The Roles of Low-Noise Stations, Arrays and Ocean-Bottom Seismometers in Monitoring UK Offshore Seismicity associated with Subsurface Storage of Carbon Dioxide

TL;DR

This study evaluates how low-noise stations, arrays, and ocean-bottom seismometers improve offshore seismic monitoring for carbon storage, highlighting OBS deployment as the most effective method despite higher costs.

Contribution

It introduces a Bayesian framework to assess seismic network enhancements, incorporating noise, detectability, and velocity model uncertainties for offshore monitoring.

Findings

OBS stations significantly improve location accuracy.

Low-noise onshore stations mainly lower detection thresholds.

Optimized OBS networks enable effective offshore seismic monitoring.

Abstract

Effective seismic monitoring of subsurface carbon dioxide storage (SCS) sites is essential for managing risks posed by induced seismicity. This is particularly challenging in offshore environments, such as the Endurance license area in the North Sea, where the UK's permanent land-based seismometer network offers limited monitoring capability due to its distance from the expected locations of seismic events. A Bayesian experimental design framework is used to assess enhancements of the network with a low-noise onshore station located at around 1~km depth in Boulby mine, the onshore North York Moors Seismic Array, an optimally-located additional on-shore monitoring site, and ocean bottom seismometers (OBS). We quantify the expected information gain about seismic source locations and introduce a practical method to incorporate signal-to-noise dependent detectability and velocity model uncertainty. We show that the Boulby station or an onshore array primarily lower the detection threshold for small-magnitude events (M=0-2), but offer limited improvement in location accuracy. An optimally-located additional land-based seismometer or local array provides little additional benefit. OBS deployments yield significant improvements in location accuracy due to their proximity to potential seismicity. Optimised networks of two to three OBS stations are effective for Endurance, while three to five OBS stations offer robust monitoring across North Sea carbon storage licence areas off England's east coast. Velocity model uncertainty remains a key limiting factor for location precision across all configurations. We conclude that deploying OBS networks is the most promising strategy for enhancing microseismic monitoring capabilities at offshore SCS sites, though potentially more expensive.