Seismic Refraction Exercise for a Hydrogeology Course Devin Castendyk State University of New York,...
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Transcript of Seismic Refraction Exercise for a Hydrogeology Course Devin Castendyk State University of New York,...
Seismic Refraction Exercise Seismic Refraction Exercise for a Hydrogeology Coursefor a Hydrogeology Course
Devin CastendykDevin Castendyk
State University of New York,State University of New York,
College at OneontaCollege at Oneonta
Basic hydrogeologic questionsBasic hydrogeologic questions
How deep is the water table?How deep is the water table? What is the stratigraphy below a site?What is the stratigraphy below a site?
Traditional solution:Traditional solution:Drill several boreholes & install wells:
Expensive ($500-$2000 per well)Labor IntensiveTime
If site is contaminated:Disposal of contaminated coreClean equipment
What students need to knowWhat students need to know What are seismic waves?What are seismic waves? Useful property: Waves travel at different velocitiesUseful property: Waves travel at different velocities
• True of different phases (e.g. solid, liquid, and gas)True of different phases (e.g. solid, liquid, and gas)
• True of different Earth materials: True of different Earth materials: Dry soil (vadose zone)Dry soil (vadose zone) ~ 1000 ft/sec ~ 1000 ft/sec Wet soil (phreatic zone)Wet soil (phreatic zone) ~ 5000 ft/sec~ 5000 ft/sec Shale = 7000-9000 ft/secShale = 7000-9000 ft/sec Sandstone = 8000-12,000 ft/secSandstone = 8000-12,000 ft/sec Granite > 17,000 ft/secGranite > 17,000 ft/sec
If we can measure the velocity of seismic waves versus If we can measure the velocity of seismic waves versus depth, we can define the water table and infer the depth, we can define the water table and infer the stratigraphy at depth without intrusive methods. stratigraphy at depth without intrusive methods.
Objective:Objective:
Depth Velocity Interpretation
0-10 ft 1000 ft/sec
10-20 ft 5000 ft/sec
> 20 ft 12,000 ft/sec
Unsaturated Soil
Saturated Soil
Bedrock
Water Table
How to measure the composition of How to measure the composition of a 1-layer, homogeneous sitea 1-layer, homogeneous site
Place a geophone (mini-seismometer) at a Place a geophone (mini-seismometer) at a known known distancedistance from a seismic source from a seismic source
Generate a seismic wave Generate a seismic wave Measure the Measure the timetime it takes the P-Wave to it takes the P-Wave to
move from the source to the geophonemove from the source to the geophone Calculate and interpret the Calculate and interpret the velocityvelocity: :
How to address 2-layered site How to address 2-layered site using seismic refractionusing seismic refraction
Step 1:Step 1:
Use multiple geophones (an array) Use multiple geophones (an array) typically spaced equal distances apart typically spaced equal distances apart (e.g. 10 feet)(e.g. 10 feet)
Step 2: Step 2:
Generate a seismic waveGenerate a seismic wave Interpret resultsInterpret results
Set up the geophone arraySet up the geophone array
Geophone
““Bison Digital Instantaneous Floating Bison Digital Instantaneous Floating Point Signal Stacking Seismograph”Point Signal Stacking Seismograph”
Make an artificial seismic waveMake an artificial seismic wave
Borehole explosionBorehole explosion
Thumper truckThumper truck
Shotgun blastShotgun blast
Sledge hammerSledge hammer
Sledge hammerSledge hammer
How seismic waves refractHow seismic waves refract Time 1:Time 1:
Direct wave travels in all directions from sourceDirect wave travels in all directions from source Vibrations from Vibrations from Layer 1Layer 1 arrive at closest geophone (1) arrive at closest geophone (1)
1 2 3 4 6 7 8 1095
Soil
Bedrock
How seismic waves refractHow seismic waves refract Time 2:Time 2:
Wave front reaches layer boundary Wave front reaches layer boundary Vibrations from Vibrations from Layer 1Layer 1 recorded by next closest geophone (2) recorded by next closest geophone (2)
1 2 3 4 6 7 8 1095
Soil
Bedrock
How seismic waves refractHow seismic waves refract Time 3:Time 3:
Wave front travels Wave front travels fasterfaster through through Layer 2Layer 2 (bedrock) than (bedrock) than Layer 1Layer 1 (soil). (soil). This causes the interface between the layers to vibrate in advance of This causes the interface between the layers to vibrate in advance of
the wave front in Layer 1 (seismic refraction).the wave front in Layer 1 (seismic refraction). Waves generated by the interface travel back to surface. Waves generated by the interface travel back to surface.
1 2 3 4 6 7 8 1095
Soil
Bedrock
Refracted Wave
How seismic waves refractHow seismic waves refract Time 4:Time 4:
Refracted waves reach geophones ahead of direct Refracted waves reach geophones ahead of direct waves from source. waves from source.
