Processing Grid Data

Filtering Single Profiles

To interpret first results directly in the field, the Malå monitor provided basic filters to enhance the signal. The profiles however were then downloaded as raw data. A set of three filters was applied to each individual transect, to improve the data’s characteristics, remove noise and enhance the signal vertically.

 DC-Removal – Mala equipment sometimes produced a constant electrical DC-(direct

current)-offset, which created a shift in the mean amplitude of the waveform. The DC- removal eliminated the offset of the signal amplitude from each individual “trace”, or one single return signal.

 Band Pass Filter – Although named 250MHz or 500MHz antenna, the signal emitted

consists of a wide band of frequencies comparable to a bell shape. The band pass filter allows only a defined bandwidth to pass. For this a Fourier transformation was applied to convert the signal into the frequency domain and the filter was applied. For the 250MHz antenna, a band pass filter between approximately 100-500MHz was implemented; frequencies above and below this were suppressed.

 Time Varying Gain – worked as an amplifier of the signal. It compensated the amplitude

loss of the signal from spread and attenuation and enhanced the lower reflections. The application affected the interpretation strongly, as the amplifier was individually applied on profiles using linear or exponential enhancement, and the best possible outcome was chosen.

Processing Grid Data

To survey an area with GPR, a rectangular grid was set up. The instrument was run over the total outlined area in parallel lines in either east-west (EW) and west-east (WE) or north-south (NS) and south-north (SN direction. Obstacles hindering the survey such as trees, termite mounds or buildings, were marked in the plan. The decision for the spacing between the survey lines had to be a compromise between the velocity of the survey and the resolution of the results. Narrowing of the grid lines made the survey take a considerably longer time to complete, while the resolution could be improved only to some extent.581 _{Considering the size of}

some survey areas as well as the expected extent of the subsurface features, 0.5m spacing was the line spacing for all grids. In several smaller grids the survey was done in all directions, and the data later combined to improve the resolution; the results however did not improve much. Concerning the size of grids, the rule was ‘the larger the better,’ with minimum obstacles in the path of the GPR to make the results interpretable. If possible the GPR was always run to the end of the stacked tape. If this was not possible due to an irregular area, a baseline was set out from

where the GPR wheel measured. Before a profile or grid was begun, the antenna offset was set to 0. Special GPR software582 _{that integrated knowledge about ground velocity and the travel}

time to depth relation (and was initially developed exclusively for archaeological prospection

purposes) was used to integrate the data from each line in the anticipated grid and produce a depth-dependent three-dimensional map of the surveyed area. The map can be viewed as horizontal slices, layer by layer defined by the depth, known as time-slices.

Several steps of processing had to be followed to produce a 3-dimensional image of the subsurface:

a) Setting up the data frame:

 Setup of a virtual information grid file that provided x, y-location of every radargram.

 Since the GPR was run in both y, and minus-y direction for data collection, the direction of the radargram had to be corrected.

b) Processing of the data:

 0-offset repair - 0-offset is the measured time of the first return signal. An error that

occurred because of sudden jumps in the recording process due to equipment failure. This happened mainly in rough terrain.

 Gain - Linear and exponential enhancement of the signal amplitude by depth is needed to

strengthen the weakening of the return signal from dispersion over time, and to equalize the signal to depth ratio.

 Filtering - There was no need for manual additional filtering since automatic filters were

applied and it was shown over time that the data did not appreciably improved (using methods such as migration and deconvolution).

 Resampling - processing vertically downwards, starting from the newly calculated 0-

offset, where each radargram is cut into equally spaced time divisions of signal return.

c) Developing a 3D image

 Slicing – Using the data from the complete set of radargrams, the slicing process averages

“over the squared amplitude of the reflected radar signal over a horizontal spatial window

and a vertical time (depth window)”583

 Automatic kriging interpolation – a process (using a least squares estimation

algorithm) used to calculate a mean value between the data points and a central point.

 Gridding – using the information provided, with a search radius including information of

at least 3 radargrams (at 50 cm spacing an interpolation search radius of 75 centimetres was used), horizontal time slices are produced; with knowledge about the ground composition time can be converted into actual depth.

 Topographic correction – In special occasions and if the topography was an important

factor and had been measured on site, the results could be recalculated with a topographic correction.

This resulted in horizontal slices to detect and measure size, depth and form of the anomalies, from which 3D images were created and presented here in the thesis.

iv. GPR

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Alice laughed. „You must hit the trees pretty often, I should think,‟ she said. (LG, IV)