Far-offset detection of normal modes and diving waves: a case study in Valhall, southern North Sea

The use of permanent arrays for continuous reservoir monitoring has become a reality in the past decades, with Ekofisk and Valhall being its flagships. One of the possibilities when such solution is available is to passively record data while acquisitions with an active source are ongoing in nearby areas. These recordings might contain ultrafar-offset data (over 30 km), which are hardly used in standard reservoir exploration and monitoring, as they are mostly a combination of normal modes, deep reflections and diving waves. We present here data from the Valhall Life of Field Seismic array, recorded while an active seismic survey was being acquired in Ekofisk, in April 2014. Despite the lack of control on source firing time and position, analysis of the data shows that the normal modes are remarkably clear, overcoming the ambient noise in the field. The normal modes can be well explained by a two-layer acoustic model, while a combination of diving waves and refracted waves can be fairly well reproduced with a regional 1D velocity model. We suggest a method to use the far-offset recordings to monitor changes in the shallow sediments between source and receivers, both with and without a coherent seismic source in the area.

Probabilistic Moment Tensor Inversion for Hydrocarbon-Induced Seismicity in the Groningen Gas Field, The Netherlands, Part 1: Testing

ABSTRACT Since 1991, induced earthquakes have been observed and linked to gas production in the Groningen field. Recorded waveforms are complex, resulting partly from a Zechstein salt layer overlying the reservoir and partly from free-surface reverberations, internal multiples, interface conversions, guided waves, and waves diving below the reservoir. Therefore, picking of polarities or amplitudes for use in moment tensor inversion is problematic, whereas phase identification may be circumvented employing full waveform techniques. Although moment tensors have become a basic tool to analyze earthquake sources, their uncertainties are rarely reported. We introduce a method for probabilistic moment tensor estimation and demonstrate its use on the basis of a single event within the Groningen field, concentrating on detailed tests of input data and inversion parameters to derive rules of good practice for moment tensor estimation of events recorded in the Groningen field. In addition to the moment tensor, event locations are provided. Hypocenters estimated simultaneously with moment tensors are often less sensitive to uncertainties in crustal structure, which is pertinent for the application to the Groningen field, because the task of relating earthquakes to specific faults hitherto suffers from a limited resolution of earthquake locations. Because of the probabilistic approach, parameter trade-offs, uncertainties, and ambiguities are mapped. In addition, the implemented bootstrap method implicitly accounts for modeling errors affecting every station and phase differently. A local 1D velocity model extracted from a full 3D velocity model yields more consistent results than other models applied previously. For all velocity models and combinations of input data tested, a shift in location of 1 km to the south is observed for the test event compared to the public catalog. A full moment tensor computed employing the local 1D velocity model features negative isotropic components and may be interpreted as normal fault and collapse at reservoir level.

A seismotectonic study and minimum 1D velocity model for the Greater Geneva Basin, Western Switzerland

<p>In the framework of the Geothermie2020 project, the canton of Geneva and the Industrial Services of Geneva (SIG) are currently developing geothermal exploration in the Greater Geneva Basin (GGB), located in south-western Switzerland and neighbouring France. Before geothermal exploration begins, it is important to investigate the ongoing seismic activity, its relationship with local tectonic features, and the large-scale kinematics of the area. Background seismicity suggest that the local tectonic structures affecting the basin may still be active. Moderate-magnitude earthquakes have been identified along the Vuache fault, a major strike-slip structure crossing the basin. In this context we deployed a dense temporary network of 20 broadband stations around and within the GGB, during ~1.5 years, and reaching a detection threshold 0.5M<sub>L</sub>. </p><p>Using a new coherence-based detector (LASSIE), we detected and located 158 events in our area of interest. However, only 20 events were located in the GGB, with local magnitudes ranging from 0.7 to 2.2M<sub>L</sub>. We found no earthquakes in the Canton of Geneva where geothermal activities are taking place. We constructed a local minimum 1D velocity model with VELEST, using the recorded seismicity together with earthquakes from adjacent regions, in a total of 1263 P- and S-picks. The new velocity model allowed to relocate micro-seismic activity up to 11km depth along the main fault systems (i.e. Vuache, Cruseilles, Le Coin, and Arve) offsetting the GGB. We retrieved 8 new focal mechanisms for the area, using a combination of polarities and waveform inversion techniques (CSPS method). A stress inversion shows a tectonic deformation dominated by a quasi-pure strike-slip regime in the GGB, consistent with structural and geological data.</p><p>The study of microseismicity in a quiet sedimentary basin is challenging due to the scarce occurrence of seismic events combined with low signal-to-noise ratios and the often strong attenuation. However, the investigation of the sporadic (yet present) natural seismicity with dedicated dense networks could provide useful information about the GGB, even with a short-term experiment. We propose a newly-computed 1D velocity model that can be used in the GGB for seismic monitoring purposes throughout the geothermal project. This model can be easily improved later on, whenever more data is available. Monitoring the evolution and dispersion of the seismic-activity through the identified seismogenic areas during the geothermal project is essential. Quantifying the seismic rate in the basin before geothermal operations start will help to quantify the impact that geothermal energy extraction might have on the GGB. </p>

3D P-wave velocity model of Ireland's crust from controlled source tomography

<p>We present a 3D P-wave velocity model of the crust and uppermost mantle below Ireland. In the absence of local earthquakes, we used quarry and mining blasts recorded on permanent stations in the Irish National Seismic Network (INSN) and during various temporary deployments. We compiled a database of 1,100 events and around 20,000 P-wave arrivals, with each event associated with a known quarry. The source location uncertainty is therefore minimal. Both source and receiver locations are fixed in time and we used repeating events to estimate the travel time uncertainty for each source-receiver combination. We created a starting 1D velocity model from previously available data, and then used VELEST to calculate a preliminary minimum 1D velocity model. The 1D velocity model enabled us to remove outliers from the data set, and to calculate the final minimum 1D model used as the initial model in the 3D tomographic inversion. The resulting 3D P-wave velocity model will shed new light on the 3D crustal structure of Ireland.</p>

Minimum 1D Velocity Model and Local Magnitude Scale for Myanmar

ABSTRACT Earthquake monitoring in Myanmar has improved in recent years because of an increased number of seismic stations. This provides a good quality dataset to derive a minimum 1D velocity model and local magnitude (ML) scale for the Myanmar region, which will improve the earthquake location and magnitude estimates in this region. We combined and reprocessed earthquake catalogs from the Department of Meteorology and Hydrology of Myanmar and the International Seismological Centre. Additional waveform data from various sources were processed as well. A total of 419 earthquakes were selected based on azimuthal gap, minimum number of stations, and root mean square travel‐time residuals. A set of initial seismic velocity models was derived from various seismic velocity models. These models were randomly perturbed and used as initial models in a coupled hypocenter and 1D seismic velocity inversion procedure. We compared the average mean travel‐time residuals from the initial and inverted models. The best final model showed an improvement of location standard errors compared to the old model. Furthermore, the local magnitude scale inversion for the Myanmar region was performed using 194 earthquakes having a minimum of two amplitude observations. The following ML scale was obtained ML=logA(nm)+1.485×logR(km)+0.00118×R(km)−2.77+S. This scale is valid for hypocentral distance up to 1000 km and magnitudes up to ML 6.2.