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GEOPHYSICAL SURVEY REPORT

Project

Trial Geophysical Survey to Investigate Sub-Surface

Geology

Location

Barton-on-Sea, Hampshire

Client

Halcrow

Unit 1 Link Trade Park Penarth Road Cardiff CF11 8TQ United Kingdom Tel: +44 (0)8707 303050 Fax: +44 (0)8707 303051 Web: www.terradat.com

Job reference: 3468 Date: March 2012 Version: 1

Geophysical Survey Report- 3468

Barton-on-Sea March 2012 2

GEOPHYSICAL SURVEY REPORT

Project

Geophysical Survey to Investigate Sub-Surface Geology

Location

Barton-on-Sea, Hampshire

Client

Halcrow

Project Geophysicist: A Lewis, BEng MSc _________________

Reviewer: C Bird, BSc FGS _________________

Job Reference: 3468

Date: March 2012



CONTENTS

1 ....... INTRODUCTION ..................................................................................................... 4

1.1 Site description and history ............................................................................. 4

1.2 Geological setting ........................................................................................... 5

1.3 Survey objectives ............................................................................................ 7

1.4 Survey design ................................................................................................. 7

1.5 Quality control ................................................................................................. 7

2 ....... SURVEY DESCRIPTION ......................................................................................... 8

2.1 Survey layout and topographic survey ............................................................ 8

2.2 Electromagnetic survey (GEM-2) .................................................................... 9

2.2.1 ..... Electromagnetic survey field activity 9

2.2.2 ..... Electromagnetic survey data processing 10

2.3 Ground Penetrating Radar .............................................................................. 10

2.3.1 ..... GPR survey field activity 10

2.3.2 ..... GPR survey data processing 11

2.4 Resistivity survey ............................................................................................ 11

2.4.1 ..... Resistivity survey field activity 11

2.4.2 ..... Resisitivity data processing 12

3 ....... RESULTS AND DISCUSSION ................................................................................. 13

3.1 Ground Conductivity (Figure 2) ....................................................................... 13

3.2 GPR (Figure 3) ............................................................................................... 14

3.3 Resistivity Tomography (Figure 4) .................................................................. 15

3.3.1 ..... Line 1 15

3.3.2 ..... Line 2 16

3.3.3 ..... Line 3 16

4 ....... CONCLUSIONS....................................................................................................... 17

Figures

Appendices



1 INTRODUCTION

This report describes a trial geophysical survey carried out on the 19th and 20th March 2012.

The work was undertaken as part of a geological investigation of the sea cliffs at Barton-on-

Sea, Hampshire, on behalf of Halcrow (the client). The initial aim of the survey was to map

gravel channels incised into the shallow, clay-rich, solid geology. It was thought that these

channels may be acting as preferential drainage pathways that, in turn, may be accelerating

cliff edge erosion. Once the initial results of the trial were examined alongside on-site

observations, it was realised that the gravel channels were underlain by a relatively clay-

deficient sandy lithology. The focus of the investigation, subsequently, turned to the deeper

geology in order to delineate where the sand units become significantly clayey enough to

inhibit ground water flow, potentially creating a slip plane for cliff movement and subsequent

collapse.

1.1 Site description and history

Two sites for investigation were identified by the client where cliff edge collapses have

occurred in recent years. The location of the geophysical surveys at each site can be seen in

Figure 1 (page 9).

Site 1

The first site was located on the cliff top between the Cliff House Hotel to the west and a

tarmac car park, off Marine Drive, to the east. It comprised of a relatively flat grassy area

approximately 200m long by 50m wide. The trial area was 140m x 45m at the east end of the

area that included the zone of recent collapse (Plate 1).

Plate 1 – Site 1 looking west from car park



Site 2

The second site was situated in front of the Cliff House Hotel where cliff edge collapses have

brought the cliff to within 40m of the building (Plate 2).

