Lab 3: Hammer Seismic Refraction Survey
Landon Mutch
Robin Connelly
Introduction
Objective:
To conduct and learn how to do a hammer – seismic refraction survey at the University of
Victoria in order to determine the structure of the ground below the surface. Also, to constrain how
many layers there from- 20 – 20 m of depth, the speed of the waves at each layer, to determine if the
layers are dipping or not, and to interpret first arrival tomes for direct waves and head waves.
Background/Theory:
Each swing of the hammer and impact with the plate is called a shot. There are 12 geophones
set up along a straight line measured at even intervals which act as sensors to detect P-wave energy
(Hutchinson, Leonard, 2015). The sensors send back information about the waves to the seismogram
which records the first arrival times at each geophone (Hutchinson, Leonard, 2015). Forward and
Reverse surveys are conducted in order to determine if the layers that the waves are propagating
through are dipping, if so there will be a change in timing for the waves to reach the sensor from one
survey to the other. If the layers are perfectly horizontal then there should be very little change in arrival
times between them. The wave data is recorded on carbon paper and from the images, first arrivals at
each sensor can be picked and with distance between the source and each geophone, linear regression
can be used to create a travel time graph (Hutchinson, Leonard, 2015). Each linear regression should fall
within the error calculate using the equations below and the data collected (Hutchinson, Leonard, 2015).
From the graph, slopes can be used to calculate the velocities of the direct wave and the head waves.
Once the velocities are calculated the thickness and dips (if any) of each layer can be constrained. The
velocities vary depending on lithology of the material below ground and from the velocities the likely
geology can also be determined (Lucinda’s Course Slides).
𝑓𝑖𝑡
𝑁
(𝑑𝑖𝑜𝑏𝑠 − 𝑑𝑖 )2
1 𝛴𝑖=1
𝜎= √
𝑁
𝑁 − 2 𝛴𝑖=1
(𝑥𝑖 − x̅ )2
% 𝑒𝑟𝑟𝑜𝑟 =
Experimental Techniques
Equipment:


Seismograph: Geometrics SmartSeis
12 geophones
𝜎
× 100
𝑚







Geophone cable on spool
Charged 12V battery
Battery cable
Steel plate
Sledgehammer with attached cable
Shovel
Tape measure
Experimental Setting:
The experiment took place on the University of Victoria property at the corner of Cedar Hill
Cross Rd. and Gordon Head Rd. (Figure 1). The area was grassy with some rock outcrops, bushes and
trees along the inside edge of the property and the experiment took place along a fairly flat walking
path, worn down to dirt. Along the path the earth was compacted down as was discovered when digging
holes for the steel plate. It was a sunny day with no wind.
Figure 1: Map of Survey Location Boxed in red.
Methods and Procedures:
Upon arrival sensor path was chosen along walking trail and the measuring tape was laid out.
The group decided on a geophone spacing of half a meter for the first round of surveys. Each of the 12
geophones, were spiked into the ground and their cords were connected to the geophone cable. This
cable was carefully laid out with each power source along the cable lined up with the appropriate
sensor.
A groove was dug out at a half meter from the first geophone for the input plate to be wedged
down into the ground so that it would not bounce around during hammer blows. This ensured the safety
of the individual swinging the hammer and that there was no interference from the plate movement
with the seismograph.
The hammer power cord was connected to the geometrics seismograph and the instrument was
configured for files name, geophone spacing, and shot offset. Each survey set was given a file name of
480-5 to 480-9 and there was no offset. The geophones were tested to ensure they were all connected
and properly working by having someone walk along the survey line and observing the seismograph
screen for recorded waves at each sensor.
Forward Survey: Once everything was hooked up and working the first set of data collected was
for the forward survey. The sledge hammer was swung down to hit the plate with significant force. This
was done several times until the waves recorded on the seismograph showed less and less alteration
from the initial display. The wave display for each shot and data recorded was stacked in order to
improve the signal to noise ratio of the survey. Once the first survey was complete the wave graph was
printed off and labeled forward.
Reverse Survey: At this point the shot plate, sledge hammer and seismograph (with the hammer
unplugged), was moved to the other end of the survey a half meter from the 12th geophone. The
seismograph was reconfigured and the same procedure was followed as for the Forward survey
measurements.
