Geophysical survey methods: How To Manage the Due Diligence Risk

One of the primary reasons for conducting a due diligence assessment on a property that

will either be purchased or sold is to understand the potential hazards and liabilities

associated with the past and present activities conducted on-site. In summary, the goal is

to “manage risk”.

The main issue is the clear understanding of what operations occurred and what the

subsurface holds.

MANAGING RISK AND UNCERTAINTY

Georadar section on an area nearby

a river with location of old bridge

arcades

10 m

1 m

LOCATION OF THE POTENTIAL RISKS

UXO (unexploded ordnance) location with

Magnetometer – CIRPARK Prj – Turin, 2014

In many instances, a geophysical

investigation can be part of the

solution to the problem. These non-

invasive exploratory methods can

greatly reduce the risk of missing

critical subsurface problems or

encountering a buried hazard by

providing a screening tool and

strategy for proposed follow-up

subsurface investigations.

How to identify an unknown buried

object (potential risk and hazard)

on a property?

Invasive investigations (borehole

drilling, excavations, etc.) are:

• Sometime not permitted by the

current site owner or facility

operations

• Limited at a single point and

without any certainty to locate

buried object

• location of buried utilities or

other subsurface hazards may

render invasive methods as

unsafe or impossible

• Non-invasive collection of data

• Cost-effective

• Quick mobilization

• Fast on-site set-up and breakdown

• Identification of hazardous prior to subsurface work

• Permitting generally not required

• Wide and comprehensive area of coverage

• No generation of impacted media

BENEFITS OF GEOPHYSICAL ASSESSMENTS

EM survey for location of waste disposal

Sacmi Prj – Scandiano (Italy), 2014

There are several types of geophysical methods available, all of which can be used independently or in

tandem with each other. Fr the due diligence risk assessment we can consider two different geophysical

survey: LARGE SCALE SURVEY AND (IF NECESSARY) SMALL SCALE SURVEY

Large-scale geophysical survey (surface mapping of the underground properties) – MAGNETIC ELECTROMAGNETIC MAPPING

GEOPHYSICAL METHODS

Location of the main targets (buried tanks, underground pipes, unknown waste disposals, any

underground anomalies)

satisfying detail NO YES

small-scale geophysical survey (anomalies characterization) GROUND PROBING RADAR, ELECTRICAL TOMOGRAPHY

Direct ground investigation/remediation

FIRST PHASE

SECOND PHASE

Frequency Domain Electro Magnetic induction (FDEM) involves generating

an electromagnetic field (primary field) which induces current in the earth

which in turn causes the subsurface to create a secondary magnetic field.

By measuring this secondary magnetic field and the difference with the

primary field, subsurface soil properties and the main features (buried

objects) can be detected. This method measures the magnitude and phase

of induced electromagnetic currents, which are related to the subsurface

electrical conductivity. Electrical conductivity is different for soil types, ad it is

a function of the soil and rock matrix, percentage of saturation, and the

conductivity of the pore fluids. EM instruments provide two measurements

simultaneously, the electrical conductivity data and the in-phase component,

which responds to magnetic susceptibility and metal.

FDEM has distinct advantages over many other techniques. Because no

contact with the ground is required, FDEM can cover a large area quickly

and therefore with cost saving.

LARGE-SCALE GEOPHYSICAL SURVEY

Common applications of FDEM include

• the mapping of buried wastes, metal drums, UST tanks, and metal utilities

• Detect archeological remnants

• Locate UXO (unexploded Ordnance)

FD EM Survey

The survey area is divided in regular grid. The spacing between line is usually 1 or 2 m

a Transmitter

Receiver

target

Primary EM Field

Secondary EM Field

Example 1 - EM SURVEY TO LOCATE ILLEGAL UNDERGROUND DUMPING

FDEM Survey

Electrical conductivity

FDEM Survey

Magnetic susceptibility

Gas pipe

Illegal landfill

Natural soil Foundry waste

Detection and location of an illegal waste disposal - Brescia (ITALY)

The ground conductivity maps obtained with electromagnetic survey can easily locate buried waste or

contaminated areas. The magnetic susceptivity is particularly suitable for the detection of metals and helps a

lot in the detection of buried drums or, for example, to recognize different waste type (in the example below,

foundry wastes from ferrous foundries)

REMARKS: THE EM METHOD INVOLVE A GROUND THICKNESS OF 5-6 M

Example 2 - EM SURVEY TO LOCATE ILLEGAL UNDERGROUND DUMPING

0 50 100 150 200

Distanza [m]

0 50 100 150 200

-120

-90

-60

-30

0

D

i s

t a

n

z a

[

m

]