Plate 2 – Site 2 looking east towards Cliff House Hotel

1.2 Geological setting

The solid geology beneath the area surveyed comprises gently easterly dipping silts, sands

and clays of the Barton Group. Beneath the survey area the Barton Group is subdivided in to

the Becton Sand Formation, Chama Sand Formation and Barton Clay Formation. This group

is overlain by drift deposits of Pleistocene Plateau Gravels (fluvially derived gravels) and

‘Brickearth’ (aeolian silts and clays). The distribution of the sediments is outlined in

schematic 1 and a photo showing the shallow geology in a section of the cliffs is shown in

Plate 3, the lithological and physical properties of the formations present are outlined in table

1.



Schematic 1 – An idealised cross section of the geology at Barton-on-sea (adapted from Melville and Freshney, 1982)

Table 1 – Physical properties of the various lithologies present beneath the survey area.

Plate 3 – relationships of the shallow geological units



1.3 Survey objectives

The initial objective of the trial survey was to map gravel channels incised into underlying

clays. However, initial interpretation of resistivity results alongside on-site observations and

discussion with the client revealed that, in this location, the plateau gravels overlie the clay-

deficient Becton Sand Fm and was unlikely to act as a preferential drainage route. Of more

importance therefore, is the depth at which the significant increase in clay content may begin

to act as an aquitard creating a potential slip plane for rotational cliff slump erosion.

1.4 Survey design

Given the scope of the trial survey brief it was decided to adopt an integrated survey

approach using several geophysical techniques as listed below:-

• Electromagnetic Ground Conductivity (Geophex GEM-2) - to map variations in

ground conductivity within near surface material across the site. This survey

technique indicates areas of higher clay and water content in the shallow subsurface

that may help map any preferential drainage pathways as well as highlighting the

presence of drainage structures.

• Ground Penetrating Radar – to provide cross-sections of reflected radar signal to

identify laterally extensive subsurface geological boundaries, and near surface

services and drainage.

• Resistivity Tomography Survey – to provide cross-sections of electrical resistivity

beneath the survey lines to reveal the distribution of materials of contrasting electrical

properties. This can identify the depths to different geological units or lithologies, and

of particular importance in this project, the depth to clay rich material could act as an

aquitard.

1.5 Quality control

The geophysical data were collected in line with normal operating procedures as outlined by

the instrument manufacturer and TerraDat company policy. On completion of the survey, the

data were downloaded from the survey instrument on to a computer and backed-up

appropriately. The acquired dataset was initially checked for errors that may have been

caused by instrument noise; low batteries, positional discrepancies etc. and any field notes

were either written up or incorporated in the initial data processing stage. The dataset was

processed using the standard processing routines and once completed, the resulting plots



were subject to peer review to ensure the integrity of the interpretation. Our quality control

standards are BS EN ISO 9001: 2008 certified.

2 SURVEY DESCRIPTION

The survey was carried out using the following geophysical methods:

• Ground Conductivity

• Ground Penetrating Radar (GPR)

• Resistivity Tomography

Background information for the survey methods is provided in the appendices and

descriptions of the actual survey work carried out on site are provided in the sections below.

2.1 Survey layout and topographic survey

The initial survey area (site 1) was indicated by the client and incorporated the recent cliff

edge collapse near the car park. The survey area was approx. 140m long x 45m wide with

the car park forming the east edge. A grid was marked on the ground with 3m line spacings

from the road to near the cliff edge. The grid and other topographic features of note were

subsequently surveyed using a Topcon 7003i Total Station and referenced to on site objects

that enabled the survey to be located on the client’s digital plan for the area.

Figure 1 shows the location of the surveys undertaken. The Gem-2 survey was undertaken

over the whole area using the 3m spaced grid lines. Two GPR (100MHz and 250MHz

antennae) profiles were acquired adjacent to Resistivity Line 2 and two resistivity trial lines

(Lines 1 and 2), were undertaken both ending at the car park and levelled using the Total

Station to an accuracy of +/-1cm.

The second area of interest (site 2) was located in front of the Cliff House Hotel. At this site a

single resisitivity profile was acquired traversing in front of the hotel and ending at the

footpath in the east. The profile was located and levelled using previously established

reference points from site 1.



