Options in Survey map tab

survey map tab
Fig.1. Example map of TEM stations.

Changing station position

Change station position by dragging directly on survey map. For changing position of several stations press button, select stations by mouse, press and hold SHIFT, and move selected stations with ←↑↓→ keyboard key (1 m displacement per click).

Mapping

Chose map type in Data tab (Fig. 2), Color map window, which loads by pressing button. Mapping is possible for depth-dependent resistivity; depth- or time-dependent apparent resistivity; voltage decay. Apparent resistivity and voltage decay maps are used in preliminary geophysical interpretation of data; resistivity is mapped (Fig. 3) based on inversion results. Grid calculation in TDEM Geomodel is with GDAL library

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Tools for TDEM grid calculation
Fig.2. Tools for choosing initial data for grid calculation

Grid calculation is with GDAL algorithms “Inverse distance to a power”, “Inverse distance to a power with nearest neighbor searching”, “Moving average”, “Nearest neighbor”, “Linear interpolation”, for which following parameters can be specified:

survey map tab
Fig.3. Example resistivity map.

Inverse distance to a power.

Fig.4
Inverse distance to a power. This is default algorithm. It has following parameters:

Power: Weighting power (default 2.0).

Smoothing: Smoothing parameter (default 0.0).

Radius1: The first radius (X axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

Radius2:The second radius (Y axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

Angle: Angle of search ellipse rotation in degrees (counter clockwise, default 0.0).

max_points: Maximum number of data points to use. Do not search for more points than this number. This is only used if search ellipse is set (both radii are non-zero). Zero means that all found points should be used. Default is 0.

min_points: Minimum number of data points to use. If less amount of points found the grid node considered empty and will be filled with NODATA marker. This is only used if search ellipse is set (both radii are non-zero). Default is 0. nodata:

NODATA marker to fill empty points (default 0.0).

При нажатии на кнопку “Использовать параметры установки” выставляются размеры радиусов radius1 и radius2 равные трем размерам генераторной петли

Inverse distance to a power with nearest neighbor searching

Fig.5

Inverse distance to a power with nearest neighbor searching, ideal when max_points is used. It has following parameters:

Power: Weighting power (default 2.0).

The radius of the search circle: The radius of the search circle, which should be non-zero. Default is 1.0.

Maximum number of data points to use: Maximum number of data points to use. Do not search for more points than this number. Found points will be ranked from nearest to furthest distance when weighting. Default is 12.

Minimum number of data points to use: Minimum number of data points to use. If less amount of points found the grid node is considered empty and will be filled with NODATA marker. Default is 0.

NODATA marker to fill empty points: NODATA marker to fill empty points (default 0.0).

Moving average algorithm.

Fig.6

Moving average algorithm. It has following parameters:

radius1: The first radius (X axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

radius2: The second radius (Y axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

angle: Angle of search ellipse rotation in degrees (counter clockwise, default 0.0).

Minimum number of data points to use: Minimum number of data points to use. If less amount of points found the grid node considered empty and will be filled with NODATA marker. Default is 0.

NODATA marker to fill empty points: Default 0.0

Note, that it is essential to set search ellipse for moving average method. It is a window that will be averaged when computing grid nodes values.

При нажатии на кнопку “Использовать параметры установки” выставляются размеры радиусов radius1 и radius2 равные трем размерам генераторной петли

Nearest neighbor

Fig.7

Nearest neighbor algorithm. It has following parameters:

radius1: The first radius (X axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

radius2: The second radius (Y axis if rotation angle is 0) of search ellipse. Set this parameter to zero to use whole point array. Default is 0.0.

angle: Angle of search ellipse rotation in degrees (counter clockwise, default 0.0).

NODATA marker to fill empty points: Default 0

Необходимо отметить, что при использовании метода скользящего среднего обычно устанавливается радиус эллипса поиска. Это окно, которое будет усреднено, при расчете узла грид.

Linear interpolation

Fig.8

The Linear method performs linear interpolation by computing a Delaunay triangulation of the point cloud, finding in which triangle of the triangulation the point is, and by doing linear interpolation from its barycentric coordinates within the triangle. If the point is not in any triangle, depending on the radius, the algorithm will use the value of the nearest point or the nodata value.

It has following parameters:

The radius of the search circle: In case the point to be interpolated does not fit into a triangle of the Delaunay triangulation, use that maximum distance to search a nearest neighbour, or use nodata otherwise. If set to -1, the search distance is infinite. If set to 0, nodata value will be always used. Default is -1.

NODATA marker to fill empty points: Default 0.

Choose color scale in tab View: light blue - red, olive green - red, deep blue – dark red, green - purple, blue - orange – red or brown - blue. Options are available for changing layer transparency, choosing linear or logarithmic color scale, inverting color scale, as well as changing min and max values of color scale.

Fig.9. Tools for setting map view.

Mapping results are exported in ESRI Shapefile, Golden Software Surfer 7 Binary Grid File (.grd), PNG, GeoTIFF (.tif) and as csv files xyz.

Fig.10. Tools for exporting data obtained during mapping.

Mapping is usually run as follows:

Logarithmic color scale is recommended because resistivity distribution is commonly logarithmic. Note that logarithmic scale is not calculated for negative and zero values when mapping voltage.

Extrapolation radius controls grid filling relative to stations and is set depending on network density (1.5 sides of transmitter loop by default).

Resulting map can be exported in different vector or raster formats: ESRI Shapefile for map contour lines; Golden Software Surfer 7 Binary Grid File (.grd) for grid; PNG, GeoTIFF (.tif) for map raster images; input data as csv file xyz for grid calculation.