One more forward survey was conducted and one more reverse survey with a change in spacing
for the geophones to 2 m apart. The shot plate was placed 2 m behind the first sensor for the forward
survey and 2 m ahead of the 12th sensor for the reverse survey. Each of the four surveys run, it took
approximately 4-8 shots to satisfy the collection of data. Once all the surveys were completed the
equipment was packed away and any holes that were dug up were filled and packed back in to ensure
pedestrians using the trail later did not get injured.
Experimental Results
Data:
Table 1: Data obtained during the survey. Wave set #: 1-1 is first survey in the forward direction and first
Layer, 2-2 is second survey in the reverse direction and the second layer. 3-2 is the third survey in the
forward direction and the second layer, while 4-1 is the fourth survey in the reverse direction and the
first layer etc.
Survey 1 Forward
Survey 2 Reverse
Survey 3 Forward
Survey 4 Reverse
Location
1st
Wave set Geophone
Location
1st
Wave set Geophone Location
1st
Wave set Geophone Location
1st
Wave set
(m)
arrival
#
#
(m)
arrival
#
#
(m)
arrival
#
#
(m)
arrival
#
0.0
0
0.0
0
Source
0.0
0
0.0
0
0.5
4 1-1
1
0.5
18 2-2
1
2.0
6 3-1
1
2.0
24 4-2
1.0
7.5 1-1
2
1.0
17.5 2-2
2
4.0
12 3-1
2
4.0
23 4-2
1.5
11 1-1
3
1.5
17 2-2
3
6.0
17 3-1
3
6.0
22 4-2
2.0
11.5 1-2
4
2.0
16 2-2
4
8.0
17.5 3-2
4
8.0
21 4-2
2.5
13 1-2
5
2.5
15 2-2
5
10.0
18 3-2
5
10.0
20.5 4-2
3.0
13.5 1-2
6
3.0
14.5 2-2
6
12.0
18.5 3-2
6
12.0
20 4-2
3.5
14 1-2
7
3.5
14 2-2
7
14.0
19.5 3-2
7
14.0
19 4-2
4.0
14.5 1-2
8
4.0
13 2-2
8
16.0
20.5 3-2
8
16.0
18 4-2
4.5
14.75 1-2
9
4.5
12.5 2-2
9
18.0
21 3-2
9
18.0
17 4-2
5.0
15 1-2
10
5.0
11 2-1
10
20.0
21.5 3-2
10
20.0
14 4-1
5.5
15.5 1-2
11
5.5
7.5 2-1
11
22.0
22 3-2
11
22.0
10 4-1
6.0
16 1-2
12
6.0
4 2-1
12
24.0
22.5 3-2
12
24.0
4.5 4-1
6.5
0
Source
6.5
0
26.0
Source
26.0
0
Geophone
#
Source
1
2
3
4
5
6
7
8
9
10
11
12
Graph 1: A travel time plot for the forward and reverse surveys with geophones at a half meter spacing.
This graph shows upslope (tu) and downslope (td) first arrival times for the waves.
Surveys 1 and 2: two layer model
50
45
45
40
40
y = -7.3x + 47.6
35
35
y = 7.4286x
30
30
25
25
y = -1.4333x + 18.861
20
y = 0.975x + 10.294
20
15
15
10
10
5
5
0
0
0.0
1.0
2.0
3.0
4.0
5.0
Location (m)
WavSet_1-1
WavSet_1-2
WavSet_2-1
WavSet_2-2
6.0
First arrival time (tu) (ms)
First arrival time (td) (ms)
50
Table 2: Calculated values obtained from interpretation of Graph 1 travel time plot
Wave set
1-1
1-2
2-1
2-2
Eq. of line
y = 7.4286x
y = 0.975x + 10.294
y = -7.3x + 47.6
y = -1.4333x + 18.861
Slope
(ms/m)
7.4286
0.975
7.3
1.4333
Velocity Left y-int Right y-int Layer Vel
θ1c
(m/ms)
(td)
(tu)
(m/s)
(rad)
0.134615
0
48.2859
134.6 0.164344
1.025641
10.294
16.6315
1025.6
0.136986
47.6
0
137.0
0.697691
18.861
9.31645
697.7
1-2 > 2-2 1-2 (tu) < 2-2 (td)
136 Ave V1
α
Depth h
(rad)
(m)
0.031549
0.64
σ
%error
S.D.