Figure 1- Location of Surveys

2.2 Electromagnetic survey (GEM-2)

An electromagnetic survey involves transmitting an electromagnetic field into the subsurface

and picking up returning signal via a receiver in the same instrument. Data are acquired on a

grid covering the area of interest and a contoured plan of the variation in ground conductivity

across the site is produced. The presence of conductive materials in the subsurface such as

clay, water, mudstone, ash, some contaminants, leachate, metal etc. can be evident as

regions of high values on the ground conductivity plan while materials such as coarse

grained sediments, dry drainage zones and many bedrock types will appear as regions of

low values.

2.2.1 Electromagnetic survey field activity

The electromagnetic data were acquired using a, cart-mounted, Geophex GEM-2 (Plate 4).

An area of approximately 140m x 45m was surveyed (see Figure 1). The instrument was

connected to a Trimble dGPS system and readings were taken at approximately 0.2m

intervals along 3m spaced survey lines traversing east-west in line with the road and cliff

edge.



Plate 4 – Cart-mounted GEM-2 ground conductivity meter being used at site 1

2.2.2 Electromagnetic survey data processing

The data were downloaded from the instrument using WinGem software, and then exported

as xyz files that can be read into Excel, each data file was then processed to compensate for

any instrumental drift. The final data set was then read in to Oasis Montaj where it was

gridded, displayed and overlain with the client’s site topographic plan before being exported

to Corel Draw for final annotation.

2.3 Ground Penetrating Radar

A Ground Penetrating Radar (GPR) survey involves the transmission of a pulsed

electromagnetic (radio) wave and the recording of any returning reflection events. Readings

can either be taken as the radar unit is towed continuously or at closely spaced intervals

along the selected traverse line. The transmitted waves are focused into the ground and can

penetrate soils, rock, concrete, and many other natural and man-made materials. Given a

sufficient contrast, reflection events from geological or hydrological boundaries can be

observed together with ‘point’ sources such as buried services, rebar, voids and large

boulders.

2.3.1 GPR survey field activity

A MALA RAMAC radar system with shielded 100MHz and 250MHz antennae were used to

acquire radar profiles (Plate 5) along the line of Resistivity profile 2.



Plate 5 – GPR survey at site 1 using 100MHz antenna (left) and 250MHz antenna(right)

2.3.2 GPR survey data processing

The data processing was carried out using Reflex software. Each radargram is first

processed with an appropriate gain function to enhance features of interest and increase the

signal from weak responses at depth. The radargram is then ‘time zero’ corrected to allow a

depth conversion to be calculated using a nominal velocity of 0.1m/nanosecond that is

usually suitable for most sediment types. Additional processing routines (e.g. background

removal, deconvolution, FK filtering, migration etc) may be applied to improve the coherency

of the reflection events and remove any multiple reflections and diffractions. The final

radargrams are exported to CorelDraw for annotation and presentation.

2.4 Resistivity survey

A resistivity survey involves the injection of a D.C. electrical current into the ground at

various electrode spacings along each section using stainless steel electrodes to ensure

good electrical contact at each survey station. An electrical cross-section of the subsurface

is then derived from the recorded data. A diverse range of features such as gravel lenses

and channels, clay-rich sediments, water tables, fracture zones, in-filled solution features,

bedrock structure and man-made ground can be imaged in cross-section using a resistivity

survey. A feature may be targeted using resistivity tomography given sufficient electrical

contrast with its surroundings.

2.4.1 Resistivity survey field activity

Site 1

A 72-channel IRIS SYSCAL resistivity system (Plate 6) utilising the Wenner-Schlumberger

electrode array type was used to acquire two profiles at this site. Firstly, a 142m long



resistivity profile approximately 8m from the cliff edge, and behind the collapse area, was

acquired using 4 cables and 72 electrodes. A spacing of 2m between electrodes was

employed to achieve high data resolution to a depth of approximately 25m bgl.

A second profile was then acquired approximately 25m from the cliff edge to measure the

deeper geology using a wider electrode spacing of 4m. This produces a lower resolution

section but a greater investigation depth of approximately 40m bgl.

