L3 QAB2044

7/31/2019 L3 QAB2044

1/17

L3: Introduction

7/31/2019 L3 QAB2044

2/17

Geophysics

The objective of geophysics is to locate or detect the presence ofsubsurface structures or bodies and determine their size, shape,depth, and physical properties (density, velocity, porosity) +fluid content

study of structure and dynamic of thesolid Earth using physical methods

Non-expensive andNon-invasive methods

less localized than borehole, but

combination of borehole and geophysicalinformation returns the optimum result

7/31/2019 L3 QAB2044

3/17

Geology and Physics

constraining the Earths sub-surfacewith observations at/near the surface

apply the principles of physics to thestudy of the Earth

7/31/2019 L3 QAB2044

4/17

Geophysics

Geophysical analysisis done when

1) you cant get directaccess to the rocks

2) you want to modela large area quickly

3) you know what youare looking for

but every geophysical technique onlyestimates geology (lots of interpretation)

7/31/2019 L3 QAB2044

5/17

Active & passive

geophysical survey methods can be classified into one of two broadclasses, e.g.:

Active Passive

Electrical Resistivity, Seismic,

Electromagnetic, Radar, InducedPolarization

Depend on artificial source

Transmit a signal into the

subsurface and record what comesback

gravity, magnetic, radiometric,

certain electrical fields (SelfPotential), certain electro-magnetic fields (telluric),seismology

Depend on natural source

Measure naturally occurringphenomena

..based on the sources of the applied energy or fields

7/31/2019 L3 QAB2044

6/17

Geophysical Techniques

wide varieties of geophysical techniques and each of these measures aspecific physical property

http://armah.netfirms.com/Geophysics.html

7/31/2019 L3 QAB2044

7/17

Geophysical Techniques

Major geophysics and rock physical properties relationship, e.g.;

Geophysical parameter relation can be describe as

http://armah.netfirms.com/Geophysics.html

geometrical parameter

anomalygeology /engineeringinterest

7/31/2019 L3 QAB2044

8/17

Anomalies

Ultimately what geophysics do is to measure the intensity ofwaves or force fields as they pass through geological materials

They are looking for unpredicted variations or deviations from

normal (i.e. anomalies)

Geological interpretations are made on the basis of theseanomalies

http://www.geos.ed.ac.uk/homes/jru/Silverpitseismic.jpg

7/31/2019 L3 QAB2044

9/17

Survey scale

There are 2 scales of geophysical analysis:

small-scale

(single well; shallow penetration seismic etc.)

large-scale

(basin wide surveys etc.)

http://sepwww.stanford.edu/sep/bi

ondo/HTML/IntroRef.html

http://gsc.nrcan.gc.ca/borehole/

index_e.php

http://www.laynewater.com/bore

hole_geophysics.htmlhttps://reader009.{domain}/reader009/html5/0429/5ae5286a83ce2/5ae5287358c15.jpg

7/31/2019 L3 QAB2044

10/17

Geophysics from a differentperspective

The techniques well talk about are used in the fields of

Environmental geophysics

Engineering geophysics

Exploration geophysics

uses surface methods to measure the physical properties ofthe subsurface Earth, in order to detect or infer the presence

and position of

ore minerals, hydrocarbons, geothermal reservoirs,groundwater reservoirs, and other geological structures

7/31/2019 L3 QAB2044

11/17

Planning a survey & Targetidentification

http://www.geology.wisc.edu/courses/g594/

e.g.

determine whatinformation already exist

7/31/2019 L3 QAB2044

12/17

Planning a survey & Targetidentification


determine whatinformation already exist

7/31/2019 L3 QAB2044

13/17

Techniques & Targets


7/31/2019 L3 QAB2044

14/17

Techniques & Targets Profiling vs. mapping

What is the nature of the target? 2D or 3D?

Preliminary site evaluation necessary?

7/31/2019 L3 QAB2044

15/17


Station spacing

Must ensure that spacing is sufficient to sample anticipated signal


7/31/2019 L3 QAB2044

16/17


Limitations

Lack of sufficient contrast in physical properties

Nonuniqueness of many interpretation

Ambiguity (obvious in gravity work)

Resolution is determined by the wavelength of the signal

Noise (undesired signal)

Multiple methods

to improve uniqueness and cross-check interpretations

7/31/2019 L3 QAB2044

17/17


http://appliedgeophysics.lbl.gov/intro/figures/table_prop.jpg

Documents

L3 QAB2044