143500

145500

147500

149500

151500

Illegal disposal 1 Tyre waste illegal disposal – Old trench excavation filled with tyre wire waste (HIGH CONDUCTIVITY)

Illegal disposal 2 Old gravel pit excavation filled with rubber waste (LOW CONDUCTIVITY)

Mapping of an illegal waste disposal - Cuneo (ITALY)

SMALL-SCALE GEOPHYSICAL SURVEY Small-scale geophysical survey has the main

objective of better defining the ground

characteristics (soil profiling, waste disposal

thickess and geometry, shape of the buried

structure, etc)

There are two main methodologies:

• ELECTRICAL RESISTIVITY

TOMOGRAPHY (ERT)

• GROUND PROBING RADAR

0m 50m 100m 150m 200m-150

-100

-50

0

Resistivity Map

Resistivity section

Mapping of an illegal waste disposal - Cuneo (ITALY) Thickness of the waste disposal = 10-11 m

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 groundwater

2. Depth of bedrock

3. Mapping of contaminant plumes

4. Location of faults and rock

fractures

5. Detection of buried landfill cells

6. Location of clayey zones that

may form aquitards

7. Zones of buried constructions

debris

8. 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. Depth of penetration:

1/3 of the length of the line

ELECTRICAL RESISTIVITY TOMOGRAPHY

0 5 10 15 20 25 30 35 40 45

Distance [m]

-7

-6

-5

-4

-3

-2

-1

0

P r o

f o n

d i t à

[ 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

LNAPL

MW3 Linea ERT7

GWL

DNAPL

Underground pipe

Floating oil

Polluted water

3 3.2 3.4 3.6 3.8 4 4.2 4.4 <3

(log-resistivity)

Natural soil

Oil tak

Source of the contamination

LNAPL

Example 3 – ERT SECTION TO LOCATE THE CONTAMINATION PLUME OF PLASTIFICANT OIL IN THE GROUNDWATER TABLE

Mapping of the pollution plume due to a leakage from an oil storage tank (project not citable)

GPR (GROUND PROBING RADAR) SURVEY METHOD

The ground penetrating radar (GPR - Ground Probing Radar) is a geophysical method used to investigate the near

surface underground. Thanks to its high degree of resolution, the GPR is the most effective method for locating

cavities, underground tanks or utilities, and buried artifacts, archaeological remains and structures in general. In

addition, the GPR can be used to identify geologic contacts, substrates and surface geological features of various

kinds (fractures, groundwater levels, etc..).

The GPR method can detect underground storage

tanks and pipes, underground utilities of any type

(metal, plastic and concrete). A typical GPR survey

requires the acquisition of a series of lines arranged

in a regular grid, in order to investigate the entire

site. When a radar profile crosses a cylindrical

shape (pipe or tank) across its axis, the

electromagnetic signal undergoes a reflection effect

that results in a typical hyperbolic shape .

For any kind of work involving excavation of the ground, the exact knowledge of the location of

underground structures is a key element for proper execution of the work. In this perspective, the use

of GPR is an excellent tool to obtain all the information needed to perform the actions provided for in an

effective way, safe and without damage to the infrastructure.

Georadar section

3D reconstruction of the buried structures

UST

Underground

tank

utilities

B

A UST

GPR SURVEY METHOD

Example 4 – GPR SURVEY FOR LOCATE THE FOUNDATIONS OF SOME RADIO TRANSMITTER TOWERS

Sistema di

dispersione a

lisca di pesce

TRANSMITTER TOWER GPR SECTION – GEOMETRY RECONSTRUCTION OF THE

FOUNDATION

ACQUISIZIONE DATI

SKETCH MAP AND SECTION

RADAR ANTENNA CIVIL WORKS IN THE RIGHT POSITION

Transmitter tower replacement and restoration - Pisa airport (ITALY)

3

4

GPR section 3

Sketch map

GPR section 4

Site: restoration of a warehouse (Novara, 2010) The purpose of the survey: The survey was conducted to assess the characteristics of the pavement of an old industrial building to converted in a storage warehouse. More in detail, we detected the potential weakness zones, which may cause differential settlement or structural collapses due to concentrated loads . Design Survey: The survey methodology was conducted with GPR (400 MHz antenna) on the whole area of the building (2000 m2): The operation required the use of two technicians for about half a day of site acquisition. Results: The investigation showed the presence of several underground old pipes (concrete and fiber-concrete), 15 hidden metal plates (basement of industrial machines) and the mapping of the reinforced concrete pavement.

Example 4 –CONCRETE FLOOR ASSESSMENT OF A CONVERTED INDUSTRIAL BUILDING

pipes Reinforced concrete pavement

Small tunnel

Small tunnel

Concrete rebars

Reinforced concrete