Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface, in order to detect or infer the presence and position of concentrations of ore minerals and hydrocarbons, e.g. Seismic Method.

i) practical application of physical methods ii) measure the physical properties of rocks or

subsurface geologyiii) to detect the measurable physical differences

between rocks that contain ore deposits or hydrocarbons.

What is Exploration Geophysics

Exploration Geophysics

• There is a need to both “identify” and “map”

subsurface sediments

• Geophysics can provide some of the

information required to “map” e.g.,

information on :subsurface architecture,

stratigraphic relationships, variation in

materials

Why use geophysics?

Using Geophysics to Characterizethe Subsurface: The Principles

Using Geophysics to Characterizethe Subsurface: The Principles

• Different geophysical surveys measure different physical properties of the subsurface

subsurface reflector

arrival of reflected wave

arrival of direct wave

source instant (t=0)

ground surface

de

pth

tim

e

• Geophysics requires subsurface targets to contrast significantly with their surroundings

choose geophysical method based on an understanding or estimate

of the physical properties of the target in relation to its environment

What geophysics?

• Where subsurface architecture/stratigraphy is complex

• Where the target produces a clear measureable signal

• Where data with a high signal-to-noise ratio can be obtained

Where should geophysics be used?

• No single survey can map all depths and/or targets

What can geophysics NOT do?

All geophysical methods have an inherent limitation on resolution depends on method, instrumentation and survey parameters, as well as site conditions (geological and cultural setting)

• In conjunction with boreholes or other information that can be used to verify the results

• With strong collaboration between geologist, hydrogeologist, and geophysicist from planning through interpretation

How should geophysics be used?

Geological SurveySurface mapping

Geochemical study

Basin study

Geophysical SurveyGravity

Magnetic

Seismic regional

Compilation & Interpretation

Data

Drilling & Testing

Coring

Logging

Testing

Formation EvaluationProspect Analysis

Risk Analysis

Reserve Calculation

Economical Study

Development

3-D Seismic Survey

InterpretationAVO

Inversion

Seismic Attributes

Drilling Development Wells

Build Platform Production Facilities

Production: Exploration Works

: Development Works

Geophysics and oil exploration

Important facts in exploration geophysics

- Two types of geophysics are available based

on the source of tool.

-We use the relative variations in the physical

properties of the earth not absolute values.

- The rocks and earth materials have overlapped

values of the physical properties.

- The application of geophysical tools depend

on the target of study and nature of the objects.

Geophysical methods are divided into two types: Active and Passive

Passive methods (Natural Sources): Incorporate measurements of natural occurring fields or properties of the earth. Ex. SP, Magnetotelluric (MT), Telluric, Gravity, Magnetic.

Active Methods (Induced Sources) : A signal is injected into the earth and then measure how the earth respond to the signal. Ex. DC. Resistivity, Seismic Refraction, IP, EM, Mise-A-LA-Masse, GPR.

0 1000 2000 3000 4000 5000 6000 7000 8000

Compression Wave Velocity, Vp (m/s)

Fresh Water

Sea Water

Clay

Sand

Till

I ce

Weathered Rocks

Intact Rocks

Steel

P - Wave Velocities

P- Waves (Compression)

Velocity of seismic waves and materials

0 1000 2000 3000 4000

Shear Wave Velocity, VS (m/s)

Fresh Water

Sea Water

Clay

Sand

Till

I ce

Weathered Rocks

Intact Rocks

Steel

S - Wave Velocities

Resistivity, (ohm- meters)

Resistivity Values ( ICE, London, 1976)

} V s = 0

S- Waves (Shear)

Mineral Densities

Potential Radar exploration depth

DEPTH (M)

References:

Physical principles of exploration methods : an introductory text for geology and

geophysics students, Beck, A. E. 1981.

Seismic tomography : with applications in global seismology and exploration

geophysics, Nolet, Guust 1987.

Handbook of exploration geophysics / by Paul A. Chapel, Paul A. 1992.

Exploration geophysics of the shallow subsurface, Burger, Henry Robert 1992.

Exploration geophysics : a review Romberg, Frederick E. 1961.

Foundation of exploration geophysics, Arnaud Gerkens, J. C. d' 1989.

Basic exploration geophysics, Robinson, Edwin S. 1988.

Exploration geophysics, Polshkov, Mikhail K. 1968.