GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical...

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GEOPHYSICAL SURVEY REPORT Project Geophysical survey to map former foundations and obstructions Location Harbourside development, Port Talbot Client WPS OPUS Unit 1 Link Trade Park Penarth Road Cardiff CF11 8TQ United Kingdom Tel: +44 (0)2920 700127 Fax: +44 (0)8707 303051 Web: www.terradat.co.uk Job reference: 5906 Date: March 2018 Version: 2

Transcript of GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical...

Page 1: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

GEOPHYSICAL SURVEY REPORT

Project

Geophysical survey to map former

foundations and obstructions

Location

Harbourside development, Port Talbot

Client

WPS OPUS

Unit 1 Link Trade Park Penarth Road Cardiff CF11 8TQ United Kingdom Tel: +44 (0)2920 700127 Fax: +44 (0)8707 303051 Web: www.terradat.co.uk

Job reference: 5906 Date: March 2018 Version: 2

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Geophysical Survey Report – 5906/2

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GEOPHYSICAL SURVEY REPORT

Project

Geophysical survey to map former foundations and

obstructions

Location

Harbourside development, Port Talbot

Client

WPS OPUS

Project Geophysicist: J Thomas BSc PhD FGS _____________

Reviewer: M Bottomley BSc MSC _____________

Job Reference: 5906

Date: March 2018

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CONTENTS

1 ....... EXECUTIVE SUMMARY .......................................................................................... 5

2 ....... INTRODUCTION ...................................................................................................... 6

2.1 Site description ................................................................................................ 6

2.2 Geological setting ............................................................................................ 7

2.3 Survey objectives ............................................................................................ 7

2.4 Survey design .................................................................................................. 7

2.5 Quality control ................................................................................................. 8

3 ....... SURVEY DESCRIPTION ......................................................................................... 8

3.1 Survey layout and topographic survey ............................................................. 9

3.2 Metal detection survey (EM-61) ....................................................................... 9

3.2.1 ..... Metal detection survey (EM-61) - field activity 9

3.2.2 ..... Metal detection survey (EM-61) - data processing 10

3.3 Magnetic survey (G-858) ................................................................................. 10

3.3.1 ..... Magnetic survey - field activity 10

3.3.2 ..... Magnetic survey - data processing 11

3.4 Electromagnetic (GEM-2) ................................................................................ 11

3.4.1 ..... Electromagnetic - field activity 12

3.4.2 ..... Electromagnetic - data processing 12

3.5 Ground penetrating radar (GPR) survey (Trivue) ............................................. 12

3.5.1 ..... GPR survey - field activity 13

3.5.2 ..... GPR survey - data processing 13

4 ....... RESULTS AND DISCUSSION ................................................................................. 14

4.1 Summary discussion ....................................................................................... 16

5 ....... CONCLUSIONS ....................................................................................................... 20

Figures

Figure 1 – Location plan

Figure 2 – EM61 – Metal Detection

Figure 3 – Magnetic – Total Field

Figure 4 – GEM-2 – Quadrature phase Figure 5 - Interpretation Plan

Figure 6 – Ground Radar – Selected sections

Figure 7 – Ground Radar – Selected sections

Figure 8 – Ground Radar – Selected sections

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Appendices EM-61 metal detection

Magnetics

GEM-2

Ground Radar

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1 EXECUTIVE SUMMARY

A geophysical survey was carried out at the Harbourside development site, Port Talbot. The

survey was commissioned by WSP OPUS (the Client) on behalf of Neath Port Talbot County

Council and took place between the 31st January and 2nd February 2018. The primary

objective of the survey was to map any former foundation structures and buried obstructions.

The survey area was approximately 3.2Ha and mainly consisted of areas of hardstanding,

remnant floor slabs and other residual surface structures following demolition work.

Historically the site has been occupied by steel works, chemical plant and some light industrial

units.

Based on the survey brief, an integrated geophysical survey was carried out consisting of

metal detection, magnetic, electromagnetic and ground radar techniques. The results of the

metal detection (EM-61) and electromagnetic (GEM-2) surveys were very good. However,

both the magnetic and radar techniques were limited by interference from the slag content

and variability within the made ground.

The resulting plots display a wide range of responses across the survey area which is typical

for this type of site. These include broad zones, lineations, localised anomalies and isolated

features. The most significant features have been highlighted on the corresponding plots and

tabulated. The most notable features include the large circular response (believed to be a

former gas holder) and the numerous rectilinear zones (remnant floor slabs/buried structures).

There are also several smaller zones, which could either be buried structures or discarded

debris within the made ground. There are also several lineations, some of which seem to

correspond with the perimeter of some of the historical buildings, while others are likely to be

buried services.

