In order to allow future users access to the data, it is important to provide them also in a format that can be used without the proprietary software with which they were created. These files are included in the Archive in a preservation format, which is well documented so that data can be accessed with other software. The Archiving Body will be able to migrate the data, which means that old file formats will be read and saved in a new format (e.g. Microsoft Excel xls files to be saved as xlsx files; or AutoCAD R13 dxf files being saved as dwfx or dwg files). Such a continuous migration of preservation files makes access much simpler and faster for potential users. The folders of the Archive that contain these preservation files should be clearly labelled, for example by the prefix ‘PRESERVE’.
It is inevitable that during the export from the proprietary format to the preservation format some information is lost and it is hence essential that this is captured separately in the geophysics metadata (e.g. line spacing, orientation of subsequent lines, instrument used) as detailed in Comprehensive documentation.
Data from magnetometer surveys or earth resistance area surveys are usually assembled into a ‘composite’ by placing all collected data in their spatial raster position, usually defined in the geophysics coordinate system. The best preservation format for composite data are as XYZ text files (maybe better referred to as XYV files; for details see Appendix 1) both in their unprocessed (‘raw’) form and after final processing. It may even be useful to export the data grids that represent the smallest unit of field data (see Data acquisition). The units for XY and Z/V must be specified (e.g. meters for X and Y; nanoteslas (nT) for V). Exporting to XYZ text files is preferred over so called ‘Z text files’ (i.e. a list of only Z values) as the latter require additional information about the order of the data stream and the length of individual rows and columns.
The final processed data should also be exported as XYZ text files in georeferenced form, where XY are in map coordinates (the datum used must be specified, e.g. UTM 36N, or Irish National Grid). This allows importing the data directly into other software packages, for example into a GIS.
For GPR data the preferred preservation file format is SEG-Y rev 1 (see Appendix 1). The files should conform to the well-defined SEG standard (Society of Exploration Geophysicists 2002) and location information should be recorded in the sx/y and gx/y fields that specify the location of transmitter and receiver antennas separately (this information also allows establishing whether a bistatic antenna pair was used in longitudinal or transverse arrangement). Where data are acquired in lines, SEG-Y files for individual lines are preferred over a single large data cube as file management is then considerably easier. However, duplicate or repeat lines must be clearly identified or eliminated.
Electrical resistivity imaging
Data for Electrical Resistivity Imaging (ERI) are collected as earth resistance measurements using a set of surface electrodes that are connected in turn to a current source and a measuring unit for the electrical potential, with different electrode spacing and configuration. The electrodes may either be along a straight line (usually referred to as a 2D survey) or in a pattern on the horizontal surface (3D survey). At the moment, there is no standard format for ERI data and a simple text file should be used, for example in an AMNBV format (see Appendix 1). If an inversion process is used to estimate the ground resistivity distribution that may have caused the measured ERI this can be exported as XYZV text file (see Appendix 1).
Interpretation diagrams are usually created as overlays of the measured geophysical data and can hence be thought of as secondary data. They are often vector diagrams (lines and polygons) and standard preservation files can be used (e.g. dxf/dwg, shp, see the Guide’s sections on Vector Images, CAD and GIS). If the diagrams were produced using a defined coordinate system (to be specified as either being in geophysics or map coordinates) these files can be directly used with CAD or GIS software and this is hence the recommended approach. If they were only sketched over an unscaled image, relevant information has to be provided to enable the georeferencing of these sketches (e.g. scale, coordinates of several control points).