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L1 1 Emanuele Forte Lesson 1 Introduction to geophysical methods 3 rd September 2016 – Naxos, Greece

Lesson 1 Introduction to geophysical methods€¦ · Lesson 1 Introduction to geophysical methods ... No data inversion is ... “Potential field”and “wave field”methods Processing

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Page 1: Lesson 1 Introduction to geophysical methods€¦ · Lesson 1 Introduction to geophysical methods ... No data inversion is ... “Potential field”and “wave field”methods Processing

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Emanuele Forte

Lesson 1Introduction to geophysical methods

3rd September 2016 – Naxos, Greece

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Outline

- What is “Geophysics”

- Base concepts: (Geo)Physical parameters

Sensitivity

Resolution

- Active and Passive methods (+ & -)

- Potential and wave field methods (+ & -)

- Review of most common and useful methods

Applicability (especially to rock mass characterization)

- Conclusions and Questions

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Geophysics: GEO (geo) – PHYSICS (fusika)”is a branch of experimental physics dealing with the earth” (SEG)i.e. Exploration of the Earth analysing the physical properties of the materials

What is “Geophysics”

PHYSICAL PARAMETERS

GEOLOGY

PHYSICS MATH

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• Conductivity geoelectrical methods

• Magnetic susceptivity Magnetometry

• Mass/density Gravity;

• Acoustic impedance (vr) Seismic methods

• Combined electrical parameters (e,s) Ground Penetrating Radar –GPR

• T, P, r,... well Logging

TemperatureLenghtColour

PHYSICAL PARAMETERS

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Basic Concepts

Sensibility/Sensitivity“Two different materials can be discriminate only if the applied geophysical method is SENSITIVE to a physical

parameter DIFFERENT for the two materials”

(Geo)Physical Contrast - Anomaly APPLICABILITY of the methods

Resistivity range Wm

E.g.: ElectricalResistivity

Seismic methods

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Resolution

Maximum achievable degree of detail

or: minimum distance allowing to describe as separated two distinct

targets

Basic Concepts

Telford, 1989Sørensen, 2004

Surface hydrogeological

units 50m

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(Synthetic)Seismic Section

0

100

150

Tim

e (m

s)

50

Real geology

0

100

Th

ick

ne

ss

(m)

50

Resolution

Basic Concepts

Usually high resolution geophysical methods are the most expensive (but not always)

ThereforeMust be a TRADEOFF between

OVERALL COSTS and ACHIVEABLE RESULTS

A crucial problem is that any inappropriate method (also low-cost) cannot give any information and

represents a nonsense cost

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51 101 151 201 251 301 351 401

40ns

80ns

120ns

160ns

200ns

Tracce

Base Concepts

Sensitivity Resolution Applicability

GPR (electromagnetic waves)Ultra High Resolution Section (150-350MHz)MAX Penetration Depth 2-20mResolution 0.1-0.5m

Plio-Quaternary

MohoLower Crust

Tertiary

Mesozoic

Batolite

Mantle

0km

32Km

Deep reflection Seismic (elastic waves)Low Resolution Section (5-50Hz)MAX Penetration Depth 40KmResolution 10-100m

ResolutionALWAYS DECREASE with

depth

It’s impossible to obtain an high detail level for increasing depths Target definition

APPLICABILITY

0m

2m

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Active and Passive methods

Geophysical methods can be classified into one of two types: Passive and Active

Passive geophysical methods Measurements of naturally occurring fields or properties of the earth Spatial variations of these fields or properties and attempt to infer something about the subsurface material distibution (geology).Gravitational field, Magnetic field, Electric field (Self Potential), Electromagnetic field (Magneto Telluric), Eartquakes/natural ground movement,Temperature/Heat flow, Pressure, Radiometric decay products, ...

Gravity Survey

Active geophysical methods a “perturbation/signal” is injected into the earth tomeasure how the subsurface responds to this signal Extract subsurface physical parameter/imaging.Electrical current Many electrical methods (ERT, VES, IP, ...);Electromagnetic currents/waves Ground Penetrating Radar, Several Inductive methods (FDEM, TDEM,...); Groung displacement Seismic methods (reflection, Refraction, MASW,...); Active radiometric sources, ...

