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Evaluation of the Ground Penetrating Radar
Adapted from report written by an Earthwatch fellow Peter Engbers, October 2008, EarthWatch project, Curchill Northern Studies Centre
1. Principles of Ground Penetrating Radar (adapted from web info)
A GPR system generates electromagnetic signals and detects the electromagnetic field interaction with the surrounding material. Ground Penetrating Radar utilises an antenna (comprising a transmitter and receiver a small fixed distance apart) to send electromagnetic waves into the subsurface. The antenna is moved over the surface of the medium to be inspected. The transmitter sends a diverging beam of energy pulses into the subsurface and the receiver collects the energy reflected from interfaces between materials of differing electro-magnetic properties. The reflected energy is recorded as a "pattern" on radargrams which are displayed in real time. The radargrams, which look similar to seismic sections, constitute the raw Ground Penetrating Radar data.
The ground penetrating radar (GPR) methodology is a very high frequency Electro Magnetic wave technique used to produce high-resolution images from the subsurface. GPR is for example used for characterizing the subsurface stratigraphy, water-table, permafrost depth, and/or geology. When used at the same site, but surveyed over time, GPR is also an effective methodology for monitoring changes in the sub-surface. EM energy from the antenna can propagate at frequencies ranging from 10 MHz to 3 GHZ. The peak power of this antenna is 20 to 100 times less the wattage of a cellular phone, and the energy is directed into the ground (and not at the operator) by means of shielding on the top side of the antenna.
Fig. 1 The process of constructing a GPR profile: 1. transmit and receive electromagnetic energy, 2. the received energy is recorded as a trace at a point on the surface, 3. traces are arranged side-byside to produce a cross section of the earth recorded as the antennas are pulled along the surface. Traces are displayed as either wiggle trace, or scan plots (gray scale or color assigned to specific amplitudes).
The GPR signal is reflected back to the antenna by materials with contrasting electrical impedance, which is primarily determined by dielectric and conductivity properties of the material, its magnetic permeability, and its physical properties.
A material's dielectric properties are primarily determined by mineralogy, and water content. The greater the contrast in the real dielectric permittivity (RDP) of two materials, the greater the reflection amplitude. Typically, high-amplitude reflections occur at lithologic or mineralogic changes, or where there is a sudden change in water content (water-table or depth of ice).
Fig. 2 Dielectric properties of various material
Relevant for our area here in Churchill, we expect ground level reflection (air to ground) to be positioned at depth=0m and to appear as a "hard" kick, either a positive or negative loop depending on the signal polarity and polarity convention. Then the permafrost loop is probably a "soft" kick (if we assume water on ice) according the relative permittivities (so should have opposite polarity than the ground level reflection.
Reflections observed on GPR records are non-unique, meaning that a similar reflector can be caused by many different objects or combinations of layers.
GPR is measured as function of time. To relate to the depth below surface, a velocity estimate is required. The velocity is different between materials with different electrical properties, and a signal passed through two materials with different electrical properties over the same distance will arrive at different times. The interval of time that it takes for the wave to travel from the transmit antenna to the receive antenna is simply called the travel time. The basic unit of electromagnetic wave travel time is the nanosecond (ns), where 1 ns = 10-9 s. The velocity of an electromagnetic wave in air is 3x108 m/s (0.3 m/ns). Air is the fastest medium. The travel time of a wave in a material other than air is always smaller than 3x108 m/s (0.3 m/ns). For Time to Depth conversion, an average velocity of 0.1 m/ns is taken for our Churchill environment. Note that in reality, within a vertical section, the velocities can vary considerably (water: 0.033m/ ns, sand/clays: 0.06-0.09 m/ns, ice: 0.16 m/ns) leading to extreme variations in depth sections per time unit (Time to Depth conversion) if we vary from water saturated rock to ice. This means that the time position and loop character of a interface reflection at say 3m will vary dramatically (time scale can vary factor 10) depending the overburden to be air, water, or ice filled.
2. GPR Acquisition (text by Garry Oughtred)
Peter Kershaw briefed us on the first day at 7.45am in the large unheated laboratory. Here we were introduced to the GPR (Ground Penetrating Radar). We all knew we would grow to love this instrument over the next 11 days. We spent about an hour in a detailed lecture on the systematic setup and operation of this very data acquisition tool.
We got 3 minutes to get ready to depart for a first survey on a Tundra site, a long drive over very rough potholed roads. We saw a mother bear and her cub on our drive, which was good as we could all tick this one off our list now. We set up two 50m tapes perpendicular to each other and sampled the permafrost underground every 25cm. The instrument comprises all plastic construction with a transmitter mounted on a heavy plastic ski approximately 1m long with tall plastic handles for holding the instrument in position. This is powered by two 6 volt motor bike size batteries. The detector is identical in construction and is operated with a separation of exactly 1m with both tracks held parallel. The transmitter and detector are connected together by optical fibre, which is then connected to a signal processor which must be kept more than 5m away from the measuring instruments to reduce electrical "noise". The processor is powered by a 12 volt bike battery and is connected to a HP Palm to store the data. Operators with steel toed boots are excused from taking their turn in operating the GPR. The unit was set to send pulses of radar at 100MHz which relates to target depth of 5 m (maximum depth of 15m). Every 25cm the heavy unit is lifted and moved for a new reading which is recorded on a second data logger. When the readings are taken near to a source of steel such as a leg of an environmental monitoring station or other steel pegs that cannot be removed this must be manually noted in the data logger. A slow tedious process but one that should provide relevant data on depth of permafrost that can be periodically compared.
3. GPR Processing
GPR data from the field is processed using the pulseEKKO program (Sensors & Software Inc) provided with the GPR tools (a bit outdated, from 1996, but still functioning appropriately). Key processing steps include input data selection, topographical correction, scaling, display parameter selection, and plot file creation. Some processing issues and tests are described below.