1 2 3 4 6 7 8 1095
Soil
Bedrock
Refracted Wave
Raw data Raw data Seismic Refraction, SUNY Oneonta, May 2006Seismic Refraction, SUNY Oneonta, May 2006
Geo
phon
e N
umbe
r
P-Wave Arrival Time (milliseconds)
Data analysisData analysis
Step 1: Step 1:
Pick P-Wave arrival times (first deviation)Pick P-Wave arrival times (first deviation)
Step 2: Step 2:
Graph arrival time versus distanceGraph arrival time versus distance
Pit P-wave arrival times Pit P-wave arrival times Seismic Refraction, SUNY Oneonta, May 2006Seismic Refraction, SUNY Oneonta, May 2006
Geo
phon
e N
umbe
r
P-Wave Arrival Time (milliseconds)
Make a data tableMake a data tableDistance from Source Arrival Time
(Feet) (Milliseconds)
10 9
20 15.5
30 18
40 20
50 22.5
60 23
70 23
80 25.5
90 26.5
100 26
110 27
120 28.5
Refraction Results SUNY Oneonta
0
5
10
15
20
25
30
0 20 40 60 80 100 120
Distance (feet)
P-W
av
e A
rriv
al T
ime
(m
illis
ec
on
ds
)
P-wave velocity calculationsP-wave velocity calculations
Step 3:Step 3:
Connect points Connect points corresponding to the corresponding to the same velocity same velocity
Step 4:Step 4:
Calculate the velocity Calculate the velocity represented by each represented by each line line
12
12
tt
ddv
Refraction Results SUNY Oneonta
0
5
10
15
20
25
30
0 20 40 60 80 100 120
Distance (feet)
P-W
av
e A
rriv
al T
ime
(m
illis
ec
on
ds
)
v1=1000 ft/secv2=5000 ft/sec
v3=12,000 ft/sec
Depth of boundary between Depth of boundary between Layer 1 and Layer 2Layer 1 and Layer 2
Step 5:Step 5:Calculate the depth of the Calculate the depth of the
interface between Layer 1 and interface between Layer 1 and Layer 2.Layer 2.
The depth (The depth (zz11) from the surface to ) from the surface to the first interface is calculated the first interface is calculated using the following equation:using the following equation:
Where Where XcXc11 is the distance to the is the distance to the first intersecting velocity lines. first intersecting velocity lines. This represents the distance This represents the distance from the source where the from the source where the direct wave and the refracted direct wave and the refracted wave arrive at the same time.wave arrive at the same time.
12
1211 2 vv
vvXcz
Refraction Results SUNY Oneonta
0
5
10
15
20
25
30
0 20 40 60 80 100 120
Distance (feet)
P-W
av
e A
rriv
al T
ime
(m
illis
ec
on
ds
)
v1=1000 ft/secv2=5000 ft/sec
v3=12,000 ft/sec
Xc = 20 feet
Depth of boundary between Depth of boundary between Layer 2 and Layer 3Layer 2 and Layer 3
Step 6:Step 6:Calculate the depth of the Calculate the depth of the
interface between Layer 2 interface between Layer 2 and Layer 3.and Layer 3.
The depth (The depth (zz22) from the ) from the
surface to the first interface surface to the first interface is calculated using the is calculated using the following equation:following equation:
Where Where XcXc22 is the distance to is the distance to
the first intersecting the first intersecting velocity lines.velocity lines.
23
23212 2
8.0~vv
vvXczz
Refraction Results SUNY Oneonta
0
5
10
15
20
25
30
0 20 40 60 80 100 120
Distance (feet)
P-W
av
e A
rriv
al T
ime
(m
illis
ec
on
ds
)
v1=1000 ft/secv2=5000 ft/sec
v3=12,000 ft/sec
Xc = 20 feet
Xc2 = 50 feet
Final interpretation of Final interpretation of stratigraphic columnstratigraphic column
Depth Velocity Interpretation
0-10 ft 1000 ft/sec
10-20 ft 5000 ft/sec
> 20 ft 12,000 ft/sec
Unsaturated Soil
Saturated Soil
Limestone
Water Table
ConclusionConclusion
Seismic refraction is a useful tool in Seismic refraction is a useful tool in hydrologic investigations:hydrologic investigations: Identifies stratigraphyIdentifies stratigraphy Identifies the depth to water tableIdentifies the depth to water table Non-intrusive: No contaminated soil to Non-intrusive: No contaminated soil to
dispose of or equipment to cleandispose of or equipment to clean Inexpensive and time saving compared to Inexpensive and time saving compared to
borehole drillingborehole drilling IT’S FUN!IT’S FUN!