018-tem-loader.txt (3969B)
1 [1]Download the current release: tem_loader.zip 2 or install via the QGIS Plugin Manager: 3 https://plugins.qgis.org/plugins/tem_loader/ 4 5 If you run into problems installing or using the plugin, please get 6 in touch. 7 8 ## Motivation 9 10 The Transient Electromagnetic Method (TEM) is used for imaging 11 subsurface electrical properties from the ground surface or with 12 airborne instrumentaiton. The method is popular in ground-water 13 surveys, mineral exploration, and other geological applications 14 where differences in electrical resistivity of the subsurface are 15 of interest. 16 17 The subsurface resistivities are calculated through numerical 18 inversion, where the observed decay of an induced magnetic field 19 is matched against subsurface properties. The inverted resistivity 20 models are the end goal for the geological analysis, and are typically 21 exported from the inversion software as XYZ text files. The files 22 contain one row per sounding, with a stack of resistivity layers 23 and a depth of investigation. Reading these data usually requires 24 specialized commercial software. 25 26 TEM Loader is a QGIS plugin that does the import to QGIS. I made 27 it for teaching activities under the Danish Strategic Sector 28 Cooperation program, where we collaborate with geological surveys 29 and water authorities in african countries on geophysics and 30 geological modelling. 31 32 ## What it does 33 34 The plugin reads inversion XYZ files exported from TEMImage and 35 Aarhus Workbench. For each file it creates three layers in a group: 36 37 - Points: surface positions of each sounding. 38 - DOI: depth-of-investigation points, revealing the lower confidence 39 boundary in the model. 40 - Layers: resistivity layers as vertical 3D line segments. 41 42 These layers are styled with bundled QML files. The imported 43 geometries are visible in the QGIS 3D Map viewer and Elevation 44 Profile tool, where it is possible to draw cross-sections along any 45 line. 46 47 If the XYZ header declares an EPSG code, it is used. Otherwise the loader falls 48 back to the project CRS, and then to EPSG:4326. Three CSV files 49 (filename.points.csv, filename.doi.csv, filename.layers.csv) are written next 50 to the source file. 51 52 ## How the data becomes geometry 53 54 The parser is a small core.py module with no QGIS dependency, which 55 keeps it unit-testable. TEMImage and Aarhus Workbench headers are 56 normalised into a common schema. 57 58 Geometry is written as [3]WKT strings into the CSVs and parsed by 59 QGIS on load. For a sounding at (x, y) with surface elevation z 60 and depth of investigation doi: 61 62 z_doi = z - doi 63 point_wkt = f'POINT Z ({x} {y} {z})' 64 doi_wkt = f'POINT Z ({x} {y} {z_doi})' 65 66 Each resistivity layer is emitted as a vertical LINESTRING Z between 67 the top and bottom elevations of that layer: 68 69 z_top = z - depth_top 70 z_bot = z - depth_bottom 71 layer_wkt = f'LINESTRING Z ({x} {y} {z_top}, {x} {y} {z_bot})' 72 73 For TEMImager data the depths are accumulated from the Thick_* 74 columns. For Aarhus Workbench data the explicit DEP_TOP_* and 75 DEP_BOT_* columns are used when present. Rendered side by side along 76 a survey line, the stacked line segments form a fence diagram 77 coloured by resistivity. 78 79 TEM Loader targets QGIS 3.40 or later, and works on Windows, MacOS, 80 and Linux. 81 82 1. Download [4]tem_loader.zip. 83 2. In QGIS: Plugins → Manage and Install Plugins → Install from ZIP. 84 3. Select the file and click Install Plugin. 85 4. The plugin appears under Plugins → TEM Loader → Load TEM XYZ files…. 86 87 Source and issue tracker: [5]https://gitlab.com/qgeomodel/qgis-tem-loader. 88 89 If something does not work, or the loader rejects a file it should 90 handle, please [6]send me an email. 91 92 Examples: 93 - sTEM data from Kenya: https://andersdamsgaard.dk/img/tem-loader.png 94 - tTEM data from Denmark: https://andersdamsgaard.dk/img/tem-loader2.png 95 96 References: 97 98 [1] file:///tmp/tem_loader.zip 99 [2] mailto:andam@geus.dk 100 [3] https://www.ogc.org/standard/sfa/ 101 [4] file:///tmp/tem_loader.zip 102 [5] https://gitlab.com/qgeomodel/qgis-tem-loader 103 [6] mailto:andam@geus.dk