Multi-Channel GPR Systems

Supplemental User’s Manual

20014 Gypsy Ln Woodland Hills, CA 91364 [email protected] last updated (1/13/10) Multi-Channel GPR systems

GPR-SLICE v7.0 is equipped to handle processing for multi-channel GPR array systems. In order to access multi-channel options in GPR-SLICE requires a special extended license.

GPR-SLICE v7.0 is flexible to handle any number of antenna pairs in a multi-channel array. The most common multi-channel systems are from GSSI, Mala Geoscience, and 3D Radar of Norway. Currently the software can be used to process the 14 channel GSSI Terravision and the Mala 16 channel Cart system or Mala 8/16 channel Mira system. GPR-SLICE can also be used to process 3D Radar multi-channel files of Norway. For this manufacturer the step-frequency radargrams must first be converted to the time domain pulse radargrams, after which the data processing can be initiated from start to finish with GPR-SLICE.

The unique features added to GPR-SLICE multi-channel extended licenses are:

 automatic detection of the ground surface - 0 ns offset for each antenna pair in the array  integration of GPS navigation from a single unit on the multi- channel system with repositioning algorithm to calculate the location of each individual antenna pair from a global-single navigation track  single total station navigation is also used to reposition each multi- channel antenna pair in the array  multi-channel information file creation template for generating project files from N tracks with set antenna offsets

Several planned developments for improving the imaging of multi-channel array data include, individualized calibration matching between the antenna pair. With the current options available in GPR-SLICE, relatively clean multi-channel time slice images can be generated.

The processes for multi-channel to make time slice images involve more details in the navigation. In general the operations are to predict all the individual GPS tracks on the antenna system from a single GPS unit mounted based on the antenna offsets. Once the individual tracks are calculated the basic GPR-SLICE processes for time slice imaging can be applied just as they are for single channel systems. GPR-SLICE monitors the direction of the antenna system to accurately predict the locations of all the antenna pairs GPR-SLICE has unique capabilities to automatically detect the 0ns offset that is unique to each antenna pair in the system. These options to correct for the 0 shift are available either via a scan-by-scan algorithm or an line- by-line algorithm. Scan-by-scan will detect and remove the 0ns offset for each scan and for each antenna pair. Line-by-line operations are also available.

Terravision 14 Channel Operations

Step 1) Create an info.dat file containing the names of the multiplexed multi-channel Terravision array radargrams (Figure 1). A *.dzt.gps file following GPR-SLICE v7.0 navigation file format should exist in the \raw\ folder of the project for each Terravision track. The multi-channel button should be clicked on in the Create Info File menu. The channel separation is also added before generating the main single track information file. If a y offset of the GPS with the linear antenna array exists, it can be added here. For variable y offsets, the user will currently need to manually adjust these in the Edit Info File menu. Step 2) Convert/extract the individual channels using the GSSI Terravision button in the Convert Data menu. Individual channels within the Terravision track radargrams will be extracted and renamed and appended with a *-N where N is the antenna channel number. These radargrams are generated to the \raw\ folder in the project where the original single track radargrams reside. Step 3) Make an infoall.dat filename for example, containing the names of all the extracted channels. Use the correct channel separation with multi- channel checkbox clicked to on in the Create Info File menu. Also note, that for Terravision, the 000-0 naming convention should be used since these will be the names of the extracted channels. Step 4) Insert the infoall.dat filename in the Create Info File menu and click the Array to Nav button to generate the individual navigation files for each separate channel. After clicking the Array to Nav, click the GPS get List button to update the number of listing generated for the *.dzt.gps. Step 5) Use the GPS trace # navigation option in the in Navigation menu if the GPS log files have scan numbers associated with each GPS reading, or use the Artificial Marker navigation if the GPS is recorded equidistantly in time and scan numbers are not available. For a further discussion of this refer to the main GPR-SLICE v7.0 manual.

Note: As of 2008, GSSI has not yet provided an engineering solution with the Terravision to output scans numbers with the NMEA strings. Step 6a) Before beginning time slice operations, it is prudent to detect the 0ns offset for each antenna pair and then generate a set of editing radargrams with the offset removed. This is accomplished using the Radargram Editing menu.

A similar discussion is provided in the main GPR-SLICE manual and is also given here:

Radargram Editing

The beginnings of a radargram editing menu began with V5 and are continuing with several more essential processing items in V6. The first option available in V5 was pull down option in the Radar menu to truncate radargrams. This option allows the user to create a new set of radargrams that cut off the samples before and after the start sample and end sample. At the end of processing all the radargrams in the information, a new information file called infoedit.dat will be automatically created which contains the new samples/scan for the edited radargrams. The edited radargrams are written to the project \work\ folder.

