ert_en.pdf

ContactDott. Ing. Fabrizio Fantini

Techgea SrlOffice: Via Carlo Viola 78 Pont Saint Martin (AO)

Office: Via Modigliani 26/a 10137 TORINOwww.techgea.eu

Tel +39 011700113 Fax +39 0117077673Cell. +39 3939009152E-mail [email protected]

Geoelectrical survey

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ELECTRICAL RESISTIVITY TOMOGRAPHY

Survey on a landfill

Survey in difficult terrain

Survey on a industrial pavement

The

electrical

resistivity

tomography

(ERT)

is

used

to

map

the

subsurface

distribution

of

electrical

resistivity

by

means

of

injection

of

DC

current

in

the

ground

and

by

measuring

the

voltage

on

the

ground

surface

or

inside

boreholes.

The

characteristics

of

electric

resistivity are

tightly

correlated

to the

chemical‐

physics

characteristics

of

the

ground

materials

and,

for

this

reason,

ERT

provide

a

very

precise

vision of the subsurface

Electrical resistivity survey are applied to evaluate:1.

Presence of groundwater2.

Depth of bedrock3.

Mapping of contaminant plumes4.

Location of faults and rock fractures5.

Detection of buried landfill cells6.

Location

of

clayey

zones

that

may

form

aquitards below proposed landfill locations7.

Zones of buried constructions debris8.

Void

spaces,

such

as

large

culverts,

pipelines,

caves, or abandoned mine adits

Electrical resistivity values are digitally recorded on

a multi‐electrodes georesistivimeter and processed

using

inversion

software

(RES2DINV)

to

create

2D

sections

or

a

three‐dimensional model

of

the

underground.

The ERT sections can reach a depth of about 300 m

with a line length of about 1000 m in a pole‐dipole

electrode array (with a remote pole)






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GEOLOGICAL MODELLING

Landslide deposits (clayey matrix)

Outcropping dolomite

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130

Distanza (m)

486488490492494496498500502504506508510512514516518520522524526528530

Alti

tud

ine

(m

) X Fault

Dolomite

Argillous

shales

Clay /shale

weathering)

Soil with blocks of

dolomite

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

3.5

3.7

3.9

Scala dei colori (log‐resistivity)

3D MODEL FOR A GRAVEL PIT DESIGN

Clay bottom

50 m

350 m

gravel

BEDROCK DEPTHFAULT DETECTION

The

electrical

resistivity

method

is

applied

to

detect

the

main

geological

structures.

The

different

resistivity

values may be related to geological setting (stratigraphy) or tectonics (fault and fractures).

The section of example 1 shows a reverse fault at the contact between dolomite (high resistivity) and shale (low

resistivity).

Example 2 is related to a 3D model of a site for gravel quarry , to evaluate the potential volume of a gravel level

(design of the gravel pit). The gravel has a high electrical resistivity, while the clay at the base (located about 35

m from ground level) is conductive.

gravel

Example 1

Example 2






3/ 5

0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66

Distanza [m]

-10

-8

-6

-4

-2

0

2

Pro

fon

dità

[m

]

GROUNDWATER EXPLORATION

(log‐resistivity)

2 3 3.2 3.4 3.6 3.8 4 4.2 4.4

Groundwater flow

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350

Distanza [m]

-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Pro

fond

ità [m

]

Existing well P2

Clay

ghiaia gravel

Clay

Sand/silt

Old source

Existing source

New water well

location

INDAGINI SUPERFICIALI

Medium depth ERT surveyNew water well

New well

GWL

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

(log‐resistivity)

bedrock

Sandy gravel deposits

Morain debris (boulder and gravel)

This example is related to a hydrogeological study to

replace an old spring catchment with a water well.

(Valgrosina, Sondrio, Western Alps ‐

Italy).

The 2D ERT section pointed out a quite simple geological

model, with a shallow level of high resistivity sediments

(moraine with prevailing boulders) lying on fluvial‐glacial

deposits. Groundwater flows in a channel, and the spring

get a little part of the water. For this reason, a new water

well has been drilled in the low‐resistivity zone

The

depth

of

investigation

is

related

to

the

length of

the

ERT

line,

in

the

ratio

(usually)

of

6

:1

(line

length

=

6,

depth

=

1).

This

ratio

can

be

reduced

using

a

remote

pole

(3:1). The

example

shown

in

the

picture

above

refers

to

a

72

electrode

line with

a

maximum

depth

of

70

m,

which

purpose

was

the

best

location

for

a

new

water

well. According

to

the

ERT

section,

the

new

water

well

was

located and

drilled

in

a

gravel lens (high permeability






4/ 5

5 10 15 20 25 30 35 40 45 50 55 60

Distance [m]

-10

-8

-6

-4

-2

0

Dep

th [m

]

The application of

electrical methods for

the detection of buried

landfills (thickness of

the waste, location of

the bottom, etc.) is

fhelped by the strong

contrast between the

waste and bottom

plastic lining (if any) or

the natural soil. Urban

waste tipically have a

low resistivity, while

the plastic lining is

impermeable to

current flow and

appears as a strong

resistivity layer. The

case shown on this

page is related to a

landfill site in Swansea

(UK).

Waste on high

resistivity plastic lining

Waste disposal in an old quarry

LANDFILL SURVEY

Nucleo discarica

3D MODEL FOR THE RECONSTRUCTION OF THE THICKNESS OF THE WASTE AND RECOGNITION OF THE LANDFILL BOTTOM (PLASTIC LINING)

HDPE

Electrical

Multielectrode

equipment

Distanza [m]

Profondità[m

]






5/ 5

0 5 10 15 20 25 30 35 40 45

Distanza [m]

-7

-6

-5

-4

-3

-2

-1

0

Pro

fon

dità

[m]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

The plume of contamination usually show a strong contrast with the natural soil or groundwater. The

oil

(and

all

hydrocarbons)

have

a

high

electrical

resistivity,

and

can

be

easily

recognized

with

the

geoelectrical survey

CONTAMINANT PLUME DETECTION

LNAPL

MW3Linea ERT7

GWL

DNAPL

Underground pipe

Floating oil

Serbatoio di olioSorgente della contaminazione

Polluted

water3 3.2 3.4 3.6 3.8 4 4.2 4.4<3

(log‐resistivity)

Natural soil

Oil takSource of contamination

LNAPL

Documents

ert_en.pdf