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A brief Introduction
Chapter 1 in textbook
1. Active techniques - a signal must be generated - example: seismic waves are generated by an explosion
2. Passive techniques - detect a naturally occurring signal - example: variations in gravity over a buried object
Geophysical exploration techniques
GEOPHYSICS 224 1. Seismic exploration (reflection and refraction)
2. Gravity exploration
3. Magnetic exploration
Common Applications - oil and gas exploration - mineral exploration - diamond exploration (kimberlites) - hydrogeology - geotechnical and engineering studies - tectonic studies - earthquake hazard assessment - archaeology
(active or passive) (passive) (passive)
from wikipedia based on CIA Factbook
Key Target: 1. Hydrocarbons
!5
178 billion barrels (Wikipedia)Traditional: 40% in Alberta, then 27-28% each in Newfoundland & Saskatchewan If factor in oil sands: Alberta =98% (today = 95%, 3rd to Venezuela & Saudi Arabia)
!6from Eccles, AGS Open File Report 65, 2011 (not highly economical)
Key Target: 2. minerals
Crustal Structure of Alberta
(Pilkington et al., 2000; Hope and Eaton, 2002)
• Archean and Proterozoic basement covered by WCSB
• basement domains identified using geophysical data and rocks from oil wells
MAGNETIC DATAGRAVITY DATA
Key target: 3.Tectonics
SEISMIC DATA
Geophysical measurements
- indirect observations of subsurface geology
- surface measurements of natural or artificial signals that travel through the Earth
- surveys are regional: 10s of km up to the entire Earth
- generally resolution is 1 m or more – but can be higher for near-surface studies with sensitive instruments
General Assessment
Interpretation of geophysical data is often non-unique ! several different rock types and geometries may give the same
observation
It is important to use additional information ! borehole data, knowledge of local geology, other types of
geophysical data
- geophysics and geology are complementary
Geophysics and Geology
Geophysical observation
Material property
Subsurface geology
Validation
Validation of geophysical exploration methods
A. Geological mapping - detailed information about rocks exposed at surface - limited information about deeper structures (need areas that have undergone uplift, faulting, erosion)
B. Borehole studies - extensive information about one location (rock types, thicknesses, etc.) - point measurement, expensive, few km deep
Most of the techniques are Interpretive, based on modeled/inverted outcomes
Kola Superdeep Borehole (NW Russia, 1989) - 12,262 m depth in 19 years - drilling stopped due to high temperatures (>180C)
Al Shaheen oil well (Persian Gulf, 2008): - 12,290 m in 36 days
Sakhalin-I Odoptu OP-11 well (offshore eastern Russia, 2011) - 12,345 m in 60 days - horizontal distance of 11,475 m
(www.geo.uni-bonn.de/members/schulze)
Deepest Boreholes
(http://stratoil.wikispaces.com/Sakhalin)Yastreb drilling rig
(52 m tall)
FOR COMPARISON: - avg. continental crust = 35 km thick - avg. radius of Earth = 6371 km
!12Corine Prieto, IGC Footnote Series, Integrated Geophys. Corp.
Where seismics may not be as effective…
These methods are complementary
Seismic reflection data from northern Germany! layers of sediments that have been perturbed by a salt diapir
! salt structure coincides with an area of low gravity
Gravity data !
(Oezsen, 2004)
Seismic data
Simulated ‘ground roll’ near Antipode
Get me out of here, faaaaaassstttt…..!!
Methods of our focus (basin/crust) have far reaching applications, not just in prospecting
Global Example
Example 2: megahertz scale studies of bones (ultrasound imaging)
Waveform Fitting Using Best 1D ModelThe nature of these pulses on the ultrasound records are:
P wave in top CB
P-S conversions in top CB
S wave in top CB
Surface wave in top CB
Marrow P and S multiples
Black ---- data Red ---- synthetics
(Le, Gu, Li & Chen 2010, APL)
Main goal: to develop an understanding of: - the physical basis for 3 geophysical techniques - how each technique is applied (data collection, processing,
interpretation) - advantages & limitations of each technique (resolution, non-uniqueness)
Mathematics will be used in this class - you are not expected to memorize equations (equation sheet provided) - must know how to use equations in data analysis and interpretation - bring your calculator to classes, labs, and exams
Feel free to ask questions! - provide feedback on the level of course material (math, geology, etc.)
1.3 General Comments