Plate 6 – 72-channel IRIS SYSCAL resistivity meter Resistivity survey

Site 2

A optimum resolution to investigation depth was now employed at site 2 using 3m spaced

electrodes, resulting in a 213m long resistivity profile to a depth of approximately 30m bgl.

i.e. down to sea level. The resisitivity profile traversed in front of the Cliff House Hotel,

ending at a footpath to the east of the hotel.

2.4.2 Resisitivity data processing

Once the data is downloaded, it is processed using Res2dinv software to derive modelled

electrical cross-sections of the subsurface. The ground levels for each electrode are

incorporated in both the data processing and presentation. This data is then exported into

Surfer 9 where it is gridded and presented as finished cross sections; these sections are

then displayed as 2D colour contoured plots and annotated in Corel Draw.



3 RESULTS AND DISCUSSION

The locations of the surveys are presented in Figure 1 and the results of each method are

presented in Figures 2 to 4.

3.1 Ground Conductivity (Figure 2)

Following a review of the ground conductivity data, it was decided only to consider the

response from the highest frequency channel (47 kHz). In general terms, a localised

increase in conductivity values is usually indicative of a relative increase in clay or moisture

content of the sub-surface, though the presence of other conductive materials such as ash,

slag or metallic material may also be indicated. Extreme fluctuations in the conductivity

values (+ve/-ve) are indicative of instrument ‘overload’ due to interference from nearby metal

structures/debris. The interpretation of the conductivity data is generally based on both

published electrical properties of typical sedimentary materials (Table 2) and when available,

correlation with on-site information.

0.1

0.1 0.01

1

1

10

10

100

100

1,000

1,000

10,000

10,000

100,000

RESISTIVITY (Ohm.m)

CONDUCTIVITY (mS/m)

Sea ice

Clays Sands

Tills

Shales ConglomeratesSandstones

Salt water Fresh water

Dolomite, limestone

SEG publication Near Surface Geophysics

Lignite, coal

Permafrost

GLACIAL SEDIMENTS

SEDIMENTARY ROCKS

WATER, AQUIFERS

Table 2 – Table of resistivities/conductivities of typical sedimentary materials

The results of the ground conductivity survey are presented in Figure 2 as a colour

contoured plot of variation in the bulk conductivity of the subsurface over, approximately, the

top 5m. Using this colour scale low conductivity material (dry/clay-deficient/granular) is

represented by browns, intermediate values which represent an increase in clay and/or

moisture are represented by greens and highly conductive material which is likely to indicate

significant clay/moisture content is represented by blues.



A zone of high conductivity can be seen along the northern boundary of survey in close

proximity to Marine Road. This is thought to relate to the presence of buried services located

parallel to the path bounding the survey area.

Due to the presence of the conductive ‘Brickearth’ across the site, the returning signal of the

ground conductivity is thought to have been limited, principally, to this horizon. The similar

electro-magnetic properties of the plateau gravels and the underlying Barton Sands would

have also made it difficult to detect palaeo-channels even if this layer were thinner or less

clay-rich. The plot does however, clearly show a linear network of what are presumed to be

field drains orientated approximately northwest-southeast in the west of the survey area. A

curved resistive zone in the northwest of the survey area correlates with a raised

topographic feature apparent on the ground, and is presumed to be a man-made feature

comprised of well drained coarse material such as gravel.

The survey becomes notably less conductive towards the east of the survey area as a result

of either lower clay/moisture content in the ‘Brickearth’ or a thinning of the ‘Brickearth’

stratum and associated shallowing of the plateau gravels.

3.2 GPR (Figure 3)

A description of typical radar features are provided below:

Reflection event – A laterally continuous interface between materials of contrasting

electrical properties (controlled largely by composition and moisture content of the material).

Examples of reflecting surfaces are soil horizons, soil-rock or air-rock interfaces, water

tables, and solid metallic or non- metallic objects.

Diffraction – A diffraction hyperbolic curve usually indicates a ‘point’ source, such as a void,

buried service or an edge-feature (e.g. wall). A zone of small diffractions can indicate rebar

or granular/blocky material.

Signal character – A degree of interpretation may be made based on the observed changes

in the character of the radar signal such as attenuation, loss of penetration and

reverberation.



