Geophysical Exploration Under Harrat · Satellite . Geophysical Survey . Outlines •Give a general overview on different geophysical field methods. •Explain the basic principles

Geophysics

Hussein M Harbi

What is geophysics?

Apply the principles of physics to

study the Earth.

Geology

Physics

Mathematics

Computer skills

Sciences Need for Geophysics

Applied geophysics is based on physics principles, collect and interpret

data to extract info about the subsurface conditions for practical

purposes, including oil and gas exploration, mineral

prospecting, geothermal exploration, groundwater

exploration, geohazard, archeology,

forensic, etc .

Classification of geophysics

Based on usage

Theoretical Geophysics

Applied Geophysics

Based on Scale

Global Geophysics Deeper than 10,000 m.

Exploration Geophysics 100 : 10,000 m.

Env. & Eng. Geophysics Less than 100 m.

Archaeological Geophysics Less than 30 m.

Based on another field

Land Geophysics

Airborne Geophysics

Marine Geophysics

Well Logging

Based on field

Potential Field Methods (Gravity, Magnetic)

Wave Field Methods (Seismic, GPR)

Electromagnetic Field Methods (EM, Electric)

Based on field source

Passive Active

Outlines

• Give a general overview on different geophysical field methods.

• Explain the basic principles of geophysical survey (Geophysical Practice).

• Outline capabilities and limitation of various geophysical techniques.

• Examples.

Magnetic

Gravity

Seismic Electric

EM

GPR

Radiometric

Main Geophysical Methods

Contrast in density

Gravity

Land

Marine

Wells

Air

Magnetic Contrast in Magnetic susceptibility

Gravity Deep earth, cavities,

minerals, oil

Magnetic Structure, lithology, minerals,

archeology

Potential

Field P a s s i v e

GeoElectric

Active

Resistivity Sounding, 2D cross section

Lithology, minerals, ground water, cavities

IP Sounding, 2D cross section

Mineralization, clay

Passive SP Profiling, 2D cross section

Mineralization

Electrical conductivity

Electric

Land

Marine

Wells

Electrical Resistivity (DC Resistivity)

Electrical capacitance Electrical Induce Polarization (IP)

Electrical conductivity Self Potential (SP)

Seismic

Active

Reflection Geological

cross section Direct indication for oil

Refraction Depth, velocity

tomography shallow stratigraphy

MASW 1D and 2D Vs cross section

Soil classification

Passive

Earthquakes Velocity and attenuation

Deep earth imaging

MASW Use ambient

noise Deep Vs imaging

Contrast in density and elastic moduli

Seismic

Land

Marine

Wells

EM

Active

TDEM Sounding, Cross section

Water, Mineralization, Oil,

FDEM Profiling, Cross

section, Mapping

Mineralization, environmental, infrastructure

CSAMT Cross section Deep structure (2km)

VLF Profile Mineralization, Water

Passive

AMT Cross section Deep structure (2-10km)

MT Cross section Deep structure (10-

100km) or more

Contrast in electrical conductivity and induction

Electromagnetic

Land

Marine

Wells

Air

Contrast in dielectric constant GPR

Land

Wells

Air

Radiometric Contrast in radiations

Active Cross

section Shallow investigation, environmental,

infrastructure, archeology, cavities

Ground Penetrating Radar

Radiometric

Land

passive Mapping Environmental, Mineralization, Lithology

Outlines




• Examples.

Geophysical Survey

What is noise? What is signal?

Signal-to-noise ratio (S/N)

Geophysical Survey

What is noise? What is signal?

Geophysical Survey

Target Geometry

Required Resolutions

Method limitations Su

rvey

Land Airborne Marine Well logging

Satellite

Geophysical Survey

Outlines




• Examples.

Use natural field, need simple

equipment.

One or two operators can do the

survey.

Can be done in small area for

details (cavities, mineralization),

or for large area (crustal

deformation, geothermal energy,

deep earth structure).

Done in land, marine, airborne and

satellite.

Data are very noisy and interact

with many artifacts.

Interpretation vey ambiguous.

Resolution needs extra care of data

acquisitions and need extra

constrains.

Does not give real geology or

structural cross section.

Capabilities Limitation

Gravity

Geophysical Capabilities and Limitation

Use natural field, need simple

equipment, cost-effective.

One or two operators can do the

survey.

Can be done in small area for

details (archeology, structure

mineralization), or for large area

(mineralization, geology, structure).

Done in land, marine, airborne and

satellite.

