Storage in Instrument
The most basic and effective way of storing the measurement data in a geophysical instrument is as single measurement values in a long list that reflects the measurement sequence (e.g. in instruments from Geoscan and Bartington). Knowledge of sub grid dimensions (number of lines and their length) allows the correlation between each storage position in the list and the respective location in a data grid, leading to a display on the instrument of the current number for Grid, Line and Point, respectively.
Alternatively, measurement data and their location coordinates are stored together as a data pair for each measurement requiring more memory and a means to determine the location (e.g., in instruments by Geonics and Geometrics). If data are recorded on a regular grid the increment of ‘positions’ along a survey line for each subsequent measurement has to be specified (e.g. every 2 m) as well as the line increment (usually taken as being in the E/W direction). These increments are often defined in the geophysics coordinate system. If data are recorded along well defined lines with continuous sampling (e.g. at 10 Hz) markers (‘fiducials’) are usually inserted by the operator at specified intervals (e.g. every 5 m) and the collected data are subsequently re-sampled to a grid (see Data acquisition). In a non-gridded survey mode data acquisition is linked to location measurements from GNSS/GPS or a Total Station. This information can be stored with the geophysics data in real-time during the survey, requiring data transfer protocols to allow the communication of different instruments. Alternatively the geophysics and location data can be combined after the survey using a very accurate time stamp in both data sets which requires careful and repeated synchronisation of both instruments’ internal clocks.