

Measurement of thickness

of pavement or track

layers and mapping

location of construction or

condition changes

High-speed GPR (200–1200 MHz)

Structural assessment Mapping location, depth,

and condition of

reinforcing within

concrete

High-frequency (900–1500 MHz) GPR, continuous-wave

electromagnetic profiling

Identification of voiding,

moisture retention behind

tunnel wall lining

High-frequency (900–1200 MHz) GPR, infrared thermography

Foundation design

Stratigraphic Determination of depth to

pile-bearing layer

Seismic refraction/reflection, cross-hole seismics, seismic

tomography, surface-wave seismics, wireline geophysics

Mapping boundaries of

layers

Seismic refraction, seismic reflection, cross-hole seismics, seismic

tomography, surface-wave seismics, wireline geophysics

Measuring the stiffness of

soils

Seismic refraction, seismic reflection, P- and S-wave cross-hole,

up-hole, and down-hole seismics, cross-hole seismic tomography,

surface-wave seismics, wireline geophysics

Rock rippability Seismic refraction

Groundwater table Seismic refraction, 2D resistivity imaging, time-domain electro-

magnetic sounding, 1D resistivity soundings, wireline logging

Structural Mapping fault zones Very-low-frequency continuous-wave electromagnetic profiling,

seismic reflection

Pipeline route evaluation

Route evaluation –

geological

Determination of thickness

of overburden

Seismic refraction, seismic reflection (P- and S-wave), 2D resistivity

imaging, microgravity, continuous-wave electromagnetic, time-

domain electromagnetic sounding

Rock rippability Seismic refraction

Corrosivity of soils Resistivity, continuous-wave electromagnetic, redox potential

Continued

ENGINEERING GEOLOGY/Geophysics 491

The prevention of corrosion of buried pipelines

requires knowledge of the aggressivity of the soils

along the proposed route. A soil’s aggressiveness

depends on three main factors: its pH, redox poten-

tial, and electrical conductivity. Once these factors

have been determined suitable preventative measures

can be put in place. These can include cathodic pro-

tection, zinc coating, or the use of plastic rather than

steel pipes.

The soil electrical conductivity (or resistivity) might

be determined at selected positions along the route

using techniques such as one-dimensional resistivity

or time-domain electromagnetic sounding or in a

more continuous manner using methods such as elec-

tromagnetic profiling and capacitively coupled towed

resistivity imaging.

Resistivity soundings have the advantage of provid-

ing a profile of the variation in soil resistivity with

depth, thereby enabling aggressive layers within the

soil to be avoided by altering the depth of the pipe.

However, as with other point-sampling investigation

tools such as boreholes, areas of importance along a

Archaeological assessment Map anthropogenic features

such as buried pits, walls,

foundations, crypts

Magnetics, resistivity profiling, GPR, 2D resistivity imaging, magnetic

susceptibility

Hazards identification See below See below

Hazards identification

Geological Natural cavities and

sinkholes

Microgravity, GPR, 2D resistivity imaging, continuous wave

electromagnetic

Brownfield and current

industrial

In-ground obstructions Continuous-wave electromagnetic, magnetics, time-domain

electromagnetic, GPR, microgravity

Mineshafts Microgravity, continuous-wave electromagnetic, magnetics, 2D

resistivity imaging, GPR, infrared thermography

Detection of live cables Passive electromagnetic

Detection of unexploded

ordnance

Magnetics, time-domain electromagnetic, GPR

Mapping contaminant

sources and pathways

Continuous-wave electromagnetic, 2D resistivity imaging, spectral

induced polarization, self potential, GPR, microgravity

Mapping existence and

number of underground

storage tanks

Continuous-wave electromagnetic, magnetics, time-domain

electromagnetic, GPR, microgravity

Containment structures

Construction Quality control of

construction process

Electric leak location, GPR, resistivity imaging

Integrity of engineered

structure over time

Self potential, 2D/3D resistivity imaging, continuous-wave

electromagnetic, GPR, borehole geophysics

Buried assets

Utilities Verifying existing maps and

producing 3D plans

Passive electromagnetic, continuous-wave electromagnetic,

magnetics, time-domain electromagnetic, GPR

Underground storage tanks Pre-purchase audit Magnetics and/or time-domain electromagnetic, GPR, microgravity

(for non-metallic tanks)

Table 3 Continued

Application Example Typical methods

Figure 9 Cross-section illustrating the results of a two-dimensional resistivity imaging survey along a proposed road route in

southern Ireland. The objective of the survey was to determine the thickness of the glacial overburden overlying the limestone

bedrock. Areas of high resistivity, relating to the bedrock, appear orange and red, whilst areas of lower resistivity, due to the overlying

glacial deposits, appear green and blue. The deep area of overburden on the left of the image relates to infilling of a solution feature.

