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PIO N E E RIN G G E O P H Y SIC S F O R T H E C O N S T R U C TIO N A N D G E O T E C H NIC A L IN D U S T RIE S


Brownfield Site Characterisation EM-38 Conductivity, Soil Vapour Analysis and Resistivity Survey

Foundations, Tanks and Services

Contamination and Buried Waste

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.

G P R s urv e y

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.

A n o m a l y o v e r b a c k f i l l e d t r e n c h

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.

25m

G ro u n d C o n d u ctivity s urv e y

H i g h r e s p o n s e o v e r r e i n f o r c e d c o n c r e t e r o a d

B r o a d a r e a o f c o n d u c t i v e v a l u e s r e p r e s e n t s a n e x t e n s i v e a r e a o f

a n o m a l i e s o v e r b u r i e d t a n k s

e l e v a t e d r e s p o n s e o v e r b u r n p i t

B u r i e d p i p e s

C o n d u c t i v e d e g r a d e d h y d r o c a r b o n s ( t a n k p o s i t i o n )

5m

R e i n f o r c e d s l a b

F e n c e

S i g n a l r e v e r b e r a t i o n ( s u r f a c e m a n h o l e c o v e r ) m

m

r e s i s t i v e l a y e r c l a y d e f i c i e n t / d r y s e d i m e n t

b u r i e d s e r v i c e s m

Ma g n etic G ra dio m eter s urv e y

C o n d u ctivity pla n

m

0

Depth (m)

B u r i e d s e r v i c e s

D etaile d s ervic e m a p

M e t a l p i p e l i n e s

Elevation (m)

Metal D ete ctio n S urv e y to lo c ate b urie d ta n k s

f o u n d a t i o n s t r u c t u r e

20

40

60

80

100

Ma g n etic S urv e y pla n

10m

p o o r p e n e t r a t i o n d u e t o b u r i e d s t r u c t u r e

G ro u n d P e n etratin g R a d ar s e ctio n

120

140

160

180

200

30

30

20

20

10

10

0

0

-10

B u r i e d s e r v i c e s

I n t e r m e d i a t e c o n d u c t i v i t y z o n e ( i n c r e a s e i n c l a y / m o i s t u r e )

Distance (m)

-10

R e s i s t i v e s a n d s t o n e b e d r o c k

S h a l l o w c o n d u c t i v e z o n e a s s o c i a t e d w i t h b i o d e g r a d a t i o n o f h y d r o c a r b o n s c o n t a m i n a n t

R e sistivity s e ctio n o v er a n are a of h y dro c arb o n c o nta min atio n


General ground hazards Ma g n etic, Micro gra vity a n d G ro u n d C o n d u ctivity te c h niq u e s

Mineworkings

Karst and Solution Features 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

g r a v i t y comprising a number of different geophysical techniques that a n o m a l y target both direct (e.g. shaft lining/cap, void space) and indirect o v e r properties (e.g. localised variations in drainage patterns or infill s h a f t

B o u g er A n o m aly (gra vity) pla n

material).

Microgravity (milligals)

E l o n g a t e d g r a v i t y a n o m a l y o v e r a d i t

L i m e s t o n e

R e sistivity S e ctio n

G ra vity Profile

C l a y f i l l e d f e a t u r e s

C l a y f i l l e d s o l u t i o n f e a t u r e s

0.20

0.15

0.10

0.10

0.05

0.05

0.00

0.00

-0.05

-0.05

Residual Gradient Processed Gravity ‘low’ gravity anomaly

-0.10 -0.15 -0.20

-0.10 -0.15 -0.20

-0.25

-0.25

-50

B a c k f i l l e d b e l l p i t

R e sistivity S e ctio n to lo c ate a n cie nt min e w orkin g s

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

0

50

Distance (m)

100

Pla n s h o win g p ote ntial U X O targ ets

150

C o m bin e d R e sistivity a n d Micro gra vity S urv e y u s e d to interpret s olutio n fe ature lo c atio n 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.

Landslides L o w r e s i s t i v i t y m a t e r i a l ( c l a y r i c h ) 200

150 250

300

350

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.

0.20

0.15

R e s i s t i v e m a t e r i a l ( b e d r o c k )

i n s e r t : c o r r e s p o n d i n g M A S W v e l o c i t y s e c t i o n f o r s h e a r w a v e v e l o c i t i e s

C o m bin e d R e sistivity To m o gra p h y a n d S eis mic S urv e y s

An 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.


