Seismicity induced in geo-reservoirs can be a valuable observation to image fractured reservoirs, to characterize hydrological properties, or to mitigate seismic hazard. However, this requires accurate location of the seismicity, which is nowadays an important seismological task in reservoir engineering. The earthquake location (determination of the hypocentres) depends on the model used to represent the medium in which the seismic waves propagate and on the seismic monitoring network. In this work, location uncertainties and location inaccuracies are modeled to investigate the impact of several parameters on the determination of the hypocentres: the picking uncertainty, the numerical precision of picked arrival times, a velocity perturbation and the seismic network configuration. The method is applied to the geothermal site of Soultz-sous-Forêts, which is located in the Upper Rhine Graben (France) and which was subject to detailed scientific investigations. We focus on a massive water injection performed in the year 2000 to enhance the productivity of the well GPK2 in the granitic basement, at approximately 5 km depth, and which induced more than 7000 earthquakes recorded by down-hole and surface seismic networks. We compare the location errors obtained from the joint or the separate use of the down-hole and surface networks. Besides the quantification of location uncertainties caused by picking uncertainties, the impact of the numerical precision of the picked arrival times as provided in a reference catalogue is investigated. The velocity model is also modified to mimic possible effects of a massive water injection and to evaluate its impact on earthquake hypocentres. It is shown that the use of the down-hole network in addition to the surface network provides smaller location uncertainties but can also lead to larger inaccuracies. Hence, location uncertainties would not be well representative of the location errors and interpretation of the seismicity distribution possibly biased. This result also emphasizes that it is still necessary to properly describe the seismic propagation medium even though the addition of down-hole sensors increases the coverage of a surface network.

Earthquake absolute location errors which can be encountered in an underground reservoir are investigated. In such an exploitation context, earthquake hypocentre errors can have an impact on the field development and economic consequences. The approach using the state-of-the-art techniques covers both the location uncertainty and the location inaccuracy-or bias-problematics. It consists, first, in creating a 3-D synthetic seismic cloud of events in the reservoir and calculating the seismic traveltimes to a monitoring network assuming certain propagation conditions. In a second phase, the earthquakes are relocated with assumptions different from the initial conditions. Finally, the initial and relocated hypocentres are compared. As a result, location errors driven by the seismic onset time picking uncertainties and inaccuracies are quantified in 3-D. Effects induced by erroneous assumptions associated with the velocity model are also modelled. In particular, 1-D velocity model uncertainties, a local 3-D perturbation of the velocity and a 3-D geostructural model are considered. The present approach is applied to the site of Rittershoffen (Alsace, France), which is one of the deep geothermal fields existing in the Upper Rhine Graben. This example allows setting realistic scenarios based on the knowledge of the site. In that case, the zone of interest, monitored by an existing seismic network, ranges between 1 and 5 km depth in a radius of 2 km around a geothermal well. Well log data provided a reference 1-D velocity model used for the synthetic earthquake relocation. The 3-D analysis highlights the role played by the seismic network coverage and the velocity model in the amplitude and orientation of the location uncertainties and inaccuracies at subsurface levels. The location errors are neither isotropic nor aleatoric in the zone of interest. This suggests that although location inaccuracies may be smaller than location uncertainties, both quantities can have a cumulative effect. Besides, small velocity uncertainties applied to the whole 1-D profile can lead to large increase of the location uncertainties. However, local variations of the velocity field around the well may have negligible effects that would make such a feature undetectable with an absolute location method. Although the reference 1-D velocity model was built from well log data, the results show that it is not a good representative of a more realistic 3-D model including a fault and its associated block shift. The amplitude and distribution of the induced location inaccuracies are such that the positioning and the orientation of features delineated by seismicity are distorted and may be difficult to correctly interpret.

Within Project Tor. which is about Teleseismic Tomography across the Tomquist Zone in Germany-Denmark-Sweden, we have confirmed very significant deep lithosphere differences And modeling is substantiated via completely independent methods. In 1996-1997 our 130 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well studied crustal profile of an earlier project, and the modeling efforts were concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km The Tor data have been subjected to P-wave travel time tomography. surface wave and receiver function analysis as well as anisotropy and scattering measurements An important goal of the project was to make several independent inversions of the tomography data. and compare the results in an attempt to evaluate uniqueness, resolution and accuracy of these inversions. The comparisons of this paper involve more diversity in methods than any previous comparison. The geological outcome is a substantiation of earlier statements that, "The transition is interpreted to be sharp and steep in two places It goes all through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large to depths more than 200 km. The other lithosphere difference. of smaller scale, is found near the southern edge of the Ringkobing-Fyn High near the border between Denmark and Germany Also this transition is sharp and steep. and goes all through the lithosphere to depths around 120 km. These two sharp transitions divide the Tor region into 3 different lithosphere structures distinguishable in P-wave travel time tomography. surface wave dispersion. P- and S-wave anisotropy and partly in P-wave scattering" The mentioned broad-scale features are judged to be unambiguously determined, with well-described resolution and accuracy Unfortunately a detail like the slope of the subcrustal lithosphere transition right under the Tronquist Zone cannot be constrained even if this is where the resolution is best. and the curiosity largest. (c) 2009 Elsevier B V All rights reserved.