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2 INTRODUCTION

This report describes a geophysical survey that was carried out at the Harbourside

development site, Port Talbot. The survey was commissioned by WSP OPUS (the Client) on

behalf of Neath Port Talbot County Council and took place between the 31st January and 2nd

February 2018. The primary objective of the survey was to map any former foundation

structures and buried obstructions.

2.1 Site description

The Harbourside site is located approximately 500m south of Port Talbot town centre and is

centred on OS National Grid Reference 276580E, 189520N. The survey area was

approximately 3.2Ha and mainly consisted of areas of hardstanding, remnant floor slabs and

other residual surface structures following demolition work. For the purpose of this report, the

survey area has been divided into 6 zones as illustrated in Figure 1 and detailed below:

Zone A: Raised plateau region along the western edge of the survey area which appears to

have been partially landscaped.

Zone B: Small fenced off area containing some former floor slabs, manholes and other

structures.

Zone C: Small area with circular concrete feature and raised structure.

Zone D: Small fenced off gravel covered area, relatively flat with remnant floor slab.

Zone E: Main area with a mixture of demolition rubble hardstanding, remnant floor slabs, large

circular base and other residual structures.

Zone F: Tarmac road section.

Based on information provided by the Client and historical maps, the western portion of the

site was largely developed with buildings and railway for industrial use (Port Talbot Steel

Works). Reconfiguration of the buildings occurred around 1964 when they were then mostly

replaced by roadways in the western area and the eastern portion of the site was developed

as a chemical works. The works buildings were demolished and the site generally levelled at

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an undetermined date between 1997 and 2007. A selection of historical maps is included in

Figure 9.

2.2 Geological setting

Previous and more recent ground investigations on site have revealed the ground conditions

to comprise:

Made Ground: encountered across the site and was predominately granular and coarse in

nature, comprising a mixture of ash, slag, brick, concrete and stone with various inclusions of

deleterious material including metal, glass and wood. The thickness of the Made Ground

ranged from 1.0m bgl to 3.4m bgl, with an average thickness of 2.1m.

Superficial Deposits: In most of the exploratory holes that encountered natural strata,

variable granular and cohesive superficial deposits were encountered beneath the Made

Ground materials. The Made Ground, Alluvial Clay and Alluvial Sand deposits were underlain

by Alluvial Gravels and were proven to depths >10.0m bgl and indicated to be dense to very

dense granular strata.

Bedrock: Interbedded strong Sandstone and weathered Mudstone.

2.3 Survey objectives

The primary objectives of the survey were to:

Map the extent of residual foundation structures and buried obstructions.

2.4 Survey design

To map the location of the target structure, it was decided to adopt an integrated survey

approach comprising the following techniques:

Metal detection survey (Geonics EM-61) – to map variations in the metal (ferrous and

non-ferrous) content of the subsurface.

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Magnetic survey (Geometrics G-858) – to map the presence of ferrous objects and

disturbed material in the near surface.

Electromagnetic (Geophex GEM-2) – to map variations in electrical properties

(clay/water/metal content) of the subsurface.

Ground Penetrating Radar (GPR) survey (Utsi Trivue) – To provide detailed cross-

sectional images and depth plans that would be used to identify anomalous shallow

features.

2.5 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 was downloaded from the survey instrument onto a computer and backed up

appropriately. The acquired data set was initially checked for errors that may be 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 data set is then processed

using the standard processing routines and once completed; the resulting plots are subject to

peer review to ensure the integrity of the interpretation. Our quality control standards are BS

EN ISO 9001: 2008 certified.

3 SURVEY DESCRIPTION

The geophysical survey was carried out using the following techniques:

Metal detection survey (EM-61)

Magnetic survey (G-858)

Electromagnetic survey (GEM-2)

Ground Penetrating Radar (GPR) survey (Utsi Trivue)

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The results of the surveys are presented in the form of interpreted data plans and sections

indicating the location and physical characteristics of identified anomalous features, together

with a description.

Background information for each of 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.

3.1 Survey layout and topographic survey

A Network RTK dGPS system was used to set out a local survey grid that was preferentially

aligned approximately southeast-northwest to suit the alignment of the survey area. The

geophysical data sets were then acquired under both dGPS and local grid control.

The base map was supplied by the Client and contains a combination of both existing and

historical features. However, as the site has undergone more recent changes, there are some

discrepancies. Some additional topographical points were recorded during the geophysical

survey and these have been added to the original plan.

3.2 Metal detection survey (EM-61)

A metal detection survey is based on measuring the response of the ground due to the

propagation of an electromagnetic field. In contrast to other electromagnetic techniques, the

EM-61 is a time domain instrument, which means that measurements are made at set time

intervals following the cessation of the primary field. The magnitude of the response depends

on the quantity and distribution of metallic (ferrous and non-ferrous) materials within the

shallow subsurface (<3m deep).