Geology and Geotechnical Resistivity, Ground Conductivity and Seismic Refraction surveys

Engineering Properties 0m

30m

40m

M A S W S w a v e p l o t o v e r l a i d o n S e i s m i c P w a v e p l o t

105 410m/s 810m/s

810

100 810m/s

580 550 520 490 460 430 400 370 340 310 280 250 220 190

95

C o r r e l a t i o n b e t w e e n S w a v e a n d P w a v e s u r v e y s

2340m/s

Increasing ground stiffness

Elevation (m)

20m

Shear Wave Velocity (m/s)

110

10m

Geological Mapping

160 130

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

L o w c o n d u c t i v i t y S a n d s t o n e

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.

H i g h c o n d u c t i v i t y M u d s t o n e

b r o a d f a u l t z o n e

100

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

C o m bin e d S eis mic P-w a v e a n d S-w a v e S urv e y 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).

500m

250m

0m

G ro u n d C o n d u ctivity Ma p pin g

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. Velocity (m/s) 2000

0 2 4

Depth (m)

6 8 10

Seismic Layer 1 Seismic Layer 2 Seismic Layer 3

Seismic Layer 4

BH 02 Lithology

2

Clay: slightly sandy

4

20

d e n sity *

P oi sso n's

She ar

Yo u n g's

B ul k

(m)

(m/s)

(m/s)

g/c m3

Ra ti o

G (Mp a )

E (Mp a )

K

1

400

200

1.54

0.33

62

164

164

2

400

200

1.54

0.33

62

164

164

3

750

250

1.54

0.44

96

277

738

8

4

750

250

1.54

0.44

96

277

738

5

1050

140

1.90

0.49

37

111

2045

10

6

1050

140

1.90

0.49

37

111

2045

7

1050

140

1.90

0.49

37

111

2045

12

8

1600

800

2.18

0.33

1395

3721

3721

14

9

1600

800

2.18

0.33

1395

3721

3721

10

1600

800

2.18

0.33

1395

3721

3721

11

1600

800

2.18

0.33

1395

3721

3721

12

1600

800

2.18

0.33

1395

3721

3721

13

1600

800

2.18

0.33

1395

3721

3721

14

1600

800

2.18

0.33

1395

3721

3721

15

1600

800

2.18

0.33

1395

3721

3721

16 P-wave S-wave

S-v e l o city

Clay: gravelly with cobbles

16 18

P-v e l o city

6

Sandstone:

18 Mudstone

20

G e o l o g i c a l s t r u c t u r e d e t e r m i n e d u s i n g s e i s m i c s u r v e y

Mo d u li d e pth

Silt: sandy, some clay

12 14

0

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

D o w n-h ole s eis mic v elo city profile a n d s u m m ary ta ble of v elo city / ela stic m o d uli

0

10

20

30

Distance (m) 40

50

130

60

d i s c o n t i n u i t y d u e t o f a u l t o n e 70z 80 90

R e s i s t i v e S a n d s t o n e 120

110

100

C o n d u c t i v e M u d s t o n e

C o m bin e d s eis mic a n d re sistivity s urv e y to d etermin e g e olo gic al stru cture

100

Increasing resistivity

1000

Elevation (m)

0


Archaeological and 3D Surveys Ma g n etic G ra dio m etry a n d G ro u n d R a d ar m eth o d s

Archaeology

3D Laser Scanning 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.

An 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.

A erial orth o p h oto of a n arc h a e olo gic al e x c a v atio n

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.

M a g n e t i c g r a d i o m e t r y s u r v e y s h o w i n g a s i g n i f i c a n t r i n g d i t c h a n d h i s t o r i c f i e l d b o u n d a r i e s

Ma g n etic G ra dio m eter Plot s h o win g Rin g Ditc h

Zones of material erosion 20m

Zones of material accretion

3D a n d 2D vie w s s h o win g c o a stal ero sio n

P oint C lo u d a n d C A D E le v atio n Mo d els

M a g n e t i c g r a d i o m e t r y p l o t r e v e a l i n g t h e l o c a t i o n o f a B r o n z e A g e f a r m s t e a d G P R s u r v e y s h o w i n g t h e e x t e n t s o f a v o i d a t a N o r m a n C a s t l e

R a d ar S urv e y s h o win g fo u n d atio n s

Interpretatio n of Ma g n etic G ra dio m etry d ata

L eft to Rig ht: P oint C lo u d D ata, Ve g etatio n R e m o v al, B are E arth Mo d el of a R o a d C uttin g


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

Foundation Mapping

Reservoir Study TerraDat 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.

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.

Ma g n etic G ra dio m eter

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 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.

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


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Geophysics for construction industry  

Geophysics for construction industry