What is Das and DTS?

The invention of fiber-optic sensing technology has opened up many possibilities in mining, civil, energy, and environmental exploration. Engineers no longer need to rely on conventional electromechanical-based sensors that are wrought with many limitations.

To begin with, traditional sensors rely on discrete sensors to measure changes at predetermined points. Conversely, distributed sensing sends pulses along an optical path to determine changes at all points along the fiber.

Also, fiber-optic cable sensing is resilient to the slightest electromagnetic interference, making it the most suitable system for applications in harsh environments. Typically, distributed sensing is used to acquire acoustic data via distributed acoustic sensing (DAS), strain, and temperature changes via temperature sensing (DTS). Here’s more about Distributed Acoustic Sensor.

What is DAS?

DAS is an acronym for distributed acoustic sensing, a system used to detect acoustic frequency strain signals over long distances and challenging conditions. The fiber optic cable functions as the sensing element that provides the measurements required to process and analyze critical data within a given time period.

DAS systems rely on Rayleigh scatter-based sensing, which uses a laser pulse sent along an optic fiber. The pulse scatters within the fiber, causing it to function as a distributed interferometer with a gauge length equal to the size of the pulse.

The pulse travels the whole length of the fiber and back before another pulse is sent along the fiber optic cable. This makes the system extremely sensitive to physical parameters like vibration, temperature, and strain, which affect the fiber core’s diameter, refractive index, and length.

What is DTS?

DTS is the short form for distributed temperature sensing, which relies on Raman-based temperature measurements in conjunction with Optical Time-domain Reflectometry. For distributed temperature systems, a short light pulse is released in fiber optic cables, producing two lights- Raman forward propagating light and Raman backscattered light.

The lights have varying wavelengths and are referred to as Stokes and anti-Stokes. The amplitude of the Stokes lights hardly depends on temperature changes, while that of the anti-Stokes light strongly depends on temperature. Typical DTS systems measure spatial and temperature resolutions to determine changes in physical parameters like vibration, strain, and temperature.

Measures spatial resolution measures changes in temperature along an optical fiber, whereas temperature resolution measures precision in measuring absolute temperature changes.

Application of DAS and DTS Systems

The utilization of DTS and DAS systems in geophysical applications has increased over the last few years owing to their precision, ability to use them in harsh conditions, and considerable investment in research and development. They are often used in the oil and gas industry to:

Perform Seismic Surveying

This is a critical stage in the exploration phase as it helps identify areas with the richest resources. DAS and DTS data help engineers understand a site’s seismic response and geology. This is made possible by using fiber optic sensor cables with DAS capabilities.

The fiber optic sensor cables with DAS capabilities help analyze the rock and soil structure of the area. Typically, a seismic source is placed on the surface of the fiber optic sensors and the fiber optic sensor cable penetrates the hole. The fiber optic sensing devices collect data in real-time, providing the most accurate data possible.

Predict Geological Formations

This application requires simultaneous measurements of temperature and vibration. As such, DAS data and DTS data must be collected simultaneously to help estimate the density of the structure and the wave velocities. This will enable us to predict the presence or absence of a reservoir at the locations.

Perform Non-intrusive Fluid Mud Monitoring

For port areas that have water/mud interfaces, safe navigation is essential. Since traditional surveying methods are intrusive and provide one-dimensional data, DAS and DTS systems come in handy. They provide non-intrusive ways of monitoring such a fluid-mud layer while offering accurate measurements. DTS profiles also eliminate the guesswork involved when retrieving recordings from existing ones by performing targeted retrieval of non-physical ghost reflections.

Monitor Cold Water Injection

Distributed acoustic sensing and distributed temperature sensing systems help monitor the flow regimes and heat transport in a wellbore to determine the dynamic behavior of a reservoir section. DTS measurements help identify the location of the flow zones and their contribution from distinct zones.

Act as Ocean Floor Cables

Ocean exploration presents unique environmental challenges. Thus ocean floor cables equipped with DAS and DTS capabilities are used to record seismic data. They also analyze the pressure, stress, and strain on submarine cables, making it easy to predict earthquakes, waves and tsunamis.