The Vrancea seismogenic zone in Romania exhibits an intense intermediate‐depth seismicity, confined to a relatively small, roughly cylindrical and elongated region, whose origin is still under debate. Our three‐dimensional P and S wave velocity and density images put additional physical constraints on the existing tectonic models to a depth of 200 km. The results appear to substantiate a combination of lithospheric delamination and oceanic subduction. For our analysis, we apply the tomographic inversion method of sequential integrated inversion (SII) to P and S first arrivals from active source data collected during the VRANCEA99 and VRANCEA2001 seismic refraction experiments, local earthquake data collected during the Carpathian Arc Lithosphere X‐Tomography (CALIXTO) experiment and recent gravity measurements of the studied area. The reconstructed models, which explain both travel times and gravity data, show a subducting slab which exhibits fast Vp, fast Vs, high density, and a low Vp/Vs ratio consistent with the cold downgoing plate. We associate intermediate‐depth seismicity with the observed sharp lateral Vp/Vs variations presumably generated by contact between the dense and cold slab and the lithospheric mantle in the shallower part or the asthenosphere in the deeper part. This contrast is particularly evident between 100 and 150 km depth, where the maximum historical seismic energy release is concentrated. Our results indicate the diagnostic power of a combined interpretation of 3‐D Vp, Vs, Vp/Vs, and density models.

A self-adaptive automated parametrization approach is suggested for the sequential inversion of controlled-source seismic tomography and gravity data. The velocities and interfaces are parametrized by their Haar wavelet expansion coefficients. Only those coefficients that are well constrained by the data, as measured by the number of rays that cross the corresponding wavelet function support area and their angular coverage, are inverted for, others are set to zero. This approach results in a reasonable distribution of resolution throughout the model even in cases of irregular ray coverage and does overcome the trade-off between different types of model parameters. A modified sequential inversion approach is suggested to join the traveltimes and gravity anomalies inversion. An algorithm is developed that inverts for smooth velocity and density variations inside the seismic layer, the position of its bottom interface as well as for optimal values of the velocity-to-density regression coefficients. The algorithm makes use of direct (diving), reflected and head (critically refracted) wave traveltimes. The algorithm workflow is demonstrated on a synthetic data example.

The main aim of the TOR project is to study the lithospheric–asthenospheric boundary structure under the Sorgenfrei–Tornquist Zone, across northern Germany, Denmark and southern Sweden. Relative arrival-time residuals of teleseismic P and S phases from 51 earthquakes, recorded by 150 seismic stations along the TOR array, were used to delineate the transition zone in the studied area. The effects of crustal structures were investigated by correcting the teleseismic residuals for travel-time variations in the crust based on a 3D crustal model derived from other data. The inversion was carried out for S phases. The results were then compared with the corresponding P-wave models. As expected, the derived models show that the relatively old and cold Baltic Shield has higher velocity at depth than the younger lithosphere farther South. The models show two sharp and distinct increases in depth to velocities which are low compared to our reference model, as we move from South to North. The location and sharpness of these boundaries suggests that the features resolved are, at least partially, compositional in origin, presumably related to mantle depletion. A sharp and steep subcrustal boundary is found roughly coincident with the southern edge of Sweden. This is below where the edge of the Baltic Shield is usually placed, based on surface geological evidence (the Sorgenfrei–Tornquist Zone). Another less significant transition is recognised more or less beneath the Elbe-lineament. Relatively high d(Vp/Vs) ratios under the central part of the profile (Denmark) indicate relatively low S-velocity in an area where a gravity high supports the hypothesis of extensive mafic intrusions.

From July 1996 to August 1997 the TOR project operated 130 seismographs in North Germany, Denmark and South Sweden, with the aim of collecting signals from local, regional and teleseismic earthquakes. This data set is particularly interesting since the seismic antenna crosses the most significant geological boundary in Europe, the Tornquist Zone, which in the northern part is the border between the Baltic Shield and the younger European lithosphere. Previous studies have shown significant physical changes in the crust and upper mantle across this transition zone, including two independent teleseismic tomographic studies of the TOR data set. But these two studies disagree on the orientation of the slope of the transition. Both studies used an iterative linearized inversion method. We will in this work Preprint submitted to Elsevier Science 27 July 2005 present an inversion based on Bayesian statistics, where the solution space is examined in order to study a very large number of tomographic solutions and to examine the solution uniqueness and uncertainty. The method is applied to measurements of 3345 relative teleseismic P-phase travel times from 48 teleseismic earthquakes with good azimuthal coverage with respect to the great circle arc of the TOR array. We find the lithospheric transition to be a north east inclination of around 30° to 45° off vertical.

We present results of an array study of seismic anisotropy beneath the Bohemian Massif (BM) showing distinct lateral and directional variations of the fast shear-wave polarization and split-delay time consistent with variations in the P-residual spheres, in which directional means of relative residuals are subtracted. Our analysis of the S- and P-wave anisotropy suggests that the mantle lithosphere of the BM consists of at least three large domains with different orientation of the large-scale fabric separated by sutures cutting most likely the whole lithosphere. Boundaries of the units are characterised by a null or small shear-wave splitting, as well as by smaller values in the P-residual spheres. We present self-consistent 3D anisotropic models of the lithosphere domains of the BM resulting from joint inversion of anisotropic parameters searching for a 3D orientation of mantle fabric. While in the Saxothuringian and Sudetes parts of the BM the (a, c) foliations dip prevailingly to the N-NW, they dip to the S and SW in the southern and eastern parts of the Moldanubian.

Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian‐Karelia‐Lapland‐Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three‐dimensional shear wave velocity model using a ray‐based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition‐based shear wave velocities implies both lateral and vertical variations of the mineralogy.

The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity‐density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan‐Baikal belt), (3) three‐dimensional topographic variations of the crust‐mantle boundary with well‐located upwarpings, and (4) the lithosphere‐asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone.