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!