Locating sub-wavelength heterogeneities with DAS

Distributed Acoustic Sensing (DAS), a photonic technology that converts a fibre-optic cable into a long (tens of kilometres), high–linear-density (with measurements every few metres) array of seismo-acoustic sensors, can provide high-density, high-resolution strain measurements along the entire cable. The potential of such distributed measurements has gained increasing attention in the seismological community for a wide range of applications.

It has been shown that DAS exhibits sub-wavelength sensitivity to heterogeneities near the fibre-optic cable. This sensitivity is related to the fact that DAS measures deformation, as opposed to the displacements measured by conventional seismometers. However, this sensitivity can also create difficulties for many DAS applications, such as source location or imaging at depth. Nevertheless, it can be advantageous for retrieving information about the subsurface in the immediate vicinity of the cable.

Here, we present a method to locate small heterogeneities near a fibre-optic cable by inverting an indicator of small-scale heterogeneities: the homogenised first-order corrector. We show that this first-order corrector can be used to locate heterogeneities near the fibre-optic cable with a precision on the order of the gauge length, independently of the wavelength.

We will first briefly present the homogenisation theory, making it possible to explain and interpret the link between small-scale heterogeneities and DAS data. We will then present the proposed method to retrieve information about small scales from DAS data, followed by several numerical and field-data application examples.