3.2.1 Metal detection survey (EM-61) - field activity

The metal detection data were acquired using a Geonics EM-61 instrument (Plate 1). The

dataset was acquired at a nominal 0.2m interval along a series of 3m spaced parallel survey

lines.

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Plate 1: EM-61 instrument

3.2.2 Metal detection survey (EM-61) - data processing

The dataset is downloaded from the data logger and compiled using dedicated software

DAT61. Initial editing was carried out to remove positional errors and rogue values. The data

is then exported as an ‘XYZ’ file and translated into the appropriate coordinate system. The

next step is to bring the data into OASIS MONTAJ, where it can be edited and manipulated to

enhance any features of interest. The colour contour plots are then integrated with the base

plan information, and the resulting plans are exported to CORELDRAW for final annotation.

3.3 Magnetic survey (G-858)

A magnetic survey is a passive technique that measures local perturbations in the Earth’s

wider magnetic field, caused by the presence of magnetically susceptible objects in the

subsurface. These subtle perturbations are mainly a result of the presence of ferrous metals,

but some ferrous-bearing materials, such as fired red brick, slags, and ash can also give a

measurable response.

3.3.1 Magnetic survey - field activity

The magnetic survey was conducted using a Geometrics G-858 instrument which was

mounted on a bespoke hand cart (Plate 2). The data were recorded uni-directionally along 3m

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spaced survey lines in a near-continuous walking mode. The magnetic dataset was acquired

under EGNOS corrected GPS control (EZ-Guide) to an accuracy of +/- 0.5m.

3.3.2 Magnetic survey - data processing

The dataset is downloaded from the data logger and compiled using dedicated software

MAGMAP2000. Initial editing is then carried out to remove positional errors and rogue values.

The data is then exported as an ‘XYZ’ file and translated into the appropriate coordinate

system. The next step is to bring the data into OASIS MONTAJ, where it can be edited and

manipulated to enhance any features of interest. The colour contour plots are then integrated

with the base plan information, and the resulting plans are exported to CORELDRAW for final

annotation.

Plate 2: G-858 instrument

3.4 Electromagnetic (GEM-2)

An electromagnetic survey is based on measuring the response of the ground due to the

propagation of an electromagnetic field. The magnitude of this response depends on the type

and distribution of conductive material in the subsurface and can be broken into two

components, namely the quadrature and in-phase components. The quadrature is usually

converted to ground conductivity, which reflects the electrical properties of the subsurface (i.e.

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clay, water, ash, metal), while the in-phase component is dependent on the magnetic

susceptibility of the subsurface.

3.4.1 Electromagnetic - field activity

The conductivity data were acquired using a multi-frequency GEOPHEX GEM-2 instrument

that was mounted on a bespoke hand cart (Plate 2). The dataset was acquired at a nominal

0.5m interval along a series of 3m spaced parallel survey lines. For this particular survey, the

instrument was primarily configured to investigate depths of 3 to 5m below ground level. The

GEM-2 dataset was acquired under EGNOS corrected GPS control (EZ-Guide) to an accuracy

of +/- 0.5m.

3.4.2 Electromagnetic - data processing

The dataset is downloaded from the data logger and compiled using dedicated software

WINGEM-3. Initial editing was carried out to remove positional errors and rogue values. The

data is then exported as an ‘XYZ’ file and translated into the appropriate coordinate system.

The next step is to bring the data into OASIS MONTAJ, where it can be edited and

manipulated to enhance any features of interest. The colour contour plots are then integrated

with the base plan information, and the resulting plans are exported to CORELDRAW for final

annotation.

3.5 Ground penetrating radar (GPR) survey (Trivue)

A ground penetrating radar (GPR) survey involves the transmission and subsequent recording

of high frequency pulsed electromagnetic (radio) waves. The transmitted waves are focused

into the ground and can penetrate soils, sediment, rock, concrete, water and air. When this

energy wave encounters a buried object or a boundary between materials having different

permittivities, it may be reflected or refracted or scattered back to the surface. 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. Readings are taken at small intervals as the radar unit is pushed along the survey

line.

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3.5.1 GPR survey - field activity

A Utsi Trivue radar system (Plate 3) using shielded 250MHz, 500MHz and 1000MHz antenna

were used to collect data continuously along a series of profile lines across the survey area

(Figure 1). The combination of these frequencies offers a balance between high-resolution

and increased levels of penetration.

