A regional shear-wave velocity (V S) model has been developed for the Groningen gas field in the Netherlands as the basis for seismic microzonation of an area of more than 1000 km 2. The V S model, extending to a depth of almost 1 km, is an essential input to the modelling of hazard and risk due to induced earthquakes in the region. The detailed V S profiles are constructed from a novel combination of three data sets covering different, partially overlapping depth ranges. The uppermost 50 m of the V S profiles are obtained from a high-resolution geological model with representative V S values assigned to the sediments. Field measurements of V S were used to derive representative V S values for the different types of sediments. The profiles from 50 to 120 m are obtained from inversion of surface waves recorded (as noise) during deep seismic reflection profiling of the gas reservoir. The deepest part of the profiles is obtained from sonic logging and V P-V S relationships based on measurements in deep boreholes. Criteria were established for the splicing of the three portions to generate continuous models over the entire depth range for use in site response calculations, for which an elastic half-space is assumed to exist below a Electronic supplementary material The online version of this article (

A study was performed to find and test quantitative methods of analysing echographic signals for the differentiation of diffuse liver diseases. An on-line data acquisition system was used to acquire radiofrequency (RF) echo signals from volunteers and patients. Several methods to estimate the frequency-dependent attenuation coefficient were evaluated, in which a correction for the frequency and depth-dependent diffraction and focusing effects caused by the sound beam was applied. Using the estimated value of the attenuation coefficient the RF signals themselves were corrected to remove the depth dependencies caused by the sound beam and by the frequency-dependent attenuation. After this preprocessing the envelope of the corrected RF signals was calculated and B-mode images were reconstructed. The texture was analysed in the axial direction by first- and second-order statistical methods. The accuracy and precision of the attenuation methods were assessed by using computer simulated RF signals and RF data obtained from a tissue-mimicking phantom. The phantom measurements were also used to test the performance of the methods to correct for the depth dependencies. The echograms of 163 persons, both volunteers and patients suffering from a diffuse liver disease (cirrhosis, hepatitis, haemochromatosis), were recorded. The mutual correlations between the estimated parameters were used to preselect parameters contributing independent information, and which can subsequently be used in a discriminant analysis to differentiate between the various diseased conditions.

SUMMARY The Groningen gas field is one of the largest gas fields in Europe. The continuous gas extraction led to an induced seismic activity in the area. In order to monitor the seismic activity and study the gas field many permanent and temporary seismic arrays were deployed. In particular, the extraction of the shear wave velocity model is crucial in seismic hazard assessment. Local S-wave velocity-depth profiles allow us the estimation of a potential amplification due to soft sediments. Ambient seismic noise tomography is an interesting alternative to traditional methods that were used in modelling the S-wave velocity. The ambient noise field consists mostly of surface waves, which are sensitive to the Swave and if inverted, they reveal the corresponding S-wave structures. In this study, we present results of a depth inversion of surface waves obtained from the cross-correlation of 1 month of ambient noise data from four flexible networks located in the Groningen area. Each block consisted of about 400 3-C stations. We compute group velocity maps of Rayleigh and Love waves using a straight-ray surface wave tomography. We also extract clear higher modes of Love and Rayleigh waves. The S-wave velocity model is obtained with a joint inversion of Love and Rayleigh waves using the Neighbourhood Algorithm. In order to improve the depth inversion, we use the mean phase velocity curves and the higher modes of Rayleigh and Love waves. Moreover, we use the depth of the base of the North Sea formation as a hard constraint. This information provides an additional constraint for depth inversion, which reduces the S-wave velocity uncertainties. The final S-wave velocity models reflect the geological structures up to 1 km depth and in perspective can be used in seismic risk modelling.

