Cross-calibration of optical fibre and geophysical sensors in a controlled environment

Distributed optical fibre sensing (DOFS) has recently led to several international high impact publications ^{e.g., 1,2} demonstrating its novel application for diverse environmental observations and hazards monitoring; e.g.  for geological carbon dioxide/hydrogen storage for Net Zero, quantification of Arctic glacier and sea ice melting rates, ocean temperature, pressure and current speed measurements. DOFS sensing transforms a cable into an array of sensors, which can be used to detect and monitor multiple physical parameters such as temperature, vibration and strain, with fine spatial and temporal resolution over long distances (up to 100s of km). DOFS offers certain benefits over conventional seismic sensors such as ease and cost of deployment on the seafloor or downhole with restricted access, for a much higher number of equivalent point sensors. Although the equivalence between DOFS and seismometer signals remains uncertain. We are presenting results from a recently started project that will connect DOFS and geophysical data interpretation, using controlled laboratory environment to robustly interpret what is seen on field-scale measurements using DOFS.

DOFS measures the strain rate of vibrations in the ambient environment, but signal magnitude and characteristics depend on the nature of ambient noise and the surrounding substrate (water column, seafloor sediments, cable protective sheath, etc.).  Here, we present results from cross-calibration of optical fibre and geophysical sensors in a controlled environment in rock physics lab. This involves equivalence testing on typical geological materials, from reservoir rocks to marine sediments, to accurately compare sensor measurements so we can relate them with certainty to future studies of seafloor seismo-acoustic wave propagation phenomena and their applications.   This project has just started, and we have completed the comparison of ultrasonic velocity measurements using piezoelectric sensors and comparing that with DOFS measurements. The same experiment also looked the effect of increasing strain, wind speed and temperature. Over the next three months, we will also assess how DOFS and elastic wave measurements varies with a) sample type b) pressure and temperature c) pore fluid – air, CO2 and water. We will first measure metal samples (aluminium and brass) and then natural samples to assess the effect of heterogeneity. This will enable assessing novelty of DOFS for heterogeneity and fluid distribution mapping, providing an extra advantage compared to ultrasonic elastic wave measurement system. We will interrogate existing DOFS field data from Orkney and Eday in terms of spatiotemporal sensitivities obtained from lab testing. We will then explore the use of multi frequency measurements, which has not been done before. We will conduct experiments on ice formation to quantify ice thickness during the formation, especially from temperature and strain changes in vertically suspended optical fibre cable. The key question is ‘Can we detect the density variations in ice using DOFS?’. This project will provide the necessary proof of concept and key calibration results enabling greater credibility and de-risking of future proposals.

References:

1 Marra, G. et al. Science (2022), DOI:10.1126/science.abo1939

2 Spingys, C. P. et al. Scientific Reports (2024), DOI:10.1038/s41598-024-70720-z