19.8 DAS Array

The DAS Array (DasArray) schema (see Figure 19.6-3 ) defines the raw, FBE, spectrum sample arrays and the corresponding times arrays used in the HDF5 files.

Further the DasArray schema defines ExternalDataSetPart and derived class DasExternalDatasetPart which help the user to keep track of which HDF5 file contains which part of the DAS sample data. Use of these parts is necessary for very large acquisitions where the DAS data samples array may become so large that must be stored in multiple HDF5 files.

The raw, FBE and spectra arrays have a Dimensions attribute of type array that specifies the ordering. The array element with the highest index contains the fastest running index. For the indexes, the three options are ‘locus’, ‘time’ and ‘frequency’. Typically the acquisition system will write raw data with locus as the fastest running index (e.g., dimensions array = {‘time’,’locus’} because it acquires all active loci on the fiber for each pulse and writes these out trace after trace. However, the ordering may differ, often after pre-processing, examples are:

• The raw data array is a 2D array containing data samples for each ‘scan’ (array size = number of scans x NumberOfLoci).
• The FBE data array is a frequency band filtered version of the raw data and also a 2D array (array size = number of scans x NumberOfLoci).
• The spectrum data array is a 3D array because each Discrete Fourier Transform (DFT) provides N points, with N equal to the TransformSize (array size = number of scans x NumberOfLoci x TransformSize).

In addition, the FBE and spectra arrays have StartFrequency and EndFrequency attributes:

• For the FBE data, these contain the start and end frequency of the frequency band.
• In the spectra data set, these correspond to the minimum and maximum frequency in the spectra.

The Times arrays contain the ‘scan’ or ‘trace’ times at which the raw, FBE and spectrum arrays were acquired or processed.

• For raw data, these contain the times for which all loci in the ‘scanned’ fiber section were interrogated. The array contains one time per ‘scanning’ pulse sent into the fiber by the DAS measurement system.
• For the processed data, these are the times of the first sample in the time window used in the frequency filter or transformation function to calculate the FBE or spectrum data.

Figure 18.1.4-1 shows traces/scans as vertical columns of dots. Note that these Times arrays contain the times in Unix time (microseconds passed since 1 January 1970).