Given the higher frequency (i.e. shortest wavelength), the 1000MHz (1GHz) antenna should

provide the greatest target resolution and in good ground conditions (i.e. homogenous/dry/clay

deficient sediment) has typical penetration depths of between 0.5 to 1.0m. This is in contrast

to the 250MHz antenna, which even though it can achieve greater depths (typically up to 5m),

it’s target resolution is significantly lower (i.e. suited to larger features).

Plate 3: GPR Utsi Trivue

3.5.2 GPR survey - data processing

The data processing was carried out using GPR-SLICE software, and the first stage involves

the compilation of each separate 2D profile line. A series of editing and gain functions are then

applied to enhance features of interest and the resulting sections/time slices are converted to

depth using a nominal velocity correction. Additional processing routines (e.g. bandpass

filtering, background removal, migration, etc.) may be applied to improve the coherency of the

radar events and remove any unwanted noise such as multiple reflections, diffractions, etc.

The end product is a series of depth corrected radar sections, which emphasise the key

features/responses.

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4 RESULTS AND DISCUSSION

The results of the geophysical survey are presented as a series of colour contour plots and

sections in Figures 2 to 8 and an interpretation plan in Figure 5 (and Figure 9). A general

description of the interpretation process for each technique is given below, and a summary

discussion of the results is provided in Section 4.1

Metal detection Survey (EM-61)

The results are presented as a colour contoured plot of the response by the bottom coil in

Figure 2. The interpretation of the bottom coil response is based on the principle that metal

objects within the range of the instrument (~3m) will produce a proportional response, i.e. a

relative increase in the amplitude of the response will reflect an increase in the amount of

metal present at a given depth.

Magnetic (G-858)

The results of the magnetic survey are presented as a total field plot in Figure 3. The

interpretation of a magnetic anomaly is normally based on observing the type (pole/dipole),

amplitude and wavelength of the anomalous feature. Unfortunately, the results of the magnetic

survey are disappointing when compared to the other techniques. Even though a small

number of features are observed, the chaotic nature of the magnetic response has masked

many of the others. It is likely that the presence of slag in the made ground has caused

distortions in the local magnetic field which has affected the magnetic dataset.

Electromagnetic (GEM-2)

Normally, the results of a GEM-2 survey are presented as a colour contoured plot of ground

conductivity and In-phase response. However, following a review of the data, it was decided to

only present the quadrature-phase data (i.e. primary response) from the 30,925 MHz

frequency channel, as this seemed to be the optimum dataset in terms of representing the

underlying ground conditions (Figure 4). A relative increase in quadrature values usually

indicates a localised increase in the clay/ash/water content, which for example could signify

either a lateral change in lithology or groundwater. However, it can also represent interference

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from adjacent metallic features (both above and below ground). Extreme fluctuations in

quadrature-phase values are usually indicative of instrument ‘overload’ due to high metal

content.

Ground Penetrating Radar

The results of the radar survey are presented in Figures 5 to 8 as a series of selected cross-

sectional images. Even though all three frequency datasets were considered, it was found that

the 500MHz data set was the most coherent for the shallow near-surface.

The quality of the radar data across the site was generally mixed. There was some correlation

with the other techniques and there were obvious responses over the visible structures.

However, the variable/disturbed nature of the made ground over most of the site has limited

the consistency of the interpretation. Therefore the radar results are primarily considered as a

supplement to the metal detection and electromagnetic survey. Conversely, for the adjacent

road areas (Zone F), the quality of the data was much more consistent and it was possible to

observe a number of different features.

In cross-section (i.e. radargram), the interpretation of the radar data is based on recognising

certain responses and characteristics as detailed below in Table 1. These include reflection

boundaries, diffraction hyperbolae and changes in signal character/penetration. With regards

to signal penetration, the key factor is the relative conductivity of the ground: for example, an

increase in clay or water content would attenuate the radar signal and result in a decrease in

the signal penetration.

Buried structures in cross-section are typically identified either by a large single diffraction

event, cluster of diffractions or changes in the signal character/penetration. Depending on

ground conditions and the nature of the specific buried structure, the radar can either directly

map the target structure or identify indirect attributes associated with the target structure, i.e.

disturbed ground, fill material, etc. Regarding target detectability, the main issues include the

depth of target structure, size, nature of host material (i.e. homogeneous/heterogeneous),

moisture content and the contrast between target/host materials.

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Table 1: Responses and characteristics of GPR sectional data

4.1 Summary discussion

The interpretation of the geophysical survey is primarily based on the results of the metal

detection (EM-61) and electromagnetic (GEM-2) surveys (Figure 5). The resulting plots

display a wide range of responses across the survey area which is typical for this type of site.

These include broad zones, lineations, localised anomalies and isolated features. To discuss

each anomaly/response would be exhaustive, so it was decided to highlight the most

significant and these are revealed in the corresponding figures and tabulated in Table 2 below.

