DTS versus DAS: temperature or acoustics along a fibre
Same fibre, same kind of pulse, two very different answers. One tells you how warm every point is. The other tells you what moved.
DTS: how warm is every point
A short pulse of light goes into the fibre. A tiny fraction scatters back from every point along the glass, and part of that scattered light is shifted in colour by the vibration of the glass itself. The balance between the two shifted colours depends on temperature, and the time the light takes to come back says where it came from. Average many pulses and you have a temperature for every half metre, in degrees, with no sensor at any of those points.
- A short pulse of light goes into the fibre.
- Every point along the glass sends back a faint glow, in two colours.
- The balance of the two colours is the temperature at that point.
- The time the glow takes to come back says where it came from.
DAS: what moved, where and when
The same kind of pulse, sent thousands of times a second. This time the instrument watches the phase of the light scattered back from each section, like the hand of a clock. When anything strains the fibre, a footstep, a passing wave, flow noise, a leak, the glass stretches by a few parts per billion and the hand turns. Every half metre becomes a vibration sensor, and because the pulses come so fast, the result is a recording, not a snapshot.
- The same kind of pulse goes in, thousands of times a second.
- The light that comes back carries a phase, like the hand of a clock, for every half metre.
- When the ground moves, the glass stretches by a few parts per billion and the hand turns.
- Every half metre becomes a vibration sensor; plotted over time, events draw themselves.
What each one is good at
| DTS, temperature | DAS, acoustics | |
|---|---|---|
| What it measures | Absolute temperature at every point | Strain and vibration at every point |
| The physics | Raman scattering: the balance of two returned colours gives the temperature | Rayleigh scattering: the phase of the returned light shifts when the glass stretches |
| Fibre | Single-mode or multimode | Single-mode |
| Reach from one box | A few kilometres at fine resolution, about ten on multimode | Forty to fifty kilometres |
| Spacing along the fibre | Half a metre | Half a metre native, grouped to a chosen gauge length |
| How fast | Seconds to minutes per profile | Thousands of times a second |
| Power | Can sleep between profiles; weeks on a battery | Runs continuously; mains or generator |
| Typical questions | Where is the groundwater, how does the borehole behave, which layer conducts, where is the pipe warm | What moved, where, when: earthquakes, traffic, flow noise, a leak hissing, an injection |
| Output | Temperature profiles, netCDF and CSV | Strain-rate channels, miniSEED, SEG-Y, HDF5 |
When you want which
Temperature, when the question is a state. Where does groundwater enter, how does a borehole behave through the year, which layer conducts heat, is a pipe section warmer than it should be. These change over minutes to seasons, and a profile every few minutes on a battery is the right instrument.
Acoustics, when the question is an event. What moved, where and when: an earthquake across a telecom route, fluid entering a well during injection, flow noise behind casing, a leak hissing. These happen in seconds, and only a continuous recording catches them.
Both, when one confirms the other. A leak is a temperature change and a noise at the same chainage. An injection is a cooling front and a burst of energy at the same depth. One instrument on one single-mode fibre can give you both.
Asked before
- Can one fibre do both?
- Yes, if it is single-mode. The two measurements use different parts of the returned light and can be time-shared on the same fibre from the same instrument.
- Can DAS measure temperature?
- It sees slow temperature change as a slow strain, which is useful as a relative signal, but it has no absolute scale. For a temperature in degrees you want DTS.
- Can DTS hear a leak?
- No. It sees the temperature the leak causes, which can take minutes to hours to build. DAS hears it at once but cannot tell you how warm the ground is. Together they confirm each other at the same chainage.
- Which one for a borehole?
- Temperature for the profile, layers and long-term behaviour. Add acoustics when you need to know what is moving: injection, flow behind casing, microseismic events.
Planning a measurement? The noise calculator says what temperature difference you can expect to see, and the field power calculator what keeps the box running.
Put your fibre to work.
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