"This filter is used to remove horizontal and nearly horizontal features in the radargram by subtracting a calculated mean trace from all traces. The running average version subtracts a mean trace calculated in a window centered at the trace to be filtered. The size of the window is selected by the 'Number of traces to use in filter process' edit box. The Total average method calculates the mean trace as the mean of the whole data file."
A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface. Each trace is composed of individual "samples," the smallest measurement unit in the vertical dimension.
This IDL function is called by or for the "Subtract Mean Trace" ground-penetrating radar (GPR) image-processing filter (subtract_mean_trace.pro) to collect user input. It displays a window for the user to enter a subtraction method (either running average or total average) and for a window length (in traces) to apply to the filter if running average is the selected subtraction method. A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface.
A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface. Each trace is composed of individual "samples," the smallest measurement unit in the vertical dimension. Because of geometrical "spreading," the radar signal decreases in strength with depth as 1/r2, where r is depth.
This IDL function is called by or for the "Time-Varying Gain" ground-penetrating radar (GPR) image-processing filter (time_varying_gain.pro) to collect user input. It displays a window for the user to enter the following information:
This IDL function is called by or for the "DC Removal" ground-penetrating radar (GPR) image-processing filter (dc_removal.pro) to collect user input. It displays a window for the user to enter a start sample for calculation of each trace's DC level. "DC" stands for an electrical "direct current." A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface.
Removes horizontal and nearly horizontal features within the radargram (i.e. "ringing") by subtracting a calculated mean trace from all traces. NOTE: A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface. Each trace is composed of individual "samples," the smallest measurement unit in the vertical dimension.
Applies a time-varying (i.e. depth-varying) gain to compensate for amplitude loss due to spreading and attenuation. Each radar trace is multiplied by a gain function combining linear and exponential components, with coefficients set by the user.
There is often a constant offset in the amplitude of each radar trace caused by interference from direct current (DC) used to power the GPR instrument. This filter removes the DC component from the data, which has the effect of making the data less noisy, or smoothing the data.
The time window and ground velocity terminology and usage is modeled after RAMAC GroundVision software, which similarly requires the user to enter these parameters in order to convert time to depth for the scale bars that are displayed to the right and left of the data imagery in GroundVision. The time window is the amount of time (in nanoseconds) that the radar receiver was set to "listen" for the return pulse after each radar pulse was released from the transmitting antenna during data acquisition. If the time window is set to 40 ns, for example, that means an individual radar pulse has a maximum time duration of 20 ns to reach a reflector and 20 more ns to reflect back to the receiver in order to be recorded in the data file. The bottom of the data file therefore represents a maximum duration of 20 ns for a time window of 40 ns. The ground velocity represents (in meters per nanosecond) how fast the radar pulses traveled through the subsurface medium being imaged in the data file. Ground velocities range from slow (e.g. 0.03 m/ns for fresh water) to fast (e.g. 0.3 m/ns for air) and the user should refer to a GPR textbook or manual for the proper value related to the media being imaged. For dry snow on the Greenland ice sheet, for example, an average value of 0.236 m/ns may suffice if the value has not been measured in a snow pit, based on an average dry snow density of 0.3 grams per cubic centimeter that has been empirically related to a dry snow permittivity of 1.62 by the following publication:
The user may also select the sample of the "first arrival" of the radar pulse reaching the subsurface: depth computations will start at this sample number (y-axis). Often in GPR data, there is an obvious lack of backscatter at the top of the file that results from the empty space that occurred between the antenna and the surface. The first arrival begins at the point where obvious backscatter begins. The user may also select whether or not to adjust the first arrival travel time by the "direct wave." The direct wave is the part of the transmitted energy that travels the shortest distance between the transmitter and receiver. Due to antenna separation, the wave traveling from the transmitter directly to the receiver (i.e. the direct wave) is received some time after the actual transmission. This means that the transmitted pulse has already penetrated the medium a certain distance before the direct wave is received. The result of this is that the depth scale zero must be corrected to be accurate. The zero for the depth scale is calculated using the first arrival value, the antenna separation, and the first arrival adjustment velocity. The adjustment velocity can be set to any value. Practically however, it can be the ground velocity, the air velocity (most common), or anything in between depending on the antenna configuration.
NOTE: A "trace" is a single, vertical column of GPR data, representing the signal "traced" by a radar pulse as it travels from the instrument into the subsurface. Traces are herein described to be composed of a number of samples (vertical dimension, or y-axis), and the number of traces are used to describe the horizontal dimension, or x-axis. Note that in ENVI-terminology, "samples" are counted in the horizontal dimension, in contrast, and "lines" are counted in the vertical dimension, but we use GPR-terminology here instead to avoid confusion.
The "time window" and "ground velocity" terminology are common in the GPR literature and are herein modeled after RAMAC GroundVision software, which similarly requires the user to enter these parameters in order to convert time to depth for the scale bars that are displayed to the right and left of the data imagery in GroundVision. The time window is the amount of time (in nanoseconds) that the radar receiver was set to "listen" for the return pulse after each radar pulse was released from the transmitting antenna during data acquisition. If the time window is set to 40 ns, for example, that means an individual radar pulse has a maximum time duration of 20 ns to reach a reflector and 20 more ns to reflect back to the receiver in order to be recorded in the data file. The bottom of the data file therefore represents a maximum duration of 20 ns for a time window of 40 ns. The ground velocity represents (in meters per nanosecond) how fast the radar pulses traveled through the subsurface medium being imaged in the data file. Ground velocities range from slow (e.g. 0.03 m/ns for fresh water) to fast (e.g. 0.3 m/ns for air) and the user should refer to a GPR textbook or manual for the proper value related to the media being imaged. For dry snow on the Greenland ice sheet, for example, an average value of 0.236 m/ns may suffice if the value has not been measured in a snow pit, based on an average dry snow density of 0.3 grams per cubic centimeter that has been empirically related to a dry snow permittivity of 1.62 by the following publication:
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