Some of the responses relate to observed or recorded metallic features/structures, but many

originate from unknown buried features. Depth estimates have been provided based on the

collective response of the EM-61 and GPR.

In addition to the historical features shown on the supplied base map, a selection of historical

maps are also included and these have also been overlaid with the interpretation plan (Figure

9).

The most notable features include the large circular response (believed to be a former gas

holder) and the numerous rectilinear zones (remnant floor slabs/buried structures). There are

also several smaller zones, which could either be buried structures or discarded debris within

the made ground. The other significant responses include a number of linear features, many

of which seem to correspond to the perimeter of some of the historical buildings. It is unsure if

these are associated with the foundations or adjacent services. Many of the other linear

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features are more likely to represent buried services, e.g., the GEM-2 survey has mapped two

curved services running north-south across Zone E.

With reference to identifying the extent of hydrocarbon contamination within the vicinity of

TP05, then this is not possible due to the variable ground conditions. There is a slightly

elevated response in the GEM-2 results, but this is likely to be due to the buried structures

rather than changes in electrical properties associated with biodegradation of hydrocarbon.

The response/anomalies/features prefixed with an F are primarily associated with metal

detection and GEM-2 survey. There are some magnetic anomalies, but generally, these are

quite limited. The additional response/anomalies/features prefixed with an R relate to the radar

survey along the adjacent roadways. Even though some of these radar responses are

indicative of buried services, there is not sufficient detail/coverage to link these features and

include them on to a plan.

Feature

Eastings/ Northings

Depth range (bgl)

Metal Detection

GEM-2 Description

F1 276639.8 189482.7

0 – 1m X X

Circular zone coinciding with the former gas holder base. The GPR suggests that the reinforced concrete slab is 0.2-0.3m thick.

F2 276606.5 189493.4

0 – 1m X X

Localised rectilinear zone suggesting metallic structure. Appears to correspond with corner of historical building.

F3 276602.7 189449.3

0 – 1m X X Linear feature that correlates with buried metal pipeline.

F4 276581.1 189423.4

0 – 1m X X Localised metallic response – buried structure at corner of historical building.

F5 276535.6 189436.2

0 – 1m X X

Rectilinear response associated with floor slab in Zone D. There may be an additional buried structure to the south of the slab.

F6 276540.3 189465.6

0 – 1m X X Rectilinear response from buried feature. Seems to be associated with historical structure.

F7 276557.9 189497.3

0 – 1m X X Rectilinear response from concrete base.

Table 2: Interpreted feature description

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F8 276535.5 189488.4

0 – 1m X X Localised response from buried structure.

F9 276542.0 189516.4

0 – 1m X X Rectilinear response associated with historical building. Correlates with ‘obstruction’ during the GI.

F10 276552.9 189543.3

0 – 1m X X Rectilinear response associated with historical building.

F11 F11a

276511.5 189504.6

0 – 1m X X Linear features corresponding to the edge of historical building.

F12 276519.9 189555.2

0 – 1m X X

Localised zone within Zone A – possible discarded debris. Correlates with ‘obstruction’ during the GI.

F13 276499.2 189558.2

0 – 1m X X

Localised zone within Zone A. Seems to be associated with historical structure. Correlates with ‘obstruction’ during the GI.

F13a 276508.3 189582.9

0 – 1m X

Localised zone within Zone A. Seems to be associated with historical structure. No metallic response.

F14 276534.9 189595.5

0 – 1m X X Rectilinear response adjacent to existing floor slab.

F15 276561.1 189572.1

0 – 1m X X Localised zone within Zone B. Seems to be associated with historical structure.

F16 276587.6 189546.4

0 – 1m X X Rectilinear response that seems to lie between some historical buildings

F17 276605.0 189558.9

0 – 1m X X Localised zone within Zone E – possibly discarded debris

F18 F18a

276603.7 189519.3

0 – 1m X X Linear feature corresponding to the edge of historical building.

F19 276615.5 189517.1

0 – 3m X X Linear feature suspected to be a buried service

F19a 276601.9 189509.9

0 – 3m X Linear feature suspected to be a buried service

F20 276593.4 189536.0

0 – 1m X Linear feature suspected to be a buried service

F21 276645.8 189520.8

0 – 1m X Linear feature corresponding to the edge and centreline of historical building.

F22 276580.5 189478.3

0 – 1m X Linear feature corresponding to the edge and centre of historical building.

Table 2: Interpreted feature description (continued)

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Feature

Eastings/ Northings

Depth range (bgl

GPR EM-61 Description

R1 276669.3 189529.0

0 – 1m X Isolated diffraction - possible buried service.

R2 276674.0 189488.6

0 – 1m X Shallow reflection event– geological boundary or buried feature.

