Seismic Refraction Analysis of California Wash and Astor Pass

Stephen HeinMason Kreidler

Overview Project Objectives

Field methods

Analyses

Interpretations

Discussions

Project Objectives California Wash

Analyze the fault running through California Wash, to evaluate seismic hazards for Las Vegas

Astor Pass Examine regions that were not previously

drilled, to see if more promising geophysical prospects exist

Equipment: All lines 7 kg sledgehammer

Seismic source

100 Hz Geophones Seismic vibration monitor

Seismic cable with 48 channel Data transfer

Bison Galileo-21 Data logger

Analysis: All lines First arrival picks using Viewmat

Better images through Band-Pass filters, TEGain,

Simple calculations using a time-distance plot in excel V1 and V2 velocities:

Refractor depth:

Dip angle

Estimated low and high velocities

Possible third layers

Analysis: All lines Dr. Pullammanappallil created P-wave seismic

velocity sections from our first-arrival pick times using SeisOpt® @2D ( 2011 Optim) Cross Section with velocities

Refractor depths

Layer velocities

Seismic cable oriented East-West with a total length of 100 meters 2 meter spacing between each takeout

6 geophones per takeout oriented parallel to the seismic cable

Sledgehammer hits used to propagate waves into the ground

10 hammer hits at each source point

A total of 74 source points were taken

14 to the East of the first geophone at a spacing of 4 meters

48 at each channel along the line

12 to the West of the last geophone at a spacing of 4 meters

Total length of hammer hits: 200 meters

Field Methods: California Wash Line

1

Layout of seismic cable and geophones (Louie, 2011)

Producing waves using a sledgehammer and layout of

geophones (Louie, 2011

Results: California Wash Line 1

V1: 666-1104.7 m/s with an average of 885.4 m/s

V2: 1301.2 m/s

Refractor depth between 2.4-15 m

Dip angle of 1.3 degrees

Optim's SeisOpt®@2DTM Results: California Wash Line 1

SeisOpt®

Interpretation: California Wash Line1 P-Wave velocity was 600-900 m/s, consistent

with simple calculations average of 880 m/s

V2 of ~ 1400 m/s consistent with simple calculations

Estimated refractor depth of 1-10 m, also consistent

Very low eastward dip in first refractor

Seismic cable oriented East-West with a total length of 48 meters 1 meter spacing between each takeout

6 geophones per takeout oriented perpendicular to the seismic cable

Sledgehammer hits used to propagate waves into the ground

10 hammer hits at each source point

A total of 48 source points were taken

48 at each channel along the line

Total length of hammer hits: 48 meters

Field Methods: California Wash Line

2

Example of geophone arrangement and wave propagation at line 2 (Louie, 2011)

Results: California Wash Line 2

V1: 216.7-326.1 m/s with an average of 271.4 m/s

V2: 1274.5 m/s

Refractor depth between 0.69-0.90 m

Dip angle of 0.24 degrees

Optim's SeisOpt®@2DTM Results: California Wash Line 2

SeisOpt®

Interpretation: California Wash Line2 P-Wave velocity was 200-400 m/s, consistent

with simple calculations average of 270 m/s

V2 of ~ 1200 m/s consistent with simple calculations

Estimated refractor depth of 1-2 m, slightly higher than estimated in simple calculations

Minimal evidence of any dip in the first refractor

Interpretation of California Wash Lines 1

and 2 Conformation of good results by overlaying CW2 onto CW1

Discussion: California Wash Lines 1 and 2

Results from Line 1 give a good indication of the dip of the fault running through the wash

Line 2 shows evidence of the amount of slip that occurred that last time on this fault, by the amount of recent, slower velocity, sediment on top of older, higher velocity alluvium

Seismic cable oriented East-West with a total length of 144 meters 3 meter spacing between each takeout

6 geophones per takeout oriented parallel to the seismic cable

Sledgehammer hits used to propagate waves into the ground

10 hammer hits at each source point

A total of 60 source points were taken

48 at each channel along the line

12 to the West of the last geophone, at a spacing of 6 meters

Total length of hammer hits: 220 meters

Field Methods: Astor Pass

Layout of seismic cables and geophones at Astor Pass

Results: Astor Pass V1 of 275-400 m/s with

an average of 337.5 m/s

V2 of 1090 m/s

Refractor depth between 15-30 m

Dip angle of 1-5 degrees

0 20 40 60 80 100 120 140 1600.00

0.05

0.10

0.15

0.20

0.25

0.30

Source Downdip TimesSource Updip TimesDowndip Source

Receiver X Coordinate, meter

Tim

e,

seco

nd

Astor Pass Time-Distance plot

Optim's SeisOpt®@2DTM Results: Astor Pass

SeisOpt®

Interpretation: Astor Pass P-Wave velocity was 300-400 m/s, consistent

with simple calculations of 275-400 m/s

V2 of ~ 1000 m/s consistent with simple calculations of 1090

Estimated refractor depth of 12-16 m, also consistent with previous calculations

No dip in first refractor

Basic Interpretation

Discussion: Astor Pass

Results from SeisOpt® did not agree with the hypothesis of seeing faster velocities near the middle of the plot where tufa layers were thought to exist.

These results actually lean towards a hypothesis that most if not all of the tufa is already on the surface.

Possible Sources of Errors

Wind creating noise on the seismic line.

People walking around on the seismic line while recording data.

First arrival picks may have been off slightly; however, refractor depth and data was consistent throughout the plot, suggesting picks were valid.

Conclusion of Astor Pass Refraction Microtremor results agreed with

our Refraction results of not seeing faster velocities near the tufa mounds.