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)