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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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