Drift correction
Various sections had to be drift corrected (Timezero Adjust) as the first reflection was at times below 2 m depth, was drifting away (changing from left to right), or appeared to have opposite polarity. Timezero Adjust shifts the tracks so that the first reflection (assumed to be ground level) is set a time zero (according to manual, although it looks closer to Depth =0). I tested and found the most stable correction using a threshold of +2000.
Polarity
Polarity of produced GPR sections is not clear. Is a positive number (black loop) a "hard" kick (increase of permittivity) or a "soft" kick (decrease of permittivity)? At several instances, zero depth, which should in principle represent ground level (air to ground interface should be a "hard" kick) appears as a black loop. Sometimes it is a white loop. Note that while recording, polarity depends on system set-up, as the electric unit orientation controls the recording polarity (always have pulseEKKO name on the electronic units pointed in the direction of antenna movement).
Scaling
Appropriate Amplitude Scaling needs to be selected to see the relevant features. With no scaling, it appears that typically below 50-80 ns (3-4 m), the formation is immediately very transparent. Automatic Gain Control (AGC) which scales amplitudes to common level (within specified window) allows that a weak signal (low amplitude) can still be "seen". Applying AGC here however shows that the data below 60-100 ns (3-4 m) is not coherent and thus not meaningful (compare fig. 4.
Fig. 3. Example of gain function. Data between 25 and 100 ns has high amplitudes.
The pictures on the following page show scaling tests on GPR line BFR0810. First figure shows no scaling, data as recorded, second figure shows Automatic Gain Control - AGC scaling applied, and three figures below show Spreading & Exponential Compensation - SEC scaling applied with three different attenuation factors. The SEC scaling with attenuation factor 0.1 dB/m (as applicable for water saturated formation) seems the most appropriate and reasonable setting.
Fig. 4. Comparison of various scaling techniques on line BFR0810
Final Processing parameters
Gains ¦ SEC Gain Max: 500 Atten: 0.100 dB/m Start Value: 1.000
Filters ¦ Trace Ave: 1 Point Ave: 1 Trace Diff: OFF Correction: DEWOW
Selectn ¦ Time : 0 to 360 ns Position Range: Full Skip Trc: 0
Velocity¦ Depth Axis: 0.100 m/ns Elev Axis: 0.100 m/ns
Options ¦ Depth Ax: ON Comment : ON ID Page : ON Reverse: OFF
¦ TIMEZERO DRIFT CORRECTED. THRESHOLD = 2000 (only files with extention T0)
All recorded sections of October 2008 have been processed according to this protocol. Also the October 2006 data has been reprocessed following the same workflow in order to attempt to compare them.
4. Further steps and interpretation
1. Calibration and validation - ground truthing
The GPR section shows loops that represent reflections at interfaces of various lithologies (with relative Permittivity or EM "hardness" variation). Loops could represent many interfaces such as sand/peat, air/water, or water/ice.
Calibration and validation with "hard" measurements such as soil coring or soil profiling is required to answer which interface is the interface with ice (top Permafrost). The least would be to use a permafrost probe. It is suggested to do such calibration and validation at various points of the GPR survey simultaneous with the acquisition (e.g. each 5 m, depending on epected variability).
2. Interpretation
After calibration, once it is clear which loop represents the depth of permafrost, the GPR section can be interpreted for depth top permafrost all along the profile.
Fig. Depth of permafrost from probes overlaid on PPA0810 section (Timezero Adjust - T0 drift corrected)
As an example, the various figures shows sections (if needed after drift correction) where we did simultaneously permafrost probing (see right picture). Superimposing the permafrost profile on the GPR sections shows coincidence of the first negative loop with the top permafrost. But would this be the right polarity? The ground level reflection (air to ground) at depth=0m appears as a black loop (assume "hard" kick). Then the interpreted permafrost loop (negative=white) is a "soft" kick (assuming water on ice) which is consistent with the relative permittivities.
Fig. Depth of permafrost from probes overlaid on PPD0810R section
Fig. Depth of permafrost from probes overlaid on PPA0810R section
Fig. Depth of permafrost from probes overlaid on AIR0810R section
3. Repeatability analysis
A repeatability test would analyse how good we can repeat a survey (without any changes in the subsurface) and thus what differences are attributable to the repeatability of the technique (tool warming, tool settings, air temperature, operators, acquisition protocol, etc.) and what to variations in the subsurface. A simple repeatability test could be done by coming back the next day or week, and have different operators repeating a few meters of the survey.
4. Timelapse comparison
Finally, timelapse analysis and interpretation would compare the same sections shot in different seasons or years and analyse the differences in terms of changes of the permafrost depth. Under favorable conditions, visual comparison could be sufficient to interpret change of permafrost depth. Shifting the timelapse sections to common datum (Timezero Adjust) is essential. In more complex cases, more detailed timelapse analysis and processing may be required looking at time shifts and amplitude differences independently. As an example, below figures compare the PPA sections for 2006 and 2008.
Fig. PPA section 2006 with Timezero Adjust (PPA0610T0_SEC)
Fig. PPA section 2008 with Timezero Adjust (PPA0810T0_SEC)
At first glance, it may look comparable, but time scale and frequency wise, it is quite different. A change of water table as well as frost depth (causing considerable velocity changes) could possibly explain such differences. Although we interpret in 2008 the first negative loop (white) as top permafrost (as calibrated by 2008 permafrost probing), it is not clear how to interpret the 2006 section. It may be that the first white loop is also the top permafrost, but that is not certain as character and shape is quite different. Permafrost probing in 2006 would have been needed to validate this.
A series of timelapse plots comparing October 2008 with October 2006 are made for most surveys.
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