R3 276678.9 189464.8

1 – 3m X Isolated diffraction - possible buried service.

R4 276680.1 189456.4

0 – 1m X Cluster of shallow diffractions – buried service, structure or disturbed ground.

R5 276660.0 189433.3

1 – 3m X Isolated diffraction - possible buried service.

R6 276630.3 189426.2

0 – 1m X X Isolated diffraction - possible buried service. Correlation with EM-61 response.

R7 276588.3 189413.1

0 – 1m X Localised diffractions and increased penetration - buried service, structure or disturbed ground.

R8 276575.0 189407.8

0 – 1m X

Localised zone with increased penetration, diffractions & reflections. - buried service, structure or disturbed ground.

R9 276556.6 189410.0

0 – 1m X

Localised diffractions and increased penetration - buried service, structure or disturbed ground

R10 276538.9 189404.6

0 – 1m X

Localised zone with increased penetration, diffractions & reflections. - buried service, structure or disturbed ground

Table 2: Interpreted feature description (continued)

Page 20: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Geophysical Survey Report – 5906/2

Harbourside, Port Talbot March 2018 20

5 CONCLUSIONS

The geophysical survey has provided a non-invasive means for investigating the

subsurface with a high degree of spatial coverage. By considering a number of

geophysical methods, it has significantly enhanced the ability to characterise the

subsurface as the techniques are based on different physical principles and provide

complementary information for interpretation.

The interpretation of the geophysical survey is primarily based on the results of the

metal detection (EM-61) and electromagnetic (GEM-2) survey. Unfortunately, the

magnetic survey seemed to be influenced by the slag material within the made ground

and thereby masking the response from the target structures. As far as the ground

radar is concerned, the variable/disturbed nature of the made ground over most of the

site has limited the consistency of the interpretation.

The resulting plots display a wide range of responses across the survey area which is

typical for this type of site. These include broad zones, lineations, localised anomalies

and isolated features. The most significant features have been highlighted on the

corresponding plots and tabulated. The most notable features include the large circular

response (believed to be a former gas holder) and the numerous rectilinear zones

(remnant floor slabs/buried structures). There are also several smaller zones, which

could either be buried structures or discarded debris within the made ground. There

are also several lineations, some of which seem to correspond to the perimeter of

some of the historical buildings, while others are likely to be buried services.

We would normally recommend some follow up invasive work to ground-truth the main

anomalies identified by the geophysical survey. However as the ground investigation

work has already been done, then it may not be necessary. If any additional

information becomes available, then it may be possible to extend the interpretation

further and calibrate the datasets

Page 21: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Geophysical Survey Report – 5906/2

Harbourside, Port Talbot March 2018 21

Disclaimer

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

purposes only. 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.

Page 22: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

FIGURES

Page 23: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 11:500 at A4

LOCATION PLAN

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref:

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

KEY NOTES/OBSERVATIONS

Club

HARBOURSIDE DEVELOPMENT

PORT TALBOT

JT/5906/1

Survey area

Radar Profiles

1: Base topographic map supplied by Client

Zone A

Zone F

Zone BZone E

Zone D

Zone C

Page 24: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 21:1000 at A3

EM61 - BOTTOM COIL

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/2

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

KEY

COLOUR SCALE

Metal response

(mV)

NOTES

1: Base topographic map supplied by Client

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F15

F12

F13

F14

F16

F17

F18

F19

F11

Survey area

Feature #(please refer to Table for full description)

F7

Page 25: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 31:1000 at A3

MAGNETIC - TOTAL FIELD

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/3

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

KEY

COLOUR SCALE

Total Field

(nT)

NOTES

1: Base topographic map supplied by Client

F1

F4

F5

F6

F9

F14

F11

F13

F17

F10

Survey area

Feature #(please refer to Table for full description)

F7

Page 26: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 41:1000 at A3

GEM2 - QUAD PHASE

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/4

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

KEY

COLOUR SCALE

Quadrature Phase

(ppm)

NOTES

1: Base topographic map supplied by Client

F1

F2

F3

F4

F5

F6

F7F8

F9

F10

F15

F14

F16

F17

F18

F19

F11

F20

F19a

F21

F22

F11a

F18a

Survey area

Feature #(please refer to Table for full description)

F7

Page 27: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Club

Scale:

FIGURE 51:1000 at A3

INTERPRETATION PLAN

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 13 MAR 2018

Drawn by/Ref: JT/5906/5

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

Survey area

Not surveyed due toobstructions/fences

Feature #

Feature # (radar)

Obstruction encountered during GI

(please refer to Table for full description)

(taken from Exploratory hole location plan - OPUS)

KEY

NOTES

1: Base topographic map supplied by Client

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

F15

F14

F16

F17

F18

F19

F11

F20

F19a

F21

F22

F11a

F18a

F7

F12

F13

Note: The geophysical anomalies eitherrepresent the peak or increased response ofa given feature. The shape and lateral extentof a feature is based on the limitations of thetechnique. Any response thought to originatefrom a mapped buried service or surfacestructures have not been included on theinterpretive plan.