492 ENGINEERING GEOLOGY/Geophysics

route may be missed. Techniques such as continuous

wave electromagnetic profiling enable more continu-

ous coverage but require multiple passes with differ-

ent instrument configurations to determine variations

in resistivity with depth. As with other route investi-

gations, it is often beneficial to undertake an initial

reconnaissance survey using a profiling method and

then target areas of interest with soundings.

Hazard Identification

Knowledge of potential subsurface hazards is

critical to the safe development and maintenance of

Figure 10 Seismic refraction survey to map depth to bedrock along a proposed road route.

Figure 11 GPR pavement survey, illustrating two types of pavement construction.

ENGINEERING GEOLOGY/Geophysics 493

an engineering structure. A lack of information on

the location and nature of both natural and man-

made hazards, such as caves, mineshafts, buried land-

fills, and unexploded ordnance, could potentially be

catastrophic and could lead to costly delays.

p0205An example of how geophysics can be used to

mitigate the risk posed by natural sinkholes is illus-

trated in Figure 14. A previous site investigation,

incorporating four boreholes located one in each

corner of the site, had determined that the depth to

chalk bedrock and the condition of the chalk were

essentially similar in each borehole. The regulator

was informed and permission was given to construct

the landfill cell. Not long after commencement, the

earthworks contractor encountered large sinkhole

features in the chalk, and the works were halted.

The regulator requested that a geophysical survey be

carried out to clarify the extent of the problem. A

ground-conductivity map of the 1 ha area highlighted

numerous sinkhole features as ground-conductivity

highs. These features were then further investigated

using resistivity imaging and cone penetrometer

testing to determine the depth of the sinkholes.

A hypothetical exercise was carried out to determine

how many boreholes would have been required to

match the detail provided by the geophysical survey.

Figure 12 Comparison of geophysical (resistivity in green) and

geotechnical (Menard pressuremeter and Cambridge in situ pres-

suremeter) results for a borehole in London, UK. (Published with

permission of Arup 2003.)

Figure 13 Modelling the microgravity response of a buried mineshaft. (A) The blue line represents the observed gravity data, whilst

the red line represents the calculated response due to (B) a cylindrical source body.

494 ENGINEERING GEOLOGY/Geophysics

Figure 14 shows that for a three-by-three (total of

nine) or even a five-by-five (total of 25) grid of bore-

holes the information on the extent of sinkhole devel-

opment would have been patchy at best. The most

cost-effective sequence of events would have been to

carry out the ground-conductivity survey first and

then drill three or four boreholes in targeted areas

that were revealed by geophysics to be representative

of background and anomalies. Figure 15 presents an

example of the use of ground penetrating radar to

locate potentially hazardous voids within karstic

limestone. The high resolution obtained from GPR

surveys is ideal for the detection of small-scale near-

surface cavities but under good conditions, such as in

unweathered hard rock, the technique is also capable

of providing information to significant depth. In these

circumstances it is often used to complement data

obtained from more traditional techniques such as

microgravity profiling.

Projects on land associated with, or affected by,

military activities may have an increased risk of en-

countering unexploded ordnance. Examples include

Figure 14 Comparison of the results of undertaking an electromagnetic (apparent conductivity) survey (centre) across a site to map

solution features in chalk and a more traditional site-investigation approach using just boreholes (left and right). Red and magenta areas

on the apparent-conductivity map represent sinkholes filled with moist sand. The blue and green areas correspond to competent chalk.

Figure 15 The use of GPR to locate voiding within karstic limestone for a port development in the Dominican Republic. The areas of

white and black on this B-scan plot (or ‘radargram’) are indicative of clustered small-scale (less than 0.5 m in diameter) voids within

otherwise competent limestone. The vertical axis on the plot represents the two-way travel time of the GPR signal through the rock

measured in nanoseconds.

ENGINEERING GEOLOGY/Geophysics 495

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