applied geophysics – oct 9 - university of british columbia · 14/10/2009 · applied geophysics...
TRANSCRIPT
Applied Geophysics – Oct 9
Goals for today
Review time-intercept method for layer over halfspace
Low velocity zones (applet for Snell’s law)
Sources/Receivers
Global earth example
A B
A B
Seismic survey offset
…
Raw data
Direct arrivals The T-X plot
* x x x x x x x x x x x x x x x x x x x x x x x
Raw data
First refractions The T-X plot
* x x x x x x x x x x x x x x x x x x x x x x x
Raw data
Second refractions The T-X plot
* x x x x x x x x x x x x x x x x x x x x x x x
Raw data
First arrivals The T-X plot
Relations for things we want
Needed: Relations between times, distances and velocites
Direct arrivals: Easy: x=v*t t=(1/v)x 1/v is slope.
* x x x x x x x x x x x x x x x x x x x x x x x
Relations for things we want
Refractions – geometry must be accounted for: Relations between times, distances, angles and velocites. Relate x, z, θ
Relate v1, v2, θ (Snell’s law)
Low Velocity Zone
Snell’s law for decreasing velocity
http://staff.washington.edu/aganse/raydemo/RayDemo2.med.html
Ray tracing for non-layered velocity models (Illustration of Snell’s law)
Low Velocity Zones
V2 < v1
No refracted arrival from the top of the second layer
Hidden Layer Layers that are too thin may not be seen
Arrival from layer 3 beats that from layer 2
V3>v2>v1
Sources & sensors and Raw data
Geophone:
Seismic lab #2 involves raw data & first break picking.
Detectors and Recorders Geophones and Seismographs
Set up the geophone array
Geophone
Multi-channel seismograph receiver
Borehole explosion
Thumper truck
From Lithoprobe website
Measurements - Sources
For small scale work, energy can be small.
Shotgun blast
Sledge hammer
Sledge hammer
Waves, rays and energy propagation
How does energy propagate?
Amplitude gets reduced further from the source
Energy decays and the pulse spreads out as it propagates
Global Earthquake
Global Earthquake:
http://www.iris.edu/hq/programs/education_and_outreach/visualizations US Network for global seismic monitoring (see tutorial)
Readings
Seismic refraction: GPG 3.e.7 - 3.e8
Next week
Monday: Class Cancelled Wednesday: Finish refraction seismology Friday: Quiz
EOSC 350 ‘06 Slide 27
Applied Geophysics – Oct 14
Refraction for 3 layers. Dipping Layers More complicated interfaces and approaches.
Plus-Minus method Generalized Reciprocal Method Outline the “Plus-Minus” method
Introduce advanced refraction interpretation techniques.
Refraction 3-layers: Snell’s law rules
Second refractions The T-X plot
* x x x x x x x x x x x x x x x x x x x x x x x
Intercept time method (ITM)
What’s ultimately wanted from a seismic refraction survey? Velocities
How? Yields what information? A necessary prerequisite for what?
Depth How? Yields what information?
ti
EOSC 350 ‘06 Slide 30
Dipping layers: interpretation requires two shots
Resulting T-X plot
Recall: dipping layers
X Shot point
X Shot point
Reciprocal time
Depth estimates “Slant” depths can be obtained through the intercept
times True depths can be estimated using dip-angle (see GPG
3.e.6)
Irregular Layers
What happens when boundary can no longer be approximated with a plane?
Plus-Minus Methods GRM (Generalized Reciprocal Methods)
What about the velocity of the second layer?
V1 known from first arrivals
V2 is difficult to obtain because slope is not straight
z varies
EOSC 350 ‘06 Slide 35
The Plus-Minus method Notes section 8. We want Z = depth under geophones
We could do
But we don’t know TAD
EOSC 350 ‘06 Slide 36
Delay time We want depth, z, under D. Relation is 1st to the right
aSD is the “delay time”, defined as TAB – TBC
Derivation discussed in notes section 8.
EOSC 350 ‘06 Slide 37
Plus Minus method summary We want depth, z, under D. Relation is 1st to the right aSD’s value comes from 2nd relation,
called the “Plus term”. V2 comes from slope of the 3rd
equation, called the “Minus term”.
(Note that the slope is 2/V2).
EOSC 350 ‘06 Slide 38
Plus Minus method summary We want depth, z, under D. Relation is 1st to the right aSD’s value comes from 2nd relation,
called the “Plus term”. V2 comes from slope of the 3rd
equation, called the “Minus term”. (Note that the slope is 2/V2).
In other words … Adding measured times from opposing
directions yields a relation that gives Z. Subtracted measured times can be
plotted to give V2 = 2/slope.
EOSC 350 ‘06 Slide 39
Generalized Reciprocal Method - GRM
ITM with fwd/rvrs shots, and PlusMinus are “reciprocal methods”.
Goal of GRM is to estimate velocities & depths without requiring interface segments that are flat.
Velocity and depth BOTH estimated under EACH geophone that has seen refractions.
Therefore lateral velocity variations in one layer can be seen.
But … “smooth” velocity changes (materials grading one into the other) can still not be seen.
GRM interpretation
Can be performed along a line (see GPG e.10)
Ray tracing and inversion for complicate structures Numerical programs compute the ray paths and travel
times.
Adjust the velocity so that simulated travel times match the observed travel times.
EOSC 350 ‘06 Slide 42
Ray tracing techniques
Created data must look like measured data, within error bars. Therefore error specifications are an important part of the data set.
Also “turning rays” can be accommodated. Interfaces are not necessary because steadily increasing velocity
means ray paths curve. This effect is crucial
in crustal studies using earthquake signals.
Of course, head waves are also handled properly.
EOSC 350 ‘06 Slide 43
Ray tracing
It is possible to calculate ray paths for complex earth structures, with velocity gradients, no sharp boundaries, and low velocity zones.
Limitations? Complicated to use, therefore costly in “expert” time. 2D only. Need large data sets
therefore field work is expensive.
Reading for Team Exercise
Near-surface SH-wave surveys in unconsolidated, alluvial sediments (on website)
This is a case history for a landfill in Norman, Oklahoma
Pay attention to:
7-Step Process
Understand data plots
Typical data plots
Upcoming
Team Quiz on Friday, October 16
Team exercise on Monday, October 19
Midterm on Oct. 28nd (physical properties,7 Step procedure, magnetics, seismic refraction; lab material, team exercises)