Seismic Survey with explosive

http://www.teara.govt.nz/

http://tojovikas.com

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Active Passive

Better control of noise sources through control of injected signal

Complex Field equipmentBoth “Sources” and “Receivers” must be supplied

Surveyor need only record a naturally occurring field no source need

Less control of noise because source of the signal is out of the hands of the surveyor.

Active experiments usually provide better depth control over source of anomalous signal

Field operations and logistics are generally more complex time consuming more expensive than passive experiments

Field operations are generally very time efficient wide areas cost-efficiency

Results in term of “anomalous geological contributions” difficult (impossible) identification of the source

Survey design flexibility in customizing surveys for particular problems. Many possible Source/receivers geometries

Greater survey design costs and potentially leads to increased probability of field mishaps

Only few (standard) field procedures are generally used. Relatively easy survey design

Only few (standard) field procedures are generally used. This limits the amount of customisation that can be done for specific problems.

Large quantities of data can be acquired to interpret subtle details of the earth's subsurface

The large quantity of data obtained in many active experiments can become overwhelming to process and interpret

Limited datasets (not always) can be accomplished with modest computational requirements

The data sets collected are (usually) smaller than those collected in active experiments and do not allow for as detailed an interpretation.

Active and Passive methods: Advantages and Disadvantages

+ - + -

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Active Passive

Sometimes invasive methods: sources are required (explosives, high voltages)

Not invasive systems: only sensors are required

Source type/energy can be tailored on the survey objectives. It can be perfectly repetitive with precise time and position knowledge

No source control time drifting, changes, obstacles.

Active and Passive methods: Advantages and Disadvantages

+ - + -

“Potential field” and “wave field” methods

Another very important classification refers to the following:

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“Potential field” and “wave field” methods

Potential methods measure potential fieldsNOTE: A force fields can be described as a potential field if the field is conservative (work path independent). Conservative forces can be represented mathematically by simple scalar expressions known as POTENTIALS

To recover the PHYSICAL PARAMETER DISTRIBUTION the dataset must be INVERTED

NON-LINEAR PROBLEM i.e. no by univocal correspondence between:MEASURED FIELD PROPERTIES and PHYSICAL PARAMETERS (MODELS) IN THEORY, INFINITE MODELS COULD MATCH A MEASURED FIELD

DISTIBUTION TYPICAL UNDERCONSTRAINED PROBLEM

Additional information are required (direct data, other geophysical data, ...)

The Potential fields used for geophysical exploration are:1) Gravitational 2) Magnetic 3) Electric

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“Potential field” method: conceptual EXAMPLES

The direct problem is to compute gravity potential and force when the density is known.

FORWARD modeling

The inverse problem is to determine the density using the force (the potential) on a given surface.

INVERSE modeling

Measured gravity field Mass/density distribution

Regional/global effects large scale anomalies

Measured gravity field Mass/density MODEL

Data INTERPRETATION

soilore

Local effects local anomalies

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“Potential field” and “wave field” methods

Wave field methods based on the propagation of a perturbation (wave) within the earth

The most common are Seismic (elastic waves) and Electromagnetic waves

The perturbation (or signal) travels into the subsurface, is REFLECTED/REFRACTED/SCATTERED/BACKSCATTERED/CONVERTED and therefore can be recorded at the surface (or into a

borehole) by one or more sensors as a function of the time (typically the time zero is the energizing instant)

No data inversion is required (but it is possible) direct IMAGING of the subsurface

SOURCE Record: reflection seismic section

DistanceDistance

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“Potential field” and “wave field” methods

To obtain a real subsurface IMAGING a complex processing is required, but usually final results are very close to the reality

Field experiment recorded data (with interpretation)

SaltDiapir

From: Bahorich et al., 1995

Original data

Processed data(coherency timeslice)

Faults Paleo Channel

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Seismic section after preliminary processing (Stack)

“Potential field” and “wave field” methods

Processing 1D, 2D, 3DD

ept

h [

m]

Dept

h [

m]

Seismic section after Depth migration

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Questions?