It was hoped that the low frequency 100MHz radar system would penetrate deep enough to

map the more clay rich sands many metres below the ground surface. However the trial

showed that the penetration achieved was only ~3m due to attenuation of the radar signal

caused by the presence of the conductive ‘Brickearth’ as measured by the resistivity survey

(section 3.3)

Several higher frequency 250MHz GPR profiles were acquired an example of which is

shown in Figure 3. Although the penetration is less than the 100MHz (~2m) and therefore

not of any use in mapping the geology at the site, the profile has clearly shown many shallow

man-made features/services and made ground.

3.3 Resistivity Tomography (Figure 4)

The results of the resistivity survey are presented in Figure 4 as colour contoured scaled

sections of the subsurface showing changes in resistivity. The vertical and horizontal axes,

respectively, display elevation and chainage along the profile line. The interpretation of the

modelled resistivity sections is generally based on both published electrical properties of

typical sub-surface materials (Table 2) and when available, correlation with on-site

information/observations. Table 1 also delineates the likely geo-electrical properties of the

lithologies found on site.

Historic borehole information has been digitised and overlain onto the sections, initially it

appeared that there were many boreholes coincident with the location of the resistivity

surveys, however, it transpired that most of the western boreholes were CPT locations and

of those that were coincident in the east, A1 is located 18m south of line 1 (now within the

slumped material) and #5 is located, ~30m, east of lines 1 and 2. Borehole #5 does not

include the same subdivisions currently ascribed to the ‘Upper Barton Beds’. The borehole

information may not accurately reflect the lithologies or thicknesses at the locations of the

resistivity profiles but have been included in Figure 3 as a guide.

3.3.1 Line 1

Line 1 had an electrode spacing of 2m, which has resulted in a section which displays a high

spatial resolution specifically in the very shallow sub-surface. There is an upper conductive

layer is ~2m thick with values <100Ω/m these values and this thickness are consistent with

the presence of the silty clays which form the ‘Brickearth’. Beneath this shallow conductive

material is a layer of intermediate-resistive values >100Ω/m. This layer is ~8m thick with



higher values in its upper part, these values are ~300-600Ω/m indicating clay deficient

granular material which is interpreted as representing the Plateau Gravels. Beneath this are

intermediate values which decrease with depth, these are thought to represent the Becton

Sand Fm and the decreasing values represent increasing clay content within the formation,

though it is still a sand dominated lithology. The sections have been scaled so that values

less than 100Ω/m are represented by blue colours. This has been done in order to

graphically represent where lithologies become clay-rich (based on the values present on

Table 2). It is likely that the boundary between yellow and blue represents the transition from

silty sands to sandy clays within the Chama Sand Fm (zones H2 and H1). There is a slight

dip to the west on this boundary which contradicts the overall dip of the geology in this area

(which should dip gently east) this may be indicative of a shallow channel feature within the

Chama Sand Fm. The values continue to increase with depth which both indicates and

correlates with the increasing clay content of the underlying geology.

3.3.2 Line 2

Line 2 was carried out, parallel to line 1 with an electrode spacing of 4m, this has enabled

the survey to image deeper, over a longer profile but with reduced resolution resulting in the

absence of the thin layer of conductive values representing the Brickearth. The intermediate-

resistive values here are present at a slightly greater depth (down to 20m AOD) which may

indicate a north easterly dip to the geology in this area. The upper zone of intermediate-

resistive values represent the same lithologies present on line 1, lateral variations in

resistivity are likely to reflect local lithological variations. The contact between the clay-rich

and clay-deficient lithologies exhibits a shallow ‘bowl’ feature which may represent a channel

feature. At a depth of ~10m AOD there is a significant reduction on resistivity (down to <20

Ω/m) which indicates the presence of a significantly clay-rich lithology, this correlates with

the logged depth of the Barton Clay Fm as logged in BH#5.

3.3.3 Line 3

Line 3 was carried out at an electrode spacing of 3m as an optimum compromise between

the two previous surveys and was carried out in the western site 2. The lithologies previously

discussed are present, however, the main feature present on this section is the shallowing of

the boundary between the intermediate-resistive values of the sand-dominated sediment and

the conductive values of the clay-rich lithology beneath. This correlates with the mapped

easterly dip of the geology.



