018-tem-loader.txt (3887B)
1 [1]Download the current release: tem_loader.zip 2 3 If you run into problems installing or using the plugin, please get 4 in touch. 5 6 ## Motivation 7 8 The Transient Electromagnetic Method (TEM) geophysics method for 9 imaging subsurface electrical properties from the ground surface 10 or airborne instrumentaiton. The method is popular in ground-water 11 surveys, mineral exploration, and other geological applications 12 where differences in electrical resistivity of the subsurface are 13 of interest. 14 15 The subsurface resistivities are calculated through numerical 16 inversion, where the observed decay of an induced magnetic field 17 is matched against subsurface properties. The inverted resistivity 18 models are the end goal for the geological analysis, and are typically 19 exported from the inversion software as XYZ text files. The files 20 contain one row per sounding, with a stack of resistivity layers 21 and a depth of investigation. Reading these data usually requires 22 specialized commercial software. 23 24 TEM Loader is a QGIS plugin that does the import to QGIS. I made 25 it for teaching activities under the Danish Strategic Sector 26 Cooperation program, where we collaborate with geological surveys 27 and water authorities in african countries on geophysics and 28 geological modelling. 29 30 ## What it does 31 32 The plugin reads inversion XYZ files exported from TEMImage and 33 Aarhus Workbench. For each file it creates three layers in a group: 34 35 - Points: surface positions of each sounding. 36 - DOI: depth-of-investigation points, revealing the lower confidence 37 boundary in the model. 38 - Layers: resistivity layers as vertical 3D line segments. 39 40 These layers are styled with bundled QML files. The imported 41 geometries are visible in the QGIS 3D Map viewer and Elevation 42 Profile tool, where it is possible to draw cross-sections along any 43 line. 44 45 If the XYZ header declares an EPSG code, it is used. Otherwise the loader falls 46 back to the project CRS, and then to EPSG:4326. Three CSV files 47 (filename.points.csv, filename.doi.csv, filename.layers.csv) are written next 48 to the source file. 49 50 ## How the data becomes geometry 51 52 The parser is a small core.py module with no QGIS dependency, which 53 keeps it unit-testable. TEMImage and Aarhus Workbench headers are 54 normalised into a common schema. 55 56 Geometry is written as [3]WKT strings into the CSVs and parsed by 57 QGIS on load. For a sounding at (x, y) with surface elevation z 58 and depth of investigation doi: 59 60 z_doi = z - doi 61 point_wkt = f'POINT Z ({x} {y} {z})' 62 doi_wkt = f'POINT Z ({x} {y} {z_doi})' 63 64 Each resistivity layer is emitted as a vertical LINESTRING Z between 65 the top and bottom elevations of that layer: 66 67 z_top = z - depth_top 68 z_bot = z - depth_bottom 69 layer_wkt = f'LINESTRING Z ({x} {y} {z_top}, {x} {y} {z_bot})' 70 71 For TEMImager data the depths are accumulated from the Thick_* 72 columns. For Aarhus Workbench data the explicit DEP_TOP_* and 73 DEP_BOT_* columns are used when present. Rendered side by side along 74 a survey line, the stacked line segments form a fence diagram 75 coloured by resistivity. 76 77 TEM Loader targets QGIS 3.40 or later, and works on Windows, MacOS, 78 and Linux. 79 80 1. Download [4]tem_loader.zip. 81 2. In QGIS: Plugins → Manage and Install Plugins → Install from ZIP. 82 3. Select the file and click Install Plugin. 83 4. The plugin appears under Plugins → TEM Loader → Load TEM XYZ files…. 84 85 Source and issue tracker: [5]https://gitlab.com/qgeomodel/qgis-tem-loader. 86 87 If something does not work, or the loader rejects a file it should 88 handle, please [6]send me an email. 89 90 Examples: 91 - sTEM data from Kenya: https://andersdamsgaard.dk/img/tem-loader.png 92 - tTEM data from Denmark: https://andersdamsgaard.dk/img/tem-loader2.png 93 94 References: 95 96 [1] file:///tmp/tem_loader.zip 97 [2] mailto:andam@geus.dk 98 [3] https://www.ogc.org/standard/sfa/ 99 [4] file:///tmp/tem_loader.zip 100 [5] https://gitlab.com/qgeomodel/qgis-tem-loader 101 [6] mailto:andam@geus.dk