19.7.3 DAS Calibration

The DAS Calibration objects (DasCalibration) contain a mapping of loci-to-fiber for each facility along the optical path for the DAS acquisition. For example when a well is interrogated the optical path typically consists of a surface fiber and a downhole fiber in the well each described by their own Calibration object.

Each Calibration object contains details about the optical path segment, facility and datum and then has the actual calibration points. The facility type can be: generic, pipeline, or well. “Generic” can be used for things like connecting cables. For a well, the well datum can be used to state where the datum for measured depth originates.

The CalibrationType can be used to differentiate between different calibration points. The user is free to define their own calibration types, but three special types have been pre-defined:

• A so-called ‘tap test’ is a test where an operator uses a (spark-free!) hammer to create a vibration on a known location like the well head to provide the interpreter a known point for depth calibration.

• The ‘last locus to end of fiber’ indicates that this calibration point contains the fiber distance from the last locus to the end of the fiber. This is a useful calibration point for permanent downhole installations. The operator can use this measure to create exactly the same interrogation interval along the fiber after a DAS instrument or surface cabling has been changed out of. Note that for this calibration type OpticalPathLength doesn’t mean anything and should be set to a NULL value.
• Locus calibration is where a single locus along the optical path is mapped to a length along one of the facilities being measured.

The calibration points are provided in the XML as a set of repeating elements, and in the HDF5, as an array of DAS calibration data. Either consists of four elements: a locus index, the corresponding optical path distance and facility length, and a description of the calibration type. The user can provide as many calibration points as necessary per calibration object.