4 CONCLUSIONS

• The ground conductivity survey has characterised variations within the shallow

subsurface which have highlighted lithological changes within the drift deposits and

the presence of engineered structures and services. The conductive nature of the

‘Brickearth’, the thickness of the gravel and the lack of contrast between the Plateau

Gravel and the Becton Sand Fm has meant that the ground conductivity has not

been successful in mapping the presence of palaeo-channels, however, where the

contrasts and thickness of drift deposits is favourable the method may still yield

results.

• A ground penetrating radar also relies on the transmission of an electromagnetic

wave the presence of shallow conductive material has limited its efficacy as a means

to map the deeper geology, the method has proven as an effective service mapping

tool.

• The resistivity tomography has proven the most effective geophysical method for

investigation the subsurface geology of the site allowing discrimination between

lithological units based on their clay-content. An electrode spacing of 3m has given

the ideal compromise between depth of penetration and resolution.

Disclaimer

This report represents an opinionated interpretation of the geophysical data. It is intended for

guidance with follow-up invasive investigation. Features that do not produce measurable geophysical

anomalies or are hidden by other features may remain undetected. Geophysical surveys compliment

invasive/destructive methods and provide a tool for investigating the subsurface; they do not produce

data that can be taken to represent all of the ground conditions found within the surveyed area. Areas

that have not been surveyed due to obstructed access or any other reason are excluded from the

interpretation.

Figures

Scale:

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CB/3468/2

Date:

FIGURE 2

GROUND CONDUCTIVITYSURVEY

Drawn by/Ref:

MARCH 2012

1:600 at A4

BARTON-ON-SEAGEOPHYSICAL TRIAL

Title

Project

Effect of buried service

Lineations indicate the presence of land drainage

Resistive values may indicate shallowing gravels

or a reduction in the moisture content of the Brickearth

Increase in conductivity is likely to represent

an increase in moisture content within the silts

Location of GroundConductivity survey

Curving resistive zone correlates with

curvi-linear raised topographic feature

A

B0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0

1

2

(mB

GL)

0m

Scale:

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CB/3468/3

Date:

FIGURE 3

GROUND PENETRATINGRADAR SURVEY

Drawn by/Ref:

MARCH 2012

NTS

BARTON-ON-SEAGEOPHYSICAL SURVEY TRIAL

Title

Project

A

B

Hyperbolic diffraction indicates a 'point' source,such as a void, buried service or an edge-feature

Zone of planar response indicates an area ofenginereed ground

‘Blocky’ response indicates the presenceof a granular material, likely to be gravel

242m

Scale:

Tel: +44 (0) 8707 303050

AM/3468/4

Date:

RESISTIVITY TOMOGRAPHY

Drawn by/Ref:

MARCH 2012

1:1000 at A3

FIGURE 4

BARTON-ON-SEAGEOPHYSICAL TRIAL

Title

Project

5

Brick Earth

Plateau

Zone I

Zone H

Zone F2

Silt

Gravel

Sand

Sand, clayey at lower parts

Clay

Zone Simplified description

Not to scale

0

9

10

11

12

15

21

36

70

118

136

156

186

279

341

454

604

Resistivity

( /m)W

0.1

0.1 0.01

1

1

10

10

100

100

1,000

1,000

10,000

10,000

100,000

RESISTIVITY (Ohm.m)

CONDUCTIVITY (mS/m)

Sea ice

Clays Sands

Tills

Shales ConglomeratesSandstones

Salt water Fresh water

Dolomite, limestone

SEG publication Near Surface Geophysics

Lignite, coal

Permafrost

GLACIAL SEDIMENTS

SEDIMENTARY ROCKS

WATER, AQUIFERS

W

W

E

E

BOREHOLE LEGEND

5

BH A1 offset18m to the south

W E

Line 1

Line 2

Line 3

Shallow conductive layer

represents the ‘Brickearth’

