Bob Paap

Scientific exploration of the UK and Ireland's subsurface has made important contributions to scholarship and prosperity for people and the planet, including economic growth, sustainable use of natural resources, storage of greenhouse gases, and inspiring curiosity about the Earth beneath our feet. This article outlines a vision for an array of seismological instruments spanning the UK and Ireland, UKI Array, augmented by other types of geophysical sensors, to maximise the value offered by existing equipment pools. The mission is to research natural phenomena and structure in the deep and shallow Earth, to solve problems concerning hazards and resources, to connect scientists to schools and the broader public, and thus to inspire a new generation to learn about geophysics. The vision was created through a community driven process of engagement and participation. This paper describes the concept and design of the UKI-Array; a companion paper discusses related opportunities and potential applications.

Distributed acoustic sensing (DAS) enables sampling seismic wavefields along optical fibers at a spatial resolution of less than 1 m, over distances beyond several tens of kilometers. This makes DAS a powerful tool to record seismic events densely along 2D directions, whether horizontally along the earth’s surface or vertically in boreholes. Compared with traditional seismic sensors measuring ground motion units, DAS provides uniaxial strain measurements along the fiber with often imperfectly known transfer functions between the measurements and true ground motion. This can generate uncertainties in the derivation of seismic source parameters, such as the magnitude, that require an absolute measurement of the ground motion and a known instrument response. In this study, we examine the DAS transfer function, mapping DAS data to reference velocity records obtained from multiple colocated accelerometers. Our investigation makes use of downhole recordings from the Frontier Observatory for Research in Geothermal Energy field site situated in Utah, USA. Overall, we find that the DAS response estimated at different depth positions follows a consistent trend and deviates significantly from a flat response only below 80 Hz. An average site-specific DAS system response is then used to convert microseismic event recordings into calibrated velocity records with improved amplitude accuracies. Subsequently, moment magnitudes MW are derived from the P-wave records with results matching the independent accelerometer-based estimations with high fidelity for events with MW>−1.0.

Over the past decades, the Groningen Gas Field (GGF) has been increasingly faced by induced earthquakes resulting from gas production. The seismic monitoring network at Groningen has been recently densified to improve the seismic network performance, resulting in increasing amounts of seismic data. Although traditional automated event detection techniques generally are successful in detecting events from continuous data, its detection success is challenged in cases of lower signal-to-noise ratios. The data stream coming from these networks has initiated specific interest in neural networks for automated classification and interpretation. Here, we explore the feasibility of neural networks in detecting the occurrence of seismic events. For this purpose, a three-layered feedforward neural network was trained using public data of a seismic event in the GGF obtained from the Royal Netherlands Meteorological Institute (KNMI) data portal. The first arrival times and duration of earthquake waveforms determined by KNMI for a subset of the station data, were used to detect the arrival times and event duration for the other uninterpreted station data. Subsequently various attributes were used as input for the neural network, that were based on different short term averaging/long term averaging (STA/LTA) and frequency sub-band settings. Using these input data, the network's parameters were iteratively improved to maximize its capability in successfully discriminating seismic events from noise and determine the event duration. Results show an increase of 65 % in accurately detecting seismic events and determining their duration as compared to the reference method. This clears the way for improved interpretation of signal waveforms and automated seismic event classification in the Groningen area.

Summary: Numerical simulations of seismic wave propagation usually rely on a simple source model consisting of an idealized point location and a moment tensor. In general, this is a valid approximation when the source dimensions are small relative to the distance of points at which the seismic wave motions are to be evaluated. Otherwise, a more realistic spatio-temporal source representation is required to accurately calculate ground motions at the position of monitoring stations. Here, we present a generic approach to couple geomechanical simulations to seismic wave propagation models using the concept of the equivalent force field. This approach allows the simulation of seismic wave propagation resulting from the spatio-temporal dependent earthquake nucleation and rupture processes. Within the geomechanical package two separate geomechanics codes are used to simulate both the slow loading stage leading to earthquake nucleation as well as the successive dynamic rupture stage. We demonstrate the approach to a case of induced seismicity, where fault reactivation occurs due to production from a natural gas reservoir.

Geothermal energy is a viable alternative to gas for the heating of buildings, industrial areas and greenhouses, and can thus play an important role in making the transition to sustainable energy in the Netherlands. Heat is currently produced from the Dutch subsurface through circulation of water between two wells in deep (1.5–3 km) geothermal formations with temperature of up to ∼100 °C. As the number of these so-called doublets is expected to increase significantly over the next decades, and targeted depths and temperatures increase, it is important to assess potential show-stoppers related to geothermal operations. One of these potential hazards is the possibility of the occurrence of felt seismic events, which could potentially damage infrastructure and housing, and affect public support. Such events have been observed in several geothermal systems in other countries. Here we review the occurrence (or the lack) of felt seismic events in geothermal systems worldwide and identify key factors influencing the occurrence and magnitude of these events. Based on this review, we project the findings for seismicity in geothermal systems to typical geothermal formations and future geothermal developments in the Netherlands. The case study review shows that doublets that circulate fluids through relatively shallow, porous, sedimentary aquifers far from the crystalline basement are unlikely to generate felt seismic events. On the other hand, stimulations or circulations in or near competent, fractured, basement rocks and production and reinjection operations in high-temperature geothermal fields are more prone to induce felt events, occasionally with magnitudes ofM> 5.0. Many of these operations are situated in tectonically active areas, and stress and temperature changes may be large. The presence of large, optimally oriented and critically stressed faults increases the potential for induced seismicity. The insights from the case study review suggest that the potential for the occurrence ofM> 2.0 seismicity for geothermal operations in several of the sandstone target formations in the Netherlands is low, especially if faults can be avoided. The potential for induced seismicity may be moderate for operations in faulted carbonate rocks. Induced seismicity always remains a complex and site-specific process with large unknowns, and can never be excluded entirely. However, assessing the potential for inducing felt seismic events can be improved by considering the relevant (site-specific) geological and operational key factors discussed in this article.