Noise interferometry has proven to be a powerful tool to image seismic structure. In this study we used data from 10 geophones located in a borehole at ∼3 km depth within the Groningen gas reservoir in the Netherlands. The continuous data cross correlations show that noise predominantly comes in from above. The observed daily and weekly variations further indicate that the noise has an anthropogenic origin. The direct P wave emerges from the stacked vertical component cross correlations with frequencies up to 80 Hz and the direct S wave is retrieved from the horizontal components with frequencies up to 50 Hz. The measured intergeophone travel times were used to retrieve the P and S velocity structure along the borehole, and a good agreement was found with well log data. In addition, from the S wave polarizations, we determined azimuthal anisotropy with a fast direction of N65°W±18° and an estimated magnitude of (4±2)%. The fast polarization direction corresponds to the present direction of maximum horizontal stress measured at nearby boreholes but is also similar to the estimated paleostress direction.

Summary Induced seismicity in the Groningen gas field in the Netherlands has been related to reservoir compaction caused by gas pressure depletion. In-situ measurement of compaction is therefore relevant for seismic hazard assessment. In this study we investigated the potential of passively recorded deep borehole noise data to detect temporal variations in the Groningen reservoir. Train signals recorded by an array of 10 geophones at reservoir depth were selected from the continuous noise data for two 5-month deployments in 2015. Interferometry by deconvolution was applied to the high-frequency train signals that acted as stable, repetitive noise sources. Direct inter-geophone P and S wave travel times were then used to construct the P and S velocity structure along the geophone array. The resulting models agree with independently obtained velocity profiles and have very small errors. Most inter-geophone P wave travel times showed decreasing travel times per deployment period, suggestive of compaction. However, the retrieved travel time changes are very small, up to tens of microseconds per deployment period, with uncertainties that are of similar size, about 10 microseconds. An unambiguous interpretation in terms of compaction is therefore not warranted, although the 10 microsecond error per 5-month period is probably smaller than can be achieved from active time-lapse seismic surveys that are commonly used to measure reservoir compaction. The direct P wave amplitudes of the train-signal deconvolutions were investigated for additional imprints of compaction. Whereas the P wave amplitudes consistently increased during the second deployment, suggestive of compaction, no such trend was observed for the first deployment, rendering the interpretation of compaction inconclusive. Our results therefore present hints, but no obvious effects of compaction in the Groningen reservoir. Yet, this study demonstrates that the approach of deconvolution interferometry applied to deep borehole data allows monitoring of small temporal changes in the subsurface for stable repetitive noise sources such as trains.

Distributed acoustic sensing (DAS) is a rapidly growing seismic technology, which provides near-continuous spatial sampling, low maintenance, long-term deployments, can exploit extensive cable networks already deployed in many environments. Here, we present a case study from the Rutford Ice Stream, Antarctica, showing how the ice-sheet firn layer can be imaged with DAS and seismic interferometry, exploiting noise from a power generator and fracturing at the ice stream margin. Conventional cross-correlation interferometry between DAS channels yields an unstable seismic response. Instead, we present two strategies to improve interferograms: (1) combining signals from conventional seismic instruments with DAS; (2) selective-stacking crosscorrelation. These steps yield high-quality Rayleigh wave responses. We validate our approach with a dataset acquired using a sledgehammer-and-plate source, and show an excellent agreement between the dispersion curves. The passive results display a lower frequency content (˜3Hz) than the active datasets (˜10Hz). A 1D S-wave velocity profile is inverted for the top 100m of the glacier, which contains inflections as predicted by firn densification models. Using a triangular DAS array, we repeat the noise interferometry analysis and find no visible effect of seismic anisotropy in the uppermost 80 meters of our study site. Results presented here highlight the potential of DAS and surface wave inversions to complement conventional refraction surveys, which are often used for imaging firn layer, and the potential in near-surface imaging applications in general.

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In this study, we show that passively recorded data of nearby passing trains by a deep borehole geophone array could be linked to fluctuations of the gas-water contact in the Groningen reservoir in The Netherlands. During a period of 1.5 months, changes of inter-geophone P-wave travel times were detected by deconvolution interferometry of the recorded train signals. P-to-S converted waves, obtained by deconvolution of the horizontal component by the vertical component at individual geophones, showed simultaneous variations. The observed travel-time changes could be related to fluctuations of the gas-water contact in the observation well caused by pressure variations at a well drilling 4.5 km away. The $$\sim$$ ∼ 3.5 day delay between drilling in the reservoir and the seismic response yields a hydraulic diffusivity of approximately 5 m$$^2$$ 2 /s and suggests that the pressure front is effectively propagated over such a long distance. Our observations illustrate that downhole geophone arrays can be used to monitor changes in the subsurface if repeating noise sources are available, and that unexpected effects may occur due to drilling.

Investigating the Groningen gas reservoir:From passive seismic monitoring to experiments on effects of pore pressure on fault slipOnderzoek naar het Groningse gasreservoir:Van passieve seismische monitoring tot experimenten naar de effecten van poriedruk op breukbeweging (met een samenvatting in het Nederlands) Proefschriftter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof. dr. H.R.B.M. Kummeling, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op woensdag 8 juli 2020 des middags te 4.

The Groningen gas field in the Netherlands is an ideal test bed for in-situ reservoir monitoring techniques because of the availability of both active and passive in-reservoir seismic data. In this study, we use deconvolution interferometry to estimate the reflection and transmission response using active and passive borehole data within the reservoir at ∼3-km depth and separate up- and downgoing P- and S-wave fields by f-k filtering. We validate the results using synthetic data of a 1D elastic model built from sonic logs recorded in the well. The estimated full-waveform reflection response for a virtual source at the top geophone is consistent with the synthetic response. For the virtual source at the bottom geophone, the reflection response appears to be phase delayed, though its arrivals are consistent with the local subsurface geology. Similarly, the first-order estimated local transmission response successfully approximates that of the P-wave velocity in the reservoir. The study shows that reliable subsurface information can be obtained from borehole interferometry without detailed knowledge of the medium parameters. In addition, the method could be used for passive reservoir monitoring to detect velocity, attenuation, and/or interface time-lapse variations.

<p><span><span>Measurements of the seismic properties of Antarctic ice streams are critical for constraining glacier dynamics and future sea-level rise contributions. In 2020, passive seismic data were acquired at the Rutford Ice Stream, West Antarctica, with the aim of imaging the near-surface firn layer. A DAS (distributed acoustic sensing) interrogator and 1 km of optic fibre were supplemented by 3-component geophones. Taking advantage of transient seismic energy from a petrol generator and seismicity near the ice stream shear margin (10s of km away from the DAS array), which dominated the ambient seismic noise field,  we retrieve Rayleigh wave signals from 3 to 50 Hz. The extracted dispersion curve for a linear fibre array shows excellent agreement with an active seismic surface wave survey (Multichannel Analysis of Surface Waves) but with lower frequency content. We invert the dispersion curves for a 1D S-wave velocity profile through the firn layer, which shows good agreement with the previously acquired seismic refraction survey. Using a triangular-array geometry we repeat the procedure and find no evidence of seismic anisotropy at our study site. Our study presents challenges and solutions for processing noisy but densely sampled DAS data, for noise interferometry and imaging.</span></span><span><span> </span></span></p>