0ns scan-by-scan auto-detection and radargram truncation

A new option to detect the sample number for each individual scan where the 0 ns are located has been added to the Radargram Editing menu in V6. With this new process in GPR-SLICE, the user can automatically detect the 0 ns scan-by-scan and the software will also automatically truncate the radargram and generate a new radargram with the 0ns offset shift accounted for across the entire radargram. This operation will run in batch.

One might ask why would one want to detect the 0ns scan-by-scan and have the software truncate each scan to this local value. Well, there are some datasets, where the 0ns drift occurs dramatically within the radargram profile. This situation can occur with any equipment that undergoes some kind of electronic drift/noise. Till Sonnemann from the University of Sydney has been working in Cambodia, and he recorded typical radargrams as shown: The 0 ns offset in the top left radargram shifts by about 3 ns suddenly near the 10 meter range. The other radargrams also shows significant changes in the 0ns offset between different lines. Making time slices of 0ns offset uncorrected radargrams will produce inconsistent and noisy time slices which will appear to have mosaic, striping and other problems, in addition to not showing reflections at equal depth across the site.

GPR-SLICE was equipped to handle line-by-line automatic offset detection for our multi-channel users. Since the same problems which are occasionally in single channel datasets, are almost always present in multi-channel data since each antenna pair has slightly different electronics.

The new process to do this correction is in the Radargram Editing menu called Auto 0ns Scan-by-Scan + Truncate. The process works by looking at a threshold change on the signal prior to the pulse rising. The operation will discover the smallest and the largest 0ns offset across the entire set of radargrams first and over each individual scan. When this is done a message box will appear detailing a description of the minimum and maximum 0ns offset. This information is needed prior to adjusting and truncating a new set of radargrams with a new sample length. For instance in this dataset, the minimum and maximum sample positions detected are the 26th and 43rd sample. When the process of radargram editing/truncation is finished, a new set of radargrams will be written to the \work\ folder containing radargrams which have the 0ns adjustment for each scan, and total scan length of (N sample – 43). A new information file called infoedit.dat will be written. To continue the data processing, the infoedit.dat file should be highlighted in the Edit Info File menu to make it the active information file.

The radargram editing for 0ns offset, requires several important data processing step in order to properly generate time slices. These steps are:

1) Convert the \raw\ data using Batch Gain (not Batch Gain – Wobble) and with NO gain curve- e.g. all the gain points set to 1.

2) After reversing the files if necessary, continue to the Radargram Editing menu and run the scan-by-scan or the line-by-line process to adjust for the 0ns offset and to write a set of 0ns corrected radargrams in the \radar\ folder to the \work\.

3) Highlight the infoedit.dat file in the Edit Info File menu for the new set of radargrams in the \work\ folder that have been adjusted.

4) Return to the Filter menu and run the Regain menu on the \work\ folder to make gained radargrams in the \regain\ folder 5) Run the Spectra menu to set the bandpass frequencies and then run the Bandpass operation.

6) Go to the Navigation menu and select the bandpass folder for running the desired navigation option. Proceed as usual to the Slice\Resample operations to make time slices.

Step 1 is necessary since applying gain prior to correcting for the 0ns offset would result in unequal gaining after truncation. The gaining of the radargram must be done after the 0ns adjustment is made.

Similar operations are done for line-by-line 0ns detection. In this operation, the average 0ns offset across each individual line is first calculated and this is used to make the adjustment.

Note 1: in runtime operations, only the corrected radargrams is actually displayed

Note 2: After completion of the 0ns offset adjustment, an (optional) file called a *.hoz is written to the \topo\ folder. This file exists for each radargram in the project and is the sample number containing the 0ns offset for each scan. It is similar to the horizon profiles that are generated in the Static menu. These files are tabulated for the future in case additional mouse editing of the 0ns offset is desired. This can be done in the Horizon menu. Editing the 0ns “horizon” and then running the horizon correction will allow the user to intervene to generate another user defined “0ns horizon corrected” radargram. For most general operations in the software, the user will not need to interact directly with these files.

