GLACIAL FLOODING AND DISASTER RISK MANAGEMENT KNOWLEDGE EXCHANGE AND FIELD TRAINING July 11-24, 2013 Huaraz, Peru July 18, 2013 Day 6 - Field Methods and Modules II GPR at Pastoruri Glacier Daene McKinney, Marcelo Somos-Valenzuela, Rachel Chisolm, Katalyn Voss A portable ground penetrating radar (GPR) system is used to detect ice thickness and bedrock profile for glaciers. The system includes a transmitter that emits radar waves at a frequency of 5 MHz and a receiver that detects the signal reflected from the interface between the ice and bedrock. Radar signals are transmitted and detected continuously at regular intervals as operators walk with the GPR system across the surface of the glacier (Figure 1). A constant distance of 40 m is maintained between the transmitter and receiver. The ice thickness is determined based on the two-way travel time (t) between the transmitter and receiver assuming that the signal travels at a velocity (v) of 150 x 106 m/s, a typical wave velocity for temperate ice.
Figure 1. Backpack mounted GPR system deployed in the field at Imja Lake
Transmitter Set up For security reasons, it is important to mention that the receiver must be connected to the battery just before the start of the survey. Components of the transmitter system (Figure 2) • 1 Transmitter (4 kV Kentech Pulse generator) • 1 battery (12 volts) • 1 Antenna (5, 10, 20 or 50 MHz) • 1 cable to connect battery to transmitter The transmitter gets its power from a 12 V battery, and it is connected to a Pulse generator, which is inside the transmitter box. The Pulse generator is connected to the two arms of the antenna (See figure 2).
Figure 2. GPR transmitter (left) Receiver Set Up Receiver Components (Figure 3) • 1 computer (Panasonic Toughbook) • 1 battery (12 volts) • 1 Digitizer (National Instruments NI USB-5133) • 1 GPS receiver • 1 Amplifier • 1 Antenna (same as Transmitter antenna) • 1 cable to connect battery to receiver • 1 USB cable to connect the digitizer to the computer • 1 coaxial cable to connect the receiver to the digitizer Receiver connections 1. Connect the antenna wires into the amplifier.
Â 2. Connect the coaxial cable to the amplifier and split the channel into Ch1 and Ch0 on connection to the digitizer. 3. Connect the USB cable to the digitizer and the USB port on the laptop. 4. Plug the GPS cable into the serial port of the laptop. 5. Connect the 12V battery to the amplifier and the GPS. 6. Turn on the amplifier
Figure 3: GPR receiver General Set Up Figure 4 shows how the system is used in the field. Four people can deploy the system, but it is always recommended to have a fifth person, who checks the connections (Rover). The Transmitter is located in the front of the group, one person carries one arm of the antenna preceding the person carrying the Transmitter and battery in a backpack follows them. The second arm of the Transmitter antenna is attached to a rope, which goes from the person carrying the Transmitter to the person carrying the Receiver. The rope helps to maintain the distance between the Transmitter and the Receiver. The Receiver is located behind the Transmitter; the arm of the Receiver antenna in front of the receiver is attached to the rope. Another person carried the Receiver, battery, GPS and computer. The arm of the Receiver antenna in the back drags on the glacier. One person is needed to assist carrying the rope attached to the antenna as it is shown in the figure below. The spacing of the Transmitter and Receiver is far enough that the Receiver is triggered by the airwave with an amplitude of about 1 V. For a 5MHz antenna (red webbing) this is about 40 m. Move the Transmitter and Receiver closer together if you have trouble triggering and further apart if the airwave is too large. Be sure to take a note of the distance between Transmitter and Receiver.
Receiver w/battery, GPS, amp, digitizer, & computer
Transmitter w/battery Rope
Figure 4: General set up in the field. Software Setup: 1. The laptop has been set to not go to sleep unless the lid is closed (battery lasts about 5 hours). If it goes to sleep just touch the mouse or slide the power button to re‐activate. 2. When you turn on the laptop it will automatically recognize the digitizer connected to the digitizer port (if it’s plugged in). It should open up LabVIEW and you’ll have an option to open the LabView file named “little_scope_gps_serial.vi”, which is in the directory: C:/Radar/Pastoruri/lilscope 3. Set the number of averages to 400. It takes about 3 seconds to acquire a signal. Set the trigger to less than 1V. 4. Turn the transmitter and amplifier on, plug in the GPS and see if it works by hitting start on the LabVIEW vi program. 5. Look at the waveform and see if you can discern the bed return (100 m should be ~1 microsecond). You’ll need to zoom in quite a bit by changing the vertical range on one of the channels. You may have to open the test panel in MAX to do this. 6. Make sure that you are triggering on the upslope of the airwave (the wave should go up first and then down on the display) and if it’s reversed (down first and then up), just reverse the antennas at the Receiver. Everything else should be set appropriately when the LabVIEW vi program loads. Adjust distance between the Transmitter and Receiver position if needed to get the appropriate trigger. 7. You can use the continuous acquisition button (next to the start button) on the vi to collect files without stopping. You need to be careful that the connections don’t lose power (e.g., if the battery and the antennae become disconnected). 8. When you stop the program, hit the stop button inside the vi program (near the bottom left).
9. Profile; keep in mind that the best technique for alpine glaciers is multiple across the valley profiles rather than one down the middle. 10. Use readlilscope.m to read the data with Matlab
b)# Glacier)Surface) Ice)Thickness)=)145)m) Ice)Thickness)=)37)m)
Figure 5: a) Radar profile of Artesón Glacier; b) Annotated radar profile showing the location of the bedrock and terminus and indicating the ice thickness, Wednesday, July 18, 2012 Huaraz, Peru