Advanced Seismic ImagingAdvanced Seismic ImagingGG 6770GG 6770

Variance Analysis of Seismic Variance Analysis of Seismic Refraction Tomography DataRefraction Tomography Data

ByByTravis CrosbyTravis Crosby

Outline

• Description of field experimentDescription of field experiment

• Description of refraction tomography processing Description of refraction tomography processing and inversionand inversion

• Description of variance analysisDescription of variance analysis

• Results of tomography experimentResults of tomography experiment

• Results of variance analysisResults of variance analysis

Site Location Map

Site Location Map

SITE

Line Location Map

Seismic Data Acquisition Parameters

GeophonesGeophones - 120 40-Hz vertical component at 5 meter intervals.

SourceSource - Elastic Weight Drop with 550 pound hammer activated at 20 meter intervals (every 4th geophone). 31 total shot points per line. Five impacts summed per shot point.

RecordingRecording - 0.5 ms sample rate, 2000 samples per trace, 1 second listening time.

Seismic Refraction DataCommon Shot Gather

West East

Created with Vista

Seismic Data Processing Parameters

- 3188 traveltimes picked (out of 3720 total)

- 0.6 meter grid size

- One dynamic smoothing schedule 21x11 30 iterations

- 3.9 times more knowns (traveltimes) than unknowns (smoothing schedule)

- Final residual 3.9 ms

Residual vs Iteration

Description of Variance Analysis

- Velocity data was obtained from 30 inversions where first-arrival time was randomly altered up to ± 4 ms ( ¼ period of average first arrival).

- The sample variance of each slowness cell over the 30 samples was determined.

- The variance provides an idea of how widely spread the values of each slowness cell are likely to be.

- The larger the variance, the more scattered the velocities are. It may be inferred that there is more uncertainty in these parts of the model.

- Sample variance:S² = Σ (xi – xave)² / n-1

Tomography Results – Original Data

VelocityProfile

Ray Density Plot

Variance Results

Variance

Ray Density Plot

with color thresholding

Variance Results

Variance

Ray Density Plot

without color thresholding