Linear feature - may representburied services, foundations, tracks

EM-61 (metal response)

Magnetic Survey

GEM-2 (Quad-phase)

Feature classificiation

Survey Technique

- may representdisturbed ground, buried structureor service trench

Broad feature

- may representLocalised feature

disturbed ground or buried feature/debris

R10

R9

R8

R7

R6

R4

R3

R2

R1

R7

F13a

R5

Page 28: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 6NTS at A3

SELECTED GPR PROFILES

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/6

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

Survey area

KEY

NOTES

1: Base topographic map supplied by Client

a) View 1 - Looking northeast across Zone A

b) View 2 - Looking east across Zone B

Location plan of selective radar profiles

Linear feature - may representburied services, foundations, tracks

EM-61 (metal response)

Magnetic Survey

GEM-2 (Quad-phase)

Feature classificiation

Survey Technique

- may representdisturbed ground, buried structureor service trench

Broad feature

- may representLocalised feature

disturbed ground or buried feature/debris

F16

F13

F14

F12

F11a

F11

F10

F15

F13a

F15

F10 F11a

F7

F18 F18a

F6

F17

Page 29: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 7NTS at A3

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/7

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

Survey area

Feature #(please refer to Table for full description)

KEY

NOTES

1: Base topographic map supplied by Client

F7

a) View 3 - Looking north across Zone E

b) View 4 - Looking west across Zone E and F

Location plan of selective radar profiles

SELECTED GPR PROFILES

Linear feature - may representburied services, foundations, tracks

EM-61 (metal response)

Magnetic Survey

GEM-2 (Quad-phase)

Feature classificiation

Survey Technique

- may representdisturbed ground, buried structureor service trench

Broad feature

- may representLocalised feature

disturbed ground or buried feature/debris

F1

F3

F22F7

F18a

F19

F2

F19a

R4

R3

R1

R2

Page 30: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 8NTS at A3

GROUND RADAR EXAMPLES

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 13 MAR 2018

Drawn by/Ref: JT/5906/8

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

Survey area

Not surveyed due toobstructions/fences

Feature #(please refer to Table for full description)

KEY

NOTES

1: Base topographic map supplied by Client

F7

a) View 5 - Looking north across Zone F

b) View 6 - Looking north across Zone F

Location plan of selective radar profiles

EM-61 (metal response)

Magnetic Survey

GEM-2 (Quad-phase)

Ground radar

Survey Technique

R10

R8F7

R6R9

R8F7

R6

R5

Page 31: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Scale:

FIGURE 9NTS at A3

HISTORICAL MAPS

HARBOURSIDE DEVELOPMENT

PORT TALBOT

Title:

Project:

Date: 26 FEB 2018

Drawn by/Ref: JT/5906/9

Tel: +44 (0) 2920 700127

Web:www.terradat.co.uk

Email: [email protected]

Survey area

KEY

NOTES

1: Base topographic map supplied by Client

Note: The geophysical anomalies eitherrepresent the peak or increased response ofa given feature. The shape and lateral extentof a feature is based on the limitations of thetechnique. Any response thought to originatefrom a mapped buried service or surfacestructures have not been included on theinterpretive plan.

Linear feature - may representburied services, foundations, tracks

EM-61 (metal response)

Magnetic Survey

GEM-2 (Quad-phase)

Feature classificiation

Survey Technique

- may representdisturbed ground, buried structureor service trench

Broad feature

- may representLocalised feature

disturbed ground or buried feature/debris

c) Historical map c. 1968

a) Historical map c. 1952

b) Historical map c. 1958

Website © Copyright and data base rights Old-Maps.co.uk 2018

Website © Copyright and data base rights Old-Maps.co.uk 2018

Website © Copyright and data base rights Old-Maps.co.uk 2018

Page 32: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

APPENDICES

Page 33: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Appendix - EM-61 Metal Detection Survey

Towed EM-61 with dGPS

GPS antennalinear buried

services

linear feature

Metal Response (top coil)

subtle response from former building

transmitter/

receiver coils

Constraints

Surface metal objects or structures (fences, rebar, vehicles, debris etc.) produce a strong signal that canmask more subtle response from any underlying target structures.

