Post on 14-Feb-2017
transcript
Overview ta3520Introduction to seismics
• Fourier Analysis• Basic principles of the Seismic Method• Interpretation of Raw Seismic Records• Seismic Instrumentation• Processing of Seismic Reflection Data• Vertical Seismic Profiles
Practical:• Processing practical (with MATLAB)
Signal and Noise
Signal: desiredNoise: not desired
So for reflection seismology:- Primary reflections are signal- Everything else is noise!
Signal and Noise (2)
Direct wave: noise
Refraction: noise
Reflection: (desired) signal
Signal and Noise (3)
Direct wave: noise
Refraction: noise
Reflection: signal
Multiply reflected : noise
Signal and Noise for P-wave survey
Noise:• direct wave through first layer• direct air wave• direct surface wave• S-wave• Multiply reflected wave• Refraction / Head wave
Desired signal:• primary reflected P-waves
Signal and Noise for P-wave survey
Signal
Noise=
Primary P-wave Reflected Energy
All but Primary Reflection Energy
Goal of Processing:Remove effects of All-but-Primary-Reflection Energy
Processing of Signal (Primary-reflected energy)
Goal of processing:Focus energy to where it comes from
Understanding signal and noise:wave theory
Basic physics underlying signal is captured by wave equation
Ray theory: approximation of wave equation (“high-frequency”) Resonances: modes expansion of wave equation
S-waves, P-waves: elastic form of wave equation
The seismicrecord
Body waves:
Surface waves:
A seismic shot record on land
Linear event:Direct/refraction
Linear event:Slower, Direct
Slightly hyperbolicevent: Reflection
Linear event:Very slow, Direct
A seismic shot record on land
Linear event:Direct/refraction
Linear event:Slower, Direct
Slightly hyperbolicevent: Reflection
Linear event:Very slow, Direct
Interpretation of seismic land record
thickn
ess 200 m
source detector
offset
air wave
surface wave(velocity 1850 m/s)
refraction
velocity 340 m/s
velocity 3500 m/s(P waves)
velocity 4800 m/s
thickn
ess 200 m
source detector
offset
air wave
surface wave(velocity 1850 m/s)
refraction
velocity 340 m/s
velocity 3500 m/s(P waves)
velocity 4800 m/s
Picture made within PowerPoint
Modelling of seismic land record
Observed data Modelled data
(ray theory approx.)
Modelling of seismic land record
Observed data Modelled data
(ray theory approx.)
refraction
direct P-wave
surface wavereflection
reflectiondirect air w
ave
Wassenaar-beach data: Wednesday
Wassenaar-beach data: Wednesday
Wassenaar-beach data: array data (Thursday)
A seismic shot record at Wassenaar beach
Linear event:Direct/refraction
Linear event:Slower, Direct
Many slightly hyperbolicevents: Reflections !
Linear event:Slow, Direct
Interpretation of Wassenaar record
Modelling of Wassenaar record
Observed data Modelled data
(ray theory approx.)
A seismic shot record at sea
Linear event:Refraction
Many Hyperbolic events: Reflections
Linear event: Direct
First hyperbolic event: primary reflection
Hyperbolic event at twice timeof primary reflection: multiple
Interpretation of marine record
300 msource detector
offset
direct P wave
refraction velocity 240 m/s
velocity 1500 m/s(P waves)
velocity 1500 m/s(density different)
250 m
reflection
multiple
reflection
Picture made with PowerPoint
300 m
source detector
offset
direct P wave
refraction velocity 240 m/s
velocity 1500 m/s(P waves)
velocity 1500 m/s(density different)
250 mreflection
multiple
reflection
Old: Modelling of marine record
Observed data Modelled data
(ray theory approx.)
Modelling of marine record
Observed data Modelled data
(ray theory approx.)
refractiondirect P-wave (water)
reflectionsmultiple reflectionmultiple reflection