Intermediate-resistive material represents

sand-domainted sediments of the Becton

Sand Fm and the Chama Sand Fm

Decreasing resistivity due

to increasing clay content

Boundary between sand-dominated

and clay-dominted sediments

Extremely low resistivity values correlate with

the logged depth of the Barton Clay Fm

Highest resistivity values are thought to

represent significantly clay-deficient/dry

areas within the arenaceous sediments

Highest resistivity values are thought to

represent significantly clay-deficient/dry

areas within the arenaceous sediments

Intermediate-resistive material represents

sand-domainted sediments of the Becton

Sand Fm and the Chama Sand Fm

Decreasing resistivity due

to increasing clay content

Boundary between sand-dominated

and clay-dominted sediments

Ele

vation

(m

)A

OD

Chainage (m)

Chainage (m)

Chainage (m)

Ele

vation

(m

)A

OD

Ele

va

tio

n (

m)A

OD

Appendices

ConstraintsPower lines, buildings, metal structures (fences, rebar, vehicles, debris etc.) and buried services can interferewith the electro-magnetic measurements.

Appendix - Ground conductivity (EM) survey

Scintrex CG-3Mgravitymeter

EDM surveyinstrument

General principle of EM surveyingTowed EM-38 with dGPS

Mounted EM-31 with dGPS

EM-31

GPS antenna

line marking system

transmitter receiverprimary EM field

modifiedprimary field

secondaryfield

conductor

surface

eddy currents

shallowlimestonebedrock

clay-richsediments

Ground conductivity data plot

linear feature

A nvolves the generation of an EM field at the surface andsubsequent measuring of the response as it propagates through the subsurface. The main components of the

a transmitter coil (to generate the primary EM field) and receiver coil (to measure the inducedsecondary EM field). The amplitude and phase-shift of the secondary field are recorded and are thenconverted into values for

ground conductivity or electromagnetic (EM) survey i

instrument are

ground conductivity and in-phase component (metal indicator).

The ground conductivity (EM) instruments are either hand carried or mounted/towed behind a quad bike.Readings are usually taken on a regular grid or along selected traverse lines and positional control can beprovided by dGPS if there is sufficient satellite coverage.

The selection of the particular EM instrument (EM-38/EM-31/GEM-2) is primarily based on the requiredpenetration depth of the survey. However for most conductivity surveys the GEM-2 has replaced the moreconventional EM-31 instrument due to its ability to simultaneously acquire data at different frequencies (i.e.different depth levels) and a greater depth of penetration.

The results from the EM survey can be presented as colour contoured plots of conductivity and inphase (metalresponse) data. In general terms, a relative increase in conductivity values usually indicates a local increase inclay content or water saturation. However, if there is a corresponding increase in the inphase response, theinfluence of some artificial source is likely (i.e. metal).

At the end of each survey, the survey data isdownloaded to a field computer and corrected for instrument, diurnal and positional shifts. Additional editingmay be carried out to remove any 'noisy' data values/positions.

EM-38Single frequency

Exploration depth ~1.5m

EM-31Single frequency

Exploration depth ~3 to 5m

GEM-2Multi-frequency

Exploration depth up to 10m

GPS antenna

EM-38 mountedwithin trailer

A Ground Penetrating Radar (GPR) survey involves one or two people either continuously towing a radar

system or taking readings at very closely spaced intervals along selected traverse lines. GPR systems use a

pulsed electromagnetic (radio wave) transmitted via a tuned frequency antenna that can penetrate soils, rock,

concrete, and many other natural and man-made materials. Reflection events from geological or hydrological

boundaries between sufficiently contrasting materials are recorded via a receiver antenna. A time-depth

cross-section (radargram) of the shallow subsurface is constructed as the radar system is moved along a

survey line. The radargram can be depth calibrated to enable detailed interpretation given known or measured

velocities for the materials being investigated. While viewing relatively raw radar data can prove useful in the

field there are numerous processing routines that can be employed to significantly improve the results. Final

sections are presented showing annotated features of interest with apparent depth calibration.

