Seismic refraction surveying

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					Seismic refraction surveying
The alternative equipment developed for seismic refraction surveying was not able to
be used to carry out a seismic refraction survey successfully. The main problem with
the developed equipment was the signal from the seismic source circuit, this was not
clear enough to pick accurately the instant when the source was generated, and
contaminated the signal from the geophone. The design and construction of the
alternative equipment produced is found below but to make the equipment suitable for
a seismic refraction survey, modifications need to be made. The basic idea to reduce
the costs of the equipment were to make a geophone modifying a gramophone pick-up
to measure the ground movement and record all the data using a laptop computer.

Theory

Seismic refraction surveying measures the first arrivals of seismic energy from a
seismic source. The exact time the source is produced and when the energy reaches
the receiver need to be determined to analyse the first arrival travel times. The first
arrival of seismic energy is always the direct wave or the refracted wave, the direct
are refracted waves are shown below.




                 Diagram of direct and refracted waves in a two layer medium

The direct wave travels from A to D at the slower velocity V1, and the refracted wave
travels from A to D via B and C, where it is critically refracted and travels at V2
between B and C. This means that at short AD distances the direct wave arrives first
because it has a shorter travel path, but when the separation between A and D
increases the refracted wave will arrive first because of the faster travel time between
B and C will overcome the difference in distance travelled. This means that the
velocities of the layers can be analysed and so can the depth to the interface.
Velocities are commonly calculated by plotting a travel time vs distance between
source and receiver plot, shown below.
                                Travel time vs distance plot

The velocity of the top layer can be calculated from the reciprocal of the gradient of
the direct arrivals and the velocity of the second layer can be calculated from the
reciprocal of the gradient of the refracted arrivals. The depth to the interface can be
calculated from the intercept time of the refracted arrivals and the two calculated
velocities and the equation is show below.


                                            t i v1v2
                            z                              1
                                     2(v  v )
                                            2
                                            2
                                                      2
                                                      1
                                                                2


                           Equation to calculate depth to interface

Design and construction

There are many separate aspects to the design to take into account for seismic
refraction surveying. The source of energy and how to time this, the geophone to
measure the movement of the ground, some way of recording the data and ways to
connect everything together. The basic design of the equipment setup going to be used
is shown below.
                        Separate components of the seismic design

Laptop, sound card and recording program

The laptop computer sound card is going to be used to read the data from the
gramophone pick-up and the instant he source is hit. The sound card needs to have a
stereo input so that the two signals can be kept separate, this means that the sound
card needs to have a line in socket. A program called goldwave has been used to
record the data collected. It is available to download from www.goldwave.com and
can read in the data in stereo and also has many other functions such as filters and
amplifiers that allow the data to be modified to make the first arrivals easier to
determine.

Geophone

Firstly the output from the gramophone pick-up needs to be amplified to the correct
level to be inputted into the line in socked of the sound card. This means a
preamplifier circuit needs to be constructed, to do this circuit diagram shown below.
                                                            9V




                                                                 R4
                                                                 120Ω
                                            R1


                                          4.7kΩ
                                                                         C5
                                                  2.2kΩ          R3
                                                                        100μF
                       C2       C1           R2                              C4


 Electret Input       10μF 10μF            220kΩ                           10μF
                             C3
                                                                                  Output

                            10μF
  Dynamic Input



                            Circuit diagram of preamplifier circuit

This circuit needs to be constructed, on either a push in circuit board or a circuit board
that    you     solder    to    more      information    about     the    circuit     from
http://www.epanorama.net/circuits/micamp.gif. From testing the electret input was
found to give the best results. For the gramophone pick-up to record a signal when the
ground struck to create seismic energy the needle of the pick-up needs to move
differently to the casing of the pick up. This was decided to be done using foam and a
weight shown below.
                                     Geophone design

The pick-up was super glued to the like of a small solid box, and connections made to
a 3.5mm socket (the same as on the sound card) that was also attached to the lid by
drilling a hole and tightening the nut. The output from the pick-up was wired to this
socket and a IC holder (used to attach IC to a circuit board) was used to connect to the
terminals of the pick-up because they cannot be soldered to. The foam was cut to size
and placed in the bottom of the box and the weight placed on top, making a
connection with the gramophone needle. The foam had to be cut to size to fit in the
box and make sure a connection was made between the weight and the geophone.
Indoor tests were done to find the best foam weight combination, different thicknesses
of stainless steel were used as the weights and foam was obtained from cosmetic
make up pads and also scouring sponges, but any foam could be tried and tested.

Holes in the casing of the geophone were also modified to allow tent pegs to pass
through this will increase the coupling with the ground hopefully making the signal
clearer.

Seismic Source and trigger signal

A sledge hammer was chosen to be used as the source of seismic energy and is a
common source for small scale refraction surveys. The hammer is used to strike a
solid plate on the ground. The plate is used to stop the hammer suddenly and create a
repeatable source.

The instant the plate is struck can be detected using an accelerometer switch. This is
connected to the hammer and its contacts close when there is a sudden change in
acceleration, eg when the plate is struck. This switch can be used to trigger a circuit to
supply a signal to the sound card to measure the source being created. A simple circuit
using a Zenner diode and resistor configuration was designed to produce a 250mA
signal, when the switch closes and is shown below.

         9V


                3V        R1
                                                                      To switch




                                    R2                                 Output
                6V



                                    R3

          0V

                         Circuit diagram of source signal generator

This circuit does create a signal when the source is created but it is not a very clear
instant and contaminated the geophone signal, so this circuit needs improving to make
the equipment work.

The switch is taped to the hammer in the correct direction so it makes a connection
when the plate is struck. The switch is soldered to a dual core cable with a 3.5mm
connector on the other end to allow the switch connection to be plugged in to connect
to the circuit.

Cable reel

A reel of cable needs to be constructed to connect the geophone to the computer when
it is a long distance away. This was done in the same was as for the resistivity VES
surveying cables detailed in the how to make the equipment section of the resistivity
surveying pages. A 5m extension cable reel has been modified to allow the desired
cable to be attached. For this application a dual cored cable is needed because there
are two separate parts to the signal from the geophone, a 3.5mm socket was connected
to the reel and a 3.5mm connected to the end of the wire to allow it to be plugged into
the geophone.

Junction Box

A junction box was also produced this contained all the circuitry and allowed the
hammer and the geophone to be connected, and has an output to be connected to the
computer. The input from the geophone was connected to the preamplifier and the
output from the preamplifier connected to one channel of the output 3.5mm stereo
socket. The input from the hammer switch was connected to the circuit and the output
from the source circuit connected to the other channel of the output 3.5mm stereo
socket. The sockets were labelled up to make sure they were connected up correctly
on the outside. A cable with 3.5mm stereo inputs was purchase to connect from the
junction box output to the computer.

Both the circuits in the junction box are wired to the same 9V battery power supply
and both circuits power supply controlled by a switch mounted to the front of the box
and a led to indicate when the circuits have power.

Other Extras

A connecting wire also needs to be made to connect the cable reel to the junction box.
All 3.5mm connections and sockets need to be checked carefully to make sure that
they are wired up to actually make connections and the right pins used. A multimeter
with a beep function when a circuit is made is very useful for this function. The
mulitmeter beeps when a circuit is complete so the connections can be checked, it is
best to do these checks as you go along because otherwise you will not know where
the problem is.

Testing and Problems with the Design identified

From the testing of the device some problems were found that meant a seismic
refraction survey was not able to be carried out using the alternative equipment. The
main problem was from the source circuit. It did not give a clear enough pulse when
the hammer was struck and so the exact moment the hammer was struck could not
easily identified. Also there was contamination of the geophone signal from the
source signal shown below.




Screen shot from goldwave program, crossover between channels, red is geophone signal and green is
                                         source signal.
The signal from the ground movement can still be seen in the geophone signal but
there is contamination from the source signal which means that the exact first arrival
time cannot be determined. The exact moment the hammer is struck and the signal
received at the geophone need to be recorded to allow the first arrival time to be
analysed. The very large peak in both signals does not seem to be when the hammer is
struck as there is a signal from the geophone before this that is independent from the
ground movement.

Another problem with the produced equipment was that the geophone signal was only
recorded up to a maximum distance of 10m. This is not really large enough to carry
out a seismic refraction survey so a greater separation between the source and the
geophone where the seismic energy can still be measured needs to be achieved.

Suggested modifications

To allow the geophone signal to be seen at a larger distance the amplification of the
preamplifier could be increase so even smaller movements could be measured, or the
setup of the geophone itself could be more fine tuned to pick up smaller signals trying
more weight and foam combinations.

A suggestion to improve the source signal could be to develop a circuit, that only
generates a signal after the hammer has been hit so the start can be easily identified. A
possible idea on how to get around the cross over signal is to uses a oscillator circuit
as the hammer signal, that turns on only after the hammer has been hit, this frequency
signal could then possibly be removed even if there was crossover of the signal into
the geophone circuit and the actual data recovered. These are only possible ideas of
modifications, they may or may not work, if anyone has any other ideas I would be
interested to hear them.

How to carry out an experiment

To carry out a seismic refraction survey using the designed equipment first a survey
line needs to be set out. The hammer plate placed at one end. Plug the hammer
connector into the correct socket on the junction box. Plug the output of the junction
box into the line in socket on the laptop using the stereo cable. Place the geophone at
the smallest separation from the source and secure to the ground using the tent pegs.
Connect the reel to the geophone and the reel to the geophone socket of the junction
box. Turn the laptop on and open the goldwave program and create a new file. Turn
on the junction box, checking the LED is illuminated. Now press record on the
goldwave program and hit the plate with the hammer. After the plate has been struck
press stop on the goldwave program and the data collected will have been recorded.
You may want to hit the hammer more then once and an average of the first arrival
travel times can be taken. Save the file with a name that relates to the geophone
position and then move the geophone, open a new file and repeat until data is
collected for the whole survey line.

How to analyse and interpret the data
Firstly the first arrival times need to be analysed. These can be measure if they can be
seen using the goldwave program. Make a selection that starts on the instant the
hammer was hit and finishes on the first arrival of the seismic data you will need to
zoom in to be able to see the data accurately. The goldwave program will then display
the time between the two in the bar along the bottom. More accurate timing can be
done on when the program is zoomed in even more, the cursor turns to a crosshair and
the exact time of its position is displayed in the bottom bar. The signals can be
improved by applying a combination of filters to remove background noise and also
amplified in the program to make them clearer.

Enter the first arrival times for the different separations into a spreadsheet and
calculate the average if multiple hammer blows were made. Check results for any
anomalous values and remove these. Now plot the geophone – source distance against
the first arrival travel time. It should be a plot with two straight line sections, the first
from the direct wave and the second from the refracted wave. The velocities can be
determined and also the depth to the interface as described in the Theory section.

Conclusion

The equipment produced needs modifications to allow it to successfully be applied to
carry out a seismic refraction survey. The basic principal of using a gramophone pick-
up to detect the ground movement has proved successful because seismic energy has
been detected, just the first arrival times that are needed for the refraction survey
could not be determined due to the poor quality of the source signal and the crossover
between the channels. If the source signal circuit can be improved there is a chance
that the equipment will be able to be applied to a seismic refraction survey.

				
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