EM-61 survey

isolated

anomalies

The EM-61 is a time-domain metal detector that can detect both ferrous and non-ferrous metals. Theinstrument was initially designed to map buried ordnance but is now used for general site characterisationespecially in the mapping of former foundation structures. The EM-61 is either towed by hand or mountedbehind a quad bike. Readings are usually taken on a regular grid or along selected traverse lines andpositional control can be provided by dGPS if there is sufficient satellite coverage.

The instrument functions by inducing current into the ground via a transmitter coil which causes thegeneration of secondary electromagnetic fields in any ground conductors present within the depth rangeof the instrument. By taking the measurement at a relatively long time after the start of the decay, thecurrent induced in the ground has fully dissipated and only the current in the metal is still producing asecondary field. The responses are recorded and displayed by an integrated data logger.

At the end of each survey, the data are downloaded to a field computer and corrected for instrument,diurnal and positional shifts. Additional editing may be carried out to remove any 'noisy' datavalues/positions.

The results are presented as a colour contoured plot based on the responses (mV) from the receiver coilsof the instrument. The interpretation of the data is based on the principle that metal objects within therange of the instrument (~3m) will produce a response that is proportional to the amount of metal presentat a given depth. By considering the response between the top and bottom receiver coils, it is possible toderive an estimate of source depth for a given anomaly.

General principle of EM-61 surveying

data logger

power

unit

transmitter/

receiver coils

data logger and

GPS controller

Bottom Tx/Rx

Top Rx

Secondary

FieldModified

Primary Field

metal target

Transmitting mode Receiving mode

Bottom Tx/Rx

Top Rx

linear buried

services

Eddy currentsEddy currents

metal target

Page 34: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

50m

Appendix - Magnetic Survey

Towed gradiometer with dGPS

GPS antennatop sensor

cluster of

anomalies

influence of

metal fence

Analytical signal plot

linear

feature

bottom sensor

The magnetic survey technique is based on mapping localised variations in the Earth’s magnetic field causedsub-surface magnetic materials, which range from naturally occurring magnetic minerals to man-madeferrous objects. This leads to a wide range of applications from small-scale archeology and engineeringsurveys to detect buried metallic objects, to large-scale surveys carried out to investigate regional geologicaltrends or mineralisation.

Magnetic surveys are carried out using a man-portable instrument with readings taken on a regular grid oralong selected traverse lines. The equipment functions by measuring the Earth's magnetic field to a very highprecision at each survey station. Ferrous materials in the subsurface have an induced magnetic field that issuperimposed on the Earth's field at that location creating a magnetic anomaly. The spacing of survey stationsdepends on the width of the expected anomaly, which broadens with the size, and depth of burial of thetargeted feature. Continuous profiling methods may be used for a high-resolution dataset.

Magnetometer data are stored digitally by the survey instrument and down loaded to a field computer at theend of each day. The magnetic data are then processed to enhance any identifiable anomalies and presentedon colour-contoured plots overlain with site maps (when available).

The results of the magnetic survey are usually presented as total field and analytical signal plots . The totalfield data may be used to observe the general character of the magnetic field across the survey area while thepeak values (pink) displayed on the analytical signal plot indicate the source positions for dipole type magneticanomalies. In general terms, the interpretation of a magnetic anomaly is based on observing the type(pole/dipole), amplitude and wavelength of the anomalous features.

Constraints

Metal objects or structures close to the survey area (fences, vehicles, debris etc.) produce a strong signalthat can overshadow more subtle effects of sub-surface anomalies.

Gradiometer survey

isolated

anomalies

top sensor

bottom sensor1 4 6 8 102

10

100

50

200

300

400500

1kg

of

iron

control unit/

data logger

Depth (m from sensor)

Mag

neti

cfi

eld

(nTesla

)

100kg

ofiro

n

1000kg

ofiro

n

6”

iron

pip

e

12”

iron

pip

e

Mag

neti

cfi

eld

Deeper target

Surface

Shallow target+ -

short wavelengthhigh amplitude

longer wavelengthlower amplitude

+ -

Surface

Mag

neti

cfi

eld

Magnetic anomalies Guide to anomaly size

Page 35: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

Constraints

Power 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-3M

gravitymeter

EDM survey

instrument

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

modified

primary field

secondary

field

conductor

surface

eddy currents

shallow

limestone

bedrock

clay-rich

sediments

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-38

Single frequencyExploration depth ~1.5m

EM-31

Single frequencyExploration depth ~3 to 5m

GEM-2

Multi-frequencyExploration depth up to 10m

GPS antenna

EM-38 mounted

within trailer

Page 36: GEOPHYSICAL SURVEY REPORT Geophysical survey to map … · 3 SURVEY DESCRIPTION The geophysical survey was carried out using the following techniques: Metal detection survey (EM-61)

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 Resistivity

Meter

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

Tx

Rx

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