0.19
2.6
0.37
38
0.03
0.4
0.18
13
0.71
V2
(m/s)
830
Graph 2: A travel time plot for the forward and reverse surveys with geophones at 2 meter spacing. This
graph shows upslope (tu) and downslope (td) first arrival times for the waves.
80
80
70
70
y = 2.8929x
y = -2.375x + 61.75
60
60
50
50
40
40
y = -0.4167x + 24.667
30
30
y = 0.3292x + 14.844
20
20
10
10
0
First arrival time (tu) (ms)
First arrival time (td) (ms)
Surveys 3 and 4: two layer model
0
0.0
5.0
10.0
15.0
20.0
25.0
Location (m)
WavSet_3-1
WavSet_3-2
WavSet_4-1
WavSet_4-2
Table 3: Calculated values obtained from interpretation of Graph 2 travel time plot
Wave set
3-1
3-2
4-1
4-2
Eq. of line
Slope
(ms/m)
y = 2.8929x
2.8929
y = 0.3292x + 14.844
0.3292
y = -2.375x + 61.75
2.375
y = -0.4167x + 24.667
0.4167
Velocity Left y-int Right y-int Layer Vel
θ1c
α
Depth h
(m/ms)
(td)
(tu)
(m/s)
(rad)
(rad)
(m)
0.345674
0
75.2154
345.7 0.143488 0.0169471
3.037667
14.844
23.4032
3037.7
2.68
0.421053
61.75
0
421.1
2.399808
24.667
13.8328
2399.8
2.88
3-2 > 4-2 3-2 (td) > 4-2 (tu)
383 Ave V1
σ
%error
S.D.
0.39
13.4
0.23
71
0.43
18.2
0.22
52
V3
(m/s)
2681
Figure 2: Model of sub surface layers showing velocities, thicknesses and geology
Calculations:
Velocity (V):
Once the travel time plots were graphed the equations of the lines were calculated in excel in order to
obtain the slopes of the lines. Using the slopes the velocities for the first layer were calculated for both
forward and reverse surveys and then the average was taken and converted to m/s. Snell’s Law was
used to calculate V2 and V3. (See Tables 2 and 3 for values)
1
𝑆𝑙𝑜𝑝𝑒
𝑉2 =
𝑉1
𝑆𝑖𝑛𝜃1𝑐
1
7.3
𝑉1 = ( ) 1000 = 137.0 𝑚/𝑠
Sample Calculations:
𝑉1𝑎𝑣𝑒 =
137.0𝑚/𝑠 + 134.6𝑚/𝑠
= 136 𝑚/𝑠
2
𝑉2 =
Thickness (h):
𝑉1 =
136 𝑚/𝑠
= 830𝑚/𝑠
𝑆𝑖𝑛(0.164344𝑟𝑎𝑑)
The thickness for the top layer was calculated from the velocity of layer one, the first arrival time for
both, upslope and downslope, the first critical angle and the dip angle. The same was applied for the
second layer along with subtracting the first layer thicknesses from the results. (See Tables 2 and 3 for
values)
ℎ1𝑢/𝑑 =
ℎ2𝑢/𝑑 =
𝑡𝑢/𝑑 𝑉1
1
∙
2𝑐𝑜𝑠𝜃1𝑐 𝑐𝑜𝑠𝛼 1000
𝑡𝑢/𝑑 𝑉1
1
∙
− ℎ1𝑢/𝑑
2𝑐𝑜𝑠𝜃1𝑐 𝑐𝑜𝑠𝛼 1000
Sample Calculations:
ℎ1𝑢 =
ℎ2𝑑 =
9.32𝑠 × 136𝑚/𝑠
1
∙
= 0.64𝑚
2cos(0.164)cos(0.032) 1000
14.844𝑠 × 383𝑚/𝑠
1
∙
− 0.71𝑚 = 2.17𝑚
2cos(0.143)cos(0.0169) 1000
Dip (α): (See Tables 2 and 3 for values)
𝑉1
𝑉1
𝛼 = 0.5 [𝐴𝑟𝑐𝑠𝑖𝑛 𝑉2𝑓 − 𝐴𝑟𝑐𝑠𝑖𝑛 𝑉2𝑟]
Sample Calculation:
𝛼 = 0.5 [𝐴𝑟𝑐𝑠𝑖𝑛
136𝑚/𝑠
136𝑚/𝑠
180
− 𝐴𝑟𝑐𝑠𝑖𝑛
= 1.8 𝑑𝑒𝑔𝑟𝑒𝑒𝑠
]×
697.7𝑚/𝑠
1025.6𝑚/𝑠
𝜋
Error:
𝑓𝑖𝑡
𝑁
(𝑑𝑖𝑜𝑏𝑠 − 𝑑𝑖 )2
1 𝛴𝑖=1
𝜎= √
𝑁
𝑁 − 2 𝛴𝑖=1
(𝑥𝑖 − x̅ )2
% 𝑒𝑟𝑟𝑜𝑟 =
𝜎
× 100
𝑚
Results
The results of the 2 sets of forward and reverse surveys at 2 m and 0.5 m show that the site has
3 layers and an approximate dip of 1.8 degrees for the survey conducted at 0.5 m spacing and 1 degree
for the 2 meter spacing. The calculated velocities, thicknesses, dips and likely geology for each layer are
as follows: Layer 1 has an approximate velocity of 136 m/s ± 1.5%, a thickness of 0.64 m to 0.71 m, and