In order to improve the quality of the recorded radar data, a number of processing routines can be applied to the

data using dedicated software (REFLEX). The final radar sections are converted to depth by applying a

conversion velocity, which is usually based on an average velocity value for the local sediments. Without any

additional calibration the measured depth to a particular feature is likely to be resolved within a 20% error

margin depending on the local velocity structure.

The main limitations affecting radar surveys are the presence of conductive materials near surface (e.g., clay

and water) which reduce penetration, and blocky material which scatters signal.

Constraints:

Metal Electrode

Multi-core Cable

Iris ResistivityMeter

0

Distance (m)

Depth

(m)

10

5

0

50

Dipping beds withinsand dune

Water table reflection

Diffraction curvesdue to gravel zone

Lack of radar signal penetration due to clay-rich material

Ground Radar profile over a parabolic sand dune (100MHz)

Horizon 2

Horizon 1

Horizon 3

Service

TxRx

Antenna

ControlUnit

Observed diffraction curves over asub-surface cavity within limestone

10.0

5.0

0.0

16

De

pth

(m)

0Distance (m)

Appendix - Ground Penetrating Radar (GPR)

General principle of Ground Radar GPR Survey in progress

The Resistivity technique is a useful method for characterising the sub-surface materials in terms of their

electrical properties. Variations in electrical resistivity (or conductivity) typically correlate with variations in

lithology, water saturation, fluid conductivity, porosity and permeability, which may be used to map

stratigraphic units, geological structure, sinkholes, fractures and groundwater.

The acquisition of resistivity data involves the injection of current into the ground via a pair of electrodes and

then the resulting potential field is measured by a corresponding pair of potential electrodes. The field set-up

requires the deployment of an array of regularly spaced electrodes, which are connected to a central control

unit via multi-core cables. Resistivity data are then recorded via complex combinations of current and

potential electrode pairs to build up a pseudo cross-section of apparent resistivity beneath the survey line. The

depth of investigation depends on the electrode separation and geometry, with greater electrode separations

yielding bulk resistivity measurements from greater depths.

The recorded data are transferred to a PC for processing. In order to derive a cross-sectional model of true

ground resistivity, the measured data are subject to a finite-difference inversion process via RES2DINV (ver

5.1) software.

Appendix - Resistivity Tomography

Data processing is based on an iterative routine involving determination of a two-dimensional (2D) simulatedmodel of the subsurface, which is then compared to the observed data and revised. Convergence betweentheoretical and observed data is achieved by non-linear least squares optimisation. The extent to which theobserved and calculated theoretical models agree is an indication of the validity of the true resistivity model(indicated by the final root-mean-squared (RMS) error).

The true resistivity models are presented as colour contour sections revealing spatial variation in subsurfaceresistivity. The 2D method of presenting resistivity data is limited where highly irregular or complex geologicalfeatures are present and a 3D survey maybe required. Geological materials have characteristic resistivityvalues that enable identification of boundaries between distinct lithologies on resistivity cross-sections. Atsome sites, however, there are overlaps between the ranges of possible resistivity values for the targetedmaterials which therefore necessitates use of other geophysical surveys and/or drilling to confirm the natureof identified features.

Readings can be affected by poor electrical contact at the surface. An increased electrode array length isrequired to locate increased depths of interest therefore the site layout must permit long arrays. Resolution oftarget features decreases with increased depth of burial.

Constraints:

P1/P2 = Potential electode

C1 C2P1 P2

Multicore CableMetal Electrode

C1/C2 = Current electode Resistivity meter

Current Lines

Ground Surface

V

I

Metal Electrode

Multi-core Cable


Metal electrode

Multi-core Cable


Typical field set-upGeneral resistivity principle

Modelled resistivity section

-10

-8

-6

-4

-2

0

2

4

6

Conductive zone - possible fault structure

-

0 10 20 30 40 50 60 70Distance (m)

Ele

va

tion

(m)

Near-surface resistive layer

Resistivityohm.m

50

61

74

91

111

135

165

201

245

299

365

446

544

663

809

987

10

-8

-6

-4

-2

0

2

4

6

Resistive bedrock strata

Clay-rich sediments

geophysical survey report - nfdc · geophysical survey report- 3468 barton-on-sea 4 march 2012 1...

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