22.1 DAS Terminology

Decimated Output Data Rate

An integer fraction of the ‘Output Data Rate’.

Hz (1/s)

1Khz

Decimation Factor

The integer number by which the ‘Output Data Rate’ is decimated.

Integer

5

Fiber Distance

The ‘Optical Path’ distance from the connector of the measurement instrument to the desired acoustic sample point along the fiber that is the furthest from the measurement instrument for that particular test.

m*

1234m

Start Locus Index

The first ‘Locus’ acquired by the interrogator unit. Where ‘Locus Index 0’ is the acoustic sample point at the connector of the measurement instrument.

Integer

20

Frequency Response (of the Interrogator Unit)

The Nyquist frequency (or some fraction thereof) of the ‘Interrogation Rate’ or ‘Pulse Rate’.

Hz (1/s)

20kHz

Gauge Length

The distance (length along the fiber) between a pair of pulses used in a dual-pulse or multi-pulse system.This is a distance that the DAS Interrogator Unit manufacturer designs/implements by hardware/software to affect the Interrogator Unit spatial resolution.

m*

40m

Locus* (plural: Loci) or ‘Spatial Sample’

A particular location indicating a spatial sample point along the sensing fiber at which a ‘Time Series’ of acoustic measurements is made. In figure 1A a locus is identified as ‘o’ or ‘o’.

Locus Distance

The ‘Fiber Distance’ to center of the ‘Locus’.

m*

1250m

Locus Index

The index number of a ‘Locus’ represents a spatial sample, where such numbering of the spatial sample starts from the output of the Interrogator Unit (IU) starting with ‘0’. Successive loci are numbered as positive integers which increase along the length of the sensor fiber.

The separation of consecutive loci along the sensor fiber is determined by the spatial sample interval. The dots in Figure 18.1.4-1 indicate the center of the particular spatial sample.

Integer

1

Measurement Start Time

The time at the beginning of a data ‘Sample’ in a ‘Time Series’. This is typically a GPS locked time measurement.

Clock Time

2014-08-19 07:57:11.453721 +- X

Number of Loci

The total number of ‘Loci’ (acoustic sample points) acquired by the measurement instrument in a single ‘scan’ of the fiber. The number of ‘scans’ per second is determined by the ‘Interrogation Rate’ or ‘Pulse Rate’.

Integer

100

Number of Samples

The number of ‘Samples’ in a ‘Time Series’.

Integer

12345

Optical Path

A series of fibers, connectors, etc. together forming the path for the light pulse emitted from the measurement instrument.

Output Data Rate

The rate at which the measurement system provides output data for all ‘Loci’ (Spatial Samples). This is typically equal to the Interrogation Rate/Pulse Rate or an integer fraction thereof.

Hz (1/s)

5.0 kHz

Pulse

A single burst of laser light into the fiber

Interrogation Rate/

Pulse Rate

The rate at which the Interrogator Unit interrogates the fiber sensor. For most interrogators this is informally known as the pulse rate.

Hz (1/s)

2.5 kHz

Pulse Width

The width of the ‘Pulse’ sent down the fiber.

s

10 ns

Trace / Scan

Array of sensor values for all loci interrogated along the fiber for a single ‘pulse’ (columns in Figure 18.1.4-1 ).

Sample Number

The index of a particular ‘Sample’ within its parent ‘Time Series’.

Integer

3

Sample Time

Time at which a particular ‘Sample’ was acquired

s

Spatial Resolution

The ability of the measurement system to discriminate signals that are spatially separated. It should not be confused with Spatial Sampling Interval.

m*

5.0 m

Spatial Sampling Interval*

The separation between two consecutive ‘Spatial Sample’ points on the fiber at which the signal is measured. It should not be confused with ‘Spatial Resolution’.

m*

1.0 m

Time Offset

The time offset relative to ‘Measurement Start Time’.

μs

1234567

Time Series

A data set for a particular ‘Locus’ which is sampled at the Interrogation Rate/Pulse Rate (rows in figure 2.3).

Time Series Sampling Rate

The rate at which acoustic samples are acquired for a particular ‘Locus’. This is equal to the ‘Output Data Rate’ or an integer fraction thereof (‘Decimated Output Data Rate’).

Hz (1/s)

5 kHz

Total Fiber Length

The maximum ‘Fiber Distance’ from the connector of the measurement instrument to the final end of the ‘Optical Path’. This end is either a purposefully cut or terminated end of the fiber.

m*

5120 m

Triggered Measurement

Measurement for an acquisition which requires synchronization between a transmitting source (Tx) and a recording (Rx) measurement system. This needs to be recorded for every measurement regardless of what application it will serve.