Data are very noisy and interact

with many artifacts.

Cannot be done urban areas.

Interpretation vey ambiguous.

Resolution needs extra care of data

acquisitions and need extra

constrains.

Does not give real geology or

structural cross section.


Magnetic


Can be used for sounding(very with

depth) or profiling to create 2D and

3D resistivity and chargeability cross

section.

Used intensively in mineralization,

ground water and cavities

(geohazards). Can be used in

archeology using small spacing

Can be done in land & marine.

Need extra equipment (transmitter,

receivers, electrodes, etc.) .

Need more people to do the survey

(intermediate- high cost).

Rocks and subsurface geology can

vary based on density of fractures

and degree of water saturation .



GeoElectric Dc Resistivity & IP

Give a very good subsurface

stratigraphic and structural images

in 2D and 3D.

Used intensively in hydrocarbon

exploration, structural context .

Can be use for reservoir

characterization.

Data is very huge, large-time

consuming, need intensive processing

(very expensive).

Need large crew to do the survey (high

cost). Sensitive to noise such traffic.

Processing and interpretation need

more experience.



Seismic Reflection

Give a subsurface velocity and

depth for simple stratigraphy in 2D

or 3D.

Can be used in small depth

(structure, lithology etc.) or for

deep crustal velocity imaging (using

large source-explosive).

For small survey, very cost-effective

compared to reflection methods.

Data is small and can be done using

simple interpretation.

Need open area survey layout

(depth=1/4 of the spread length).

Sensitive to noise such traffic, winds

etc.

Crew need average number of people.



Seismic Refraction

Give a the Vs subsurface velocity in

1D and 2D.

Can be used soil classifications or

engineering and geohazard analysis;

can be used also for sinkhole and

caves detection.

Cab be applied using control source

(~30m depth) or ambient noise

(more than 60m depth).

Very slow in operations.

Limited depth .

Relatively low resolution.



Seismic MASW Usually is surveyed in 1D then

summed into 2D or 3D sections.

Earthquake tomography gives the

velocity, density and attenuation for

the deep earth structure. MT gives

the conductivity distribution.

Use natural source (earthquakes) or

EM waves from ionosphere.

Used to define the crust thickness,

can be used for geothermal

explorations.

Very low resolution.

MT needs very long time for surveying.



EM& Seismic MT& Earthquake

Earth Seismic velocity

EM can be used for shallow (few meters) exploration to 100s meters.

Can be used for environmental, mineralization, ground water, metal detector, hydrocarbon, regional structures.

Cab be applied using control sources or natural sources (AMT).

Give 2D and 3D conductivity section for the subsurface.

Can be done in land, airborne, marine and well logging.

TDEM, AMT and CSAMT is a bit slow in surveying.

FDEM very sensitive to external noise.

Cannot applied in urban areas.

For deep explorations, resolution degreased.



EM

TDEM

FDEM

CSAMT

AMT

SP uses natural source; FLV used

international low frequency

transmitters.

Equipment very simple, one or two

people can do the survey.

Give the subsurface conductivity

distribution.

Used for general mineralization,

ground water exploration.

Limited depth (less than 30m).

Sensitive to near surface conductivity.



GeoElectric &EM SP & VLF

GPR gives high resolution of near

surface stratigraphy and features

imaging (few cm to 10s m).

Used in infrastructure detection,

archeology, geohazards, engineering

and environmental study.

Fast and can be used in urban areas.

Can be used in 2D and 3D model.

Limited depth (less than 30m).

Sensitive to near surface conductors.

Cannot penetrate clay.



GPR

Use the natural radiation from the

radioactive elements.

Used for geological mapping,

mineral exploration (radioactive

ore), environmental, and in well

logging for shall and sand

classification.

Fast and can be used in wells, land,

car-borne, and airborne survey.

Give estimation of the amount of

eU, eTh and K and the ration

between them.

Can be used as an indication for

other minerlizations.

Limited depth (less than 30cm).



Radiometric

Outlines



• Examples.


Examples

Integration

between

DC

Resistivity,

IP and

Seismic

Refraction

Geohazard Application

Examples

Integration

between

DC

Resistivity,

IP and

Seismic

Refraction

Environmental Application

Examples

Integration

between

DC

Resistivity,

IP

Mineralization Application

Conclusion




• Examples.

Documents

Geophysical Exploration Under Harrat · Satellite . Geophysical Survey . Outlines •Give a general overview on different geophysical field methods. •Explain the basic principles