Note: A quick button to detect (but not edit) the 0ns offset line-by-line is available in the Slice/Resample menu. The unique 0ns offset for each antenna pair will be incorporated and used in making the time slices that compensate for these changes. However, the resampled radargrams however, will not reflect any changes made to the 0ns. Step 6b) Chose an appropriate cuts/mark in the Slice/Resample menu which will give an equivalent spatially averaging along the profile tracks to the channel separation. You may have to manually calculate a reasonable value for the cuts per mark based on your radargram length and the density of GPS readings over the ground. Run the Slice/Resample operations and use the XYZ option to create the time slices. Step 7) Use the Help GPS button in the Grid Menu. Note, the x,y search radii and the blanking should be manually adjusted then to yield searches which are about 1.5-2 or even 3 times the profile or channel separation to obtain various levels of initial smoothing to the depth slices. 2) Mala Mira Multi-Channel System

For those with multi-channel extended licenses in GPR-SLICE, there is now support for processing Mala’s new Mira multi-channel system. File naming conventions in the Create Info File menu and new buttons to extract and read Mira *.pos GPS or total station navigation files are available in the Edit Info File menu. This equipment has unique naming conventions for extracted channels and navigation files. The names may look like gps_000_a000.rd3 where the first 000 is the track number followed the a000 which is the channel number in the track. The new naming conventions for this equipment can be automatically generated in the Edit Info File menus for multi-channel licenses (Figure 1). For multi-channel operations 2 information files are needed when random GPS or total station surveying is used:

Step 1) Create an information file of the first channel name for each track, e.g. info.dat (Figure 1) Step 2) Create another information file for all the extracted track names, e.g. infomain.dat. This can be easily done by setting the N channels to 16. Step 3) In the Edit Info File menu insert the info.dat file and click the Mira to Nav button to generate the GPS track from the *.g01.pos file associated with the Mira track. The information file with only the first channel profiles names have the GPS tracks named with the Mala extension *.g01.pos. To generate the *.*.rd3.gps GPR-SLICE standard GPS files the button Mira to Nav is available. The GPS Get List button will update the list of the total number of listing for each GPS track for the first channel. The GPS track can also be edited at this time if need be. Step 4) In the Edit Info File menu insert the info main.dat file and click the Array to Nav button to generate the individual GPS tracks taking into consideration the track movement as well as the offsets in the infomain.dat file. Step 5) To make time slice proceed as usual using the GPS Trace # in the Navigation menu etc. Example time slices for the Mala Mira multi- channel system are shown (data courtesy of Helder Teraco, GEOSURVEYS Portugal):

Note: Mala’s naming convention for the navigation file which is now called a *.pos file may change. I would recommend that the *.pos file have the same extension name as the first radar track as is done with other multi- channel systems. Open GL Visualization is available for all multi-channel processed datasets in the GPR-SLICE v7.0 3) IDS Stream Multi-Channel Arrays

GPR-SLICE has a beta version for doing multi-channel analysis with the IDS Stream arrays. The IDS array has several linear arrays in 2D. The operation will be used for making images using the 2 outside linear arrays definied as channels 23-9, and 39-25 in the picture below.

Step 1) The First step is to generate *.dt.gps files of the main channel and have these reside in the \raw\ folder. Using the naming convention li090002.dt and li250002.dt as for the upper and lower arrays (excluding the dual channel array in the middle), the *.dt.gps are placed into the \raw\ folder. These files represent the first track. The second track is defined as li090003.dt, li250003.dt. (note the first track for this example dataset is bogus and not used). Additional tracks continue with the same naming convention. An example of the second track in its *.dt.gps form using GPR- SLICE 20 column navigation format is shown below. X,Y,Z for this example are total station information. No scan numbers exist for this manufactures so the 5th column is set to 1:

For a survey with 3 tracks, a total of 6 navigation files are needed for the upper and lower arrays, e.g. li090002-4.dt.gps and li250002-5.dt.gps.

Step 2) Once these are present, an information file describing all channels for lower track and the upper track are made separately and called info1.dat and info2.dat. The Create Info File menu in GPR-SLICE v7.0 is used to generate the information files as shown in the figures on the next page. One should make sure to change the necessary menu items, using the IDS array definition radio buttons, etc. The offsets in X of the antenna arrays are handled in the information file.

Step 3) Insert the Y offset via the GPS track menu for each of info1.dat and info2.dat separately, e.g. -.72 for the bottom linear array and .72 for the top linear array respectively.

Step 4) Info1.dat and Info2.dat can be combined to generate info12.dat using the Append option at the bottom of the Create Info File menu. The entire Stream array will be processed together if desired using info12.dat

Step 5). In the Edit info file menu, the info12.dat file is highlighted and the button Array to Nav is clicked – generating all the *.dt.gps files for each antenna pair in the Stream array. 5) After the navigation files are generated, time slicing is done. The Stream array requires Radargram Editing operation where the 0ns is automatically detected/truncated and new radargrams are written to the \work\ folder (See earlier discussion on 0ns editing in this manual). An example of time slices made with the Stream array is shown in the following figure. Optional information: The PowerBasic for Windows code to generate *.dt.gps files of the upper and lower arrays in the Stream array from a csv file with total station navigation: