PIO N E E RIN G G E O PHYSIC S F O R TH E

C O NSTRU C TIO N A NDG E O T E C HNIC A L INDUSTRIE S

Brownfield SiteCharacterisation

Foundations, Tanks and Services

Metal Detection Survey to locate buried tanks

Ground Penetrating Radar section

G PR survey

Detailed service map

Magnetic Survey plan

Mapping subsurface structures consists of an initial survey using a combination of metal detection, magnetic, electro-magnetic and ground conductivity mapping. These techniques target both direct (i.e. metal supports/pipes, rebar, foundations, etc) and in-direct (i.e. variation in the backfill material, trench fill etc ) subsurface geophysical properties.

More detail on geophysical targets in terms of depth, size and lateral extent can be achieved by following up using methods such as: ground radar (GPR), resistivity tomography and microgravity techniques.

Contamination and Buried Waste

Ground C onductivity survey

Most former industrial sites have a legacy of buried structural hazards and contamination. Geophysical surveys can detect buried targets without the need to excavate. Many industrial contaminants such as salinisation by acids and hydrocarbon plumes leave a significant geophysical imprint on the subsurface that can be imaged.

Using a combination of magnetic and ground conductivity mapping, it is possible to rapidly locate waste material and contamination. This can be characterised vertically using profiling techniques such as resistivity tomography, or in plan view meaning less chance of missing possible targets.

Magnetic Gradiometer surveyC onductivity plan

anomalies overburied tanks

5m

High response over reinforced concrete road

Resistivity section over an area of hydrocarbon contamination

Anomaly over backfilled trench

25 m

���foundation structure

elevated response over burn pit

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Broad area of conductive values represents an extensive area of

Conductive degraded hydrocarbons(tank position)

Buried pipes

poor penetration due to buried structure

buried services Signal reverberation(surface manhole cover)

Dep

th (m

)

mmmm

10 m

Metal pipelinesBuried services

FenceReinforcedslabBuried services

Shallow conductive zone associated withbiodegradation of hydrocarbons contaminant

resistive layer - clay deficient/dry sediment Intermediate conductivity zone -(increase in clay/moisture)

Resistive sandstone bedrock

0 20 40 60 80 100 120 140 160 180 200Distance (m)

-10

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vatio

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EM-38 Conductivity, Soil Vapour Analysis and Resistivity Survey

Generalground hazards

Mineworkings Magnetic , Microgravity and Ground C onductivity techniques

Unexploded Ordnance (UXO)

Plan showing potential UX O targets

TerraDat use combined high resolution metal detection (EM-61) and magnetic mapping to locate possible non-ferrous and ferrous buried ordnance down to depths of around 3m.

The results are used for follow-up ground truthing / clearance by a qualified EOD operative. For deeper targets, it may be necessary to incrementally clear, strip overburden and then re-survey.

Karst and Solution Features

Solution features can be mapped by measuring contrasts in the geophysical properties of fill material / void space and the surrounding geology. Subtle effects of drainage associated with these features produce anomalies. Ground Conductivity, Resistivity and Microgravity are the principal methods.

Bell pits, shafts, adits, and subsidence are all legacies of mining activity that can have a significant effect on present day developments, The majority of mining activity is well documented, but in some cases the accuracy of this information can be questionable. A geophysical survey coupled with a selective intrusive investigation can provide a rapid and cost effective means for locating shallow abandoned mineworkings .

For most sites, TerraDat adopt an integrated approach comprising a number of different geophysical techniques that target both direct (e.g. shaft lining/cap, void space) and indirect properties (e.g. localised variations in drainage patterns or infill material).

LandslidesAn integrated geophysical survey can provide valuable information to assist with the investigation of the stability of potential and active landslip sites.

The non-intrusive, low environmental impact surveys are ideal for remote or sensitive sites.C ombined Resistivity Tomography and Seismic Surveys

350

300

Low resistivity material (clay-rich)

Resistive material (bedrock)

insert: corresponding MASW

velocity section for shear-wave

velocities

250200150

gravity

anomaly

over

shaft

Elongated

gravity anomaly

over adit

��

Back-filled bell pit��

B ouger A nomaly (gravity) plan

Resistivity Section to locate ancient mineworkings

150100500

Mic

rogr

avity

(mill

igal

s)

-0.25

-0.20

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0.00

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Residual GradientProcessed Gravity‘low’ gravity anomaly

-50

-0.25

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0.00

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Clay-filled features

Limestone

Clay-filled solution features

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Gravity Profile

Resistivity Section

Distance (m)

C ombined Resistivity and Microgravity Survey used to interpret solution feature location

Geology and Geotechnical

Engineering PropertiesResistivity, Ground Conductivity and Seismic Refraction surveys

Geological MappingGeophysical surveys can compliment conventional ground investigations for ground engineering projects by providing lateral continuity across a site.

Ground conductivity mapping can provide a plan of the main changes in ground conditions (e.g. depth, lithology, or structural) typically within 5m of ground level. This plan can be used to target resistivity profiles to yield cross-sectional data on features of interest.

Deeper geology can be mapped using seismic reflection, typically down to depths of around 100m.

Seismic surveys provide information on the depth and engineering strength (rippability) of Earth materials. The data is acquired using surface-based P and S-wave refraction and MASW. Data can also be acquired using boreholes (either down or cross-hole).

Model seismic velocities can be combined with density data to calculate Poisson's Ratio, plus elastic moduli such as Shear, Young's and Buk - all useful for geotechnical design.

Down-hole seismic velocity profile and summary table of velocity / elastic moduli

Ground C onductivity Mapping

Shear W

ave Velocity (m/s)

100

130

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Increasing ground stiffness

410m/s810m/s 810

95

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Ele

vatio

n (m

)

0m 10m 20m 30m 40m

810m/s

2340m/s

MASW S-wave plot overlaid on Seismic P-wave plot

Correlation betweenS-wave and P-wavesurveys

0 1000 2000

Velocity (m/s)

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Sandstone:

Mudstone

BH 02 Lithology

P-waveS-wave

SeismicLayer

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SeismicLayer

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Made ground: clayey, sandy, gravel, ash, brick, stone

Clay: slightly sandy

Silt: sandy, some clay

Clay: gravelly with cobbles

SeismicLayer

1

SeismicLayer

3

de pth P-ve locity S-ve locity de nsity * P oisson's S he a r Young's Bulk(m) (m /s) (m /s) g/cm3 Ra tio G (Mpa ) E (Mpa ) K

1 400 200 1.54 0.33 62 164 1642 400 200 1.54 0.33 62 164 1643 750 250 1.54 0.44 96 277 7384 750 250 1.54 0.44 96 277 7385 1050 140 1.90 0.49 37 111 20456 1050 140 1.90 0.49 37 111 20457 1050 140 1.90 0.49 37 111 20458 1600 800 2.18 0.33 1395 3721 37219 1600 800 2.18 0.33 1395 3721 3721

10 1600 800 2.18 0.33 1395 3721 372111 1600 800 2.18 0.33 1395 3721 372112 1600 800 2.18 0.33 1395 3721 372113 1600 800 2.18 0.33 1395 3721 372114 1600 800 2.18 0.33 1395 3721 372115 1600 800 2.18 0.33 1395 3721 3721

Moduli

C ombined Seismic P-wave and S-wave Survey0m 250m 500m

broad fault zone

Low conductivitySandstoneHigh conductivityMudstone

0 10 20 30 40 50 60 70 80 90 100Distance (m)

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Ele

vatio

n(m

)

Incr

easin

g re

sistiv

ityResistive Sandstonediscontinuity due tofault zone

Geological structuredetermined usingseismic survey

Conductive MudstoneC ombined seismic and resistivity survey to determine geological structure

Archaeological and 3D Surveys

ArchaeologyAn integrated geophysical approach is used to detect buried archaeology, the most common being magnetic gradiometry. This approach makes it possible to exploit the contrast in the physical properties of the target and the host geology. The selection of appropriate techniques is tailored to individual site requirements, depth, history etc.

Foundations and buried structures are detected using high resolution magnetic gradiometry, ground penetrating radar and electromagnetic mapping. Archaeological data analysis is carried out by TerraDat's archaeological specialist. The final deliverablea are interpretative plans indicating the location of archaeological features of interest.

Magnetic gradiometry survey

showing a significant ring ditch

and historic field boundaries

GPR survey showing the extents

of a void at a Norman Castle

Magnetic Gradiometry and Ground Radar methods

3D Laser Scanning

A erial orthophoto of an archaeological excavation

TerraDat has in-house facilities for terrestrial laser scanning and photogrammetry, including low altitude aerial capabilities. This enables rapid collection of high accuracy 3D datasets. These can be applied to modelling inaccessible rock faces, generation of bare earth models for settlement / movement monitoring, measured building surveys and 3D recording of historic structures.

Data can be integrated with LIDAR information to generate deliverables such as orthophotos, 2D elevation views, DTM (digital terrain models), structural geology (dips and strikes), TINs / contour maps and animated fly-throughs. We can also facilitate rapid mobile mapping surveys.

Left to Right: Point C loud Data , Vegetation Removal, B are E arth Model of a Road C utting

Magnetic gradiometry plot

revealing the location of a

Bronze Age farmstead

Magnetic Gradiometer Plot showing Ring Ditch

Interpretation of Magnetic Gradiometry data Radar Survey showing foundations

20m

Zones of material erosion

Zones of material accretion

3D and 2D views showing coastal erosion Point C loud and C A D E levation Models

Case Studies

Resistivity and Conductivity surveys to detect the extents of an in-filled quarry

Reservoir StudyTerraDat were commissioned by a major engineering consultancy to delineated the extents of the London Clay and underlying gravels across the site of a proposed dam and reservoir. Clay has a high electrical conductivity due to is mineralogy, whereas gravels are electrically resistive.

This contrast of geophysical properties enabled the delineation of both geological units laterally and vertically beneath the site via a combination of towed GEM-2 Ground Conductivity mapping and Electrical Resistivity Tomography.

The resulting plots were compiled in 3D GIS software alongside the results of an intrusive investigation allowing a 3D view of the site geology to be generated

100m

TerraDat were commissioned by a housing developer to map an inner-city site and establish whether foundation structures related to three tower blocks, demolished during an earlier phase of work, had actually been removed.

A magnetic survey was carried out in one day aimed at detecting any ferrous metal that may be related to these structures. The magnetic method measures localised anomalies in the Earth's magnetic field due to the presence of buried ferrous metal.

The figure below shows anomalies indicating that all 3 sets of foundations were still in place allowing the Client to accurately project costs for ameliorative works at the site.Magnetic Gradiometer

Foundation Mapping

H e a d O f f i c eUnit 1, Link Trade ParkPenarth RoadC ardiffC F11 8T QUnited K ingdomTelephone: +44 (0)8707 303050F ax: +44 (0)8707 303051e-mail: info @terradat.co .ukwww.terradat.com