is most likely soft top soil. Layer 2 has a velocity of approximately 830 m/s ± 25.5%, a thickness of 2.04 m
to 2.17 m and the geology is most likely dense soil to soft rock. Layer 3 has a velocity is calculated to be
around 2680 m/s error and from the velocity could be sandstone or limestone.
Discussion
The percent error calculated was quite variable for this survey. The geophones are very sensitive
and therefor sources of error could be from the noise created by people or animal moving around
anywhere near or at a distance from the geophones, as well as traffic diving on the nearby road and air
or wind. The noise is generally at a lower frequency which can disrupt the desired higher frequencies
from being recorded (Burger, Jones 2006) and can make deciphering which waves are the first arrivals
and which are not difficult. Though stacking the trials along with filters designed to reduce the noise by
minimizing certain frequencies (Burger, Jones 2006), they do not eliminate the noise completely.
However the errors caused by these were significantly smaller than the error in picking the first arrivals.
Other sources of error could be due to not leaving enough time before each swing of the hammer for
some of the trials as well as the hammer double hitting the plate with some of the swings. This could
cause overlap in wave fronts or extra noise. Before and after the first arrivals tiny waves are noted due
to air waves triggering the sensitive geophones. One more important factor in reducing error is to
ensure proper ground coupling for the geophones (Burger, Jones 2006). This have led to a source of
error if the geophones were not properly stuck into the ground as far as they could go or moved around
during insertion creating gaps around the spike. Before the survey very low amplitude waves can be
seen which are air waves picked up by the sensitive geophones. Other waveforms that can be seen on
the profile are reflection waveforms and ground roll. From the profile it can be seen that with time
frequency is decreasing.
Conclusion
This exercise used a seismic refraction survey in order to determine the geology, velocity,
thickness and dip angle of layers below the surface. Sensors penetrated into the earth were able to pick
up the waves created by the swing of a hammer connected to a seismogram and send back arrival times
to the seismogram which printed out a profile of the wave propagation. From this profile first arrivals
were picked and plotted on a travel time graph which was used to calculate the velocities in each layer.
With the velocities the dip and thickness could be calculated.
References:
Hutchinson, J., and Leonard, L., 2015, EOS 480 Applied Geophysics Lab Manual: Hammer Seismic
Refraction Survey, University of Victoria, p 1-5.
Burger, Jones and Sheehan, 2006, Introduction to Applied Geophysics, W.W Norton & Company, New
York
ftp://geom.geometrics.com/pub/seismic/Manuals/SmartSeisVersionC.pdf