Boolean

True, False

Triggered Time

The time of the trigger in a ‘Triggered Measurement’

Clock Time

2014-08-19 07:57:11.453721 +- X

DAS Acquisition

Collection of DAS data acquired during the DAS survey.

Acquisition ID

A universally unique identifier (UUID) for the acquisition.

Facility

Generic term for equipment being measured, e.g., a wellbore, a pipeline, etc.

Facility Length

A distance along a physical Facility from the connector of the Interrogator Unit to the physical end of the Facility.

m

10 m

Optical Path

A series of fibers, connectors, etc. together forming the path for the light pulse emitted from the Interrogator Unit.

Optical Path Distance

Fiber distance along the Optical Path measured from the connector of the Interrogator Unit.

m*

Instrument Box /

Interrogator Unit

The measurement instrument. Often referred to as Interrogator Unit or IU.

Instrument Location

Latitude – Longitude – Mean Sea Level.

If GPS antenna is not connected leave as N/A- N/A - N/A

Or WGS84

(Latitude, Longitude, MSL pairs)

(35.466088,-84.415512,-85.9)

Time Zone

UTC time zone

String

DAS Calibration

A mapping of loci to fiber distance to a physical location and/or depth along an Optical Path.

Offset End Fiber Distance

The Fiber Distance to the end of the fiber?

m*

5678 m

Offset End Fiber Locus Index

The Index of the last Locus interrogated by the measurement instrument. The position of this Locus is determined by the Offset End Fiber Distance.

Vendor Code

A string describing the data acquisition provided by the vendor to the client. This is backed by a document that the vendor issues to its clients describing the data collected. For example data mode 1 for Vendor A might mean raw ADC single pulse and Mode 2 might be raw ADC single pulse, with a high pass filter. This must be vendor specific as it’s commercially sensitive.

String

“OptaSense 010 DAS raw decimated”

Tap Test

A test to identify a certain physical location on the fiber in the field. This is often done by ‘tapping’ the wellhead, hence the name Tap Test.

Tap Test Locus Index

The locus index of the Tap Test.

Tap Test Fiber Distance

The Fiber Distance of the Tap Test.

DAS Raw Data

DAS data exchange format for describes ‘raw’ (unprocessed) DAS data provided by the vendor to a customer.

System dependent

DAS FBE Data

DAS data exchange format for Frequency Band Extracted (FBE) DAS data provided by the vendor to a customer. This DAS data type describes a dataset that contains one or more frequency bands filtered time series of a ‘raw’ DAS dataset. This could be for example the RMS of a band passed filtered time series or the level of a frequency spectrum.

System dependent

DAS Spectrum Data

DAS data exchange format for Frequency Spectrum information extracted from a (sub) set of ‘raw’ DAS data.

System dependent

DAS Job

A set one or more DAS acquisitions acquired in a defined timeframe using a common Optical Path and DAS Instrument Box.

Start Frequency

Start an individual frequency band in a DAS FBE data set. This corresponds to the frequency of the 3dB point of the filter.

Hz

1.00 Hz

End Frequency

End of an individual frequency band in a DAS FBE data set. This corresponds to the frequency of the 3dB point of the filter.

Hz

9.99 Hz

Minimum Frequency

The minimum signal frequency a measurement instrument can provide as specified by the vendor.

Hz

0.1 Hz

Maximum Frequency

The maximum signal frequency a measurement instrument can provide as specified by the vendor. This is the Nyquist frequency (or some fraction thereof) of the Time Series.

Hz

10.0 kHz

Filter

Describes the mathematical operation applied to remove unwanted components of the signal pre-processing. Filters can be described in the time domain as a time series or in the frequency domain as a response per frequency ‘sub’-band.

Filter Vendor Technique

A string describing the filter applied by the vendor to data provided to the client. This is backed by a document that the vendor issues to its clients describing the data collected. Filters applied are part of a vendor’s proprietary processing techniques.

String

“Vendor X Bandpass”

Filter Type

A string describing the type of filter applied by the vendor. Important frequency type filter classes are frequency response filters (low-pass, high-pass, band-pass, notch filters) and butterworth, chebyshev and bessel filters. The filter type and characteristics applied to the acquired or processed data is important information for end-user applications.

String

“Vendor X Bandpass”

Filter Window Size

The number of samples in the filter window applied.

Integer

128

Filter Window Overlap

The number of samples overlap between consecutive filter windows applied

Integer

64

Depth

Depths are commonly specified either as measured depth (MD) along the borehole or true vertical depth (TVD), which is the measured depth relative to a depth reference. Common datums used are Mean Sea Level (MSL) and Kelly Busing (KB).

m

1234.0 m (KB) or

1228.1 m (TVD)