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Summary Conventional models of hydraulic fracture closure make assumptions about the fracture geometry and rock mechanical behavior. In this paper we present the results of scaled laboratory experiments. We show that deviations from conventional assumptions influence leak-off volumes, mechanical behavior, and pressure decline significantly. We discuss mechanisms that lead to these deviations from assumptions during fracture closure. Introduction Analysis of the pressure decline during closure of mini-hydraulic fractures is used in field practice to infer the leak-off coefficient and least in-situ stress, needed for determining net pressure. These are important parameters for the design of a propped hydraulic fracture. A routinely used method in the industry for analyzing the pressure decline is the use of the so-called G -plot,1 which was introduced by Nolte (see, e.g., Ref. 2). This model is based on certain assumptions, among which are linear elastic rock behavior and constant fracture area after shut-in. Nolte recognized that this last assumption is in practice not always met,3,4 and he introduced diagnostic methods to detect deviations from ideal behavior. He proposed the "3/4 rule" to identify the transition from extension to recession. In scaled laboratory experiments both radius growth after shut-in and radius recession were reported in materials with, respectively, low and higher permeability.5 In this paper we present the results of scaled laboratory experiments concerning fracture closure. While in field cases the pressure is usually the only source of information, in our experiments we obtain extensive information by measuring pressure, width, radius, and volume of the fracture, using advanced acoustic monitoring techniques.6,7 This enables us to identify the processes that occur during fracture closure independently from the measured pressure decline. The objective of this paper is first to validate the conventional modeling assumptions in our experiments, and second to investigate how possible deviations from ideal behavior can influence the leak-off rate and mechanical behavior of the fracture, and subsequently pressure decline and closure times. Possible deviations we want to address are radius changes after shut-in, surface roughness of the fracture, and plastic rock deformation. Experimental Setup and Method Scaling. We performed scaled model experiments of hydraulic fracture propagation and closure. The experiments are scaled in terms of energy rates associated with fluid flow, fracture opening, and rock separation.8 Leak-off is scaled by the fracture efficiency. In order to have sufficient time for measurements during fracture propagation, the time scale for the propagation phase is about 103 seconds. The fracture radius is approximately 0.10 m. These practical conditions require a relatively high fluid viscosity and low separation energy of the rock material. There are various length scales involved in hydraulic fracturing, which can be used for scaling up our experimental fracture size to field scale. Depending on the processes that take place, a length scale must be chosen. In our experiments, it is probably best to obtain a length scale from the fracture toughness or to use the length scale for the tip region obtained by an asymptotic solution for the stress field near the hydraulic fracture tip.9 This solution takes into account the sharp pressure drop near the tip and neglects fracture toughness. Experimental Setup. Fig. 1 shows a schematic view of the experimental setup used for the hydraulic fracturing experiments. Cubic blocks of 0.30 m size are loaded in a true triaxial machine to simulate in-situ stress states; no pore pressure can be applied. We used 0.1 mm thin Teflon sheets greased with vaseline to reduce friction between the block and the loading platens. Six linear variable differential transformers (LVDTs) measure the block deformation. The hydraulic fracture is oriented transversely to the wellbore wall, which was sealed with a 0.5-mm-thick glue layer. A 3-mm-deep notch was sawn in the wellbore wall to control the fracture location. The confining stress ? c perpendicular to the plane of the fracture is always smaller than both stresses ?h directed parallel with the fracture plane, which have equal values. The deviation of the stress inside the block from the expected value is smaller than 10% in the region of interest. During fracture propagation, a high-pressure pump injects fluid into the wellbore, at whose end a dead string is mounted where the wellbore pressure is measured. An LVDT, mounted with clamps in the wellbore, measures the fracture width with a measuring error of approximately 10%. The block extension during propagation is a measure for the fracture volume. The fracturing fluid we use is silicon oil which behaves approximately Newtonian at the shear rates of interest. Fig. 1 shows a schematic view of the ultrasonic measurements. During fracture propagation, two diffractions are measured, which we interpret as coming from the fluid front and fracture tip position.7 From the travel time of these diffractions and the material velocities we can construct the ray path, and calculate the fracture radius and a measure of the size of the nonpenetrated zone. In experiments on plaster with nonpenetrated zone sizes of 1 to 2 cm, this measure of the nonpenetrated zone size agreed well with the size of the dry zone measured after opening of the block (as is visible in Fig. 2). When the fracture stops growing, the diffraction disappears. Then, the ultrasonic shear-wave transmissions gave us the radius over which the fracture was open. These measurements have an accuracy of approximately ±1 cm. The Young's modulus is determined during loading of the block in the true triaxial machine. We determined the leak-off coefficient independently in a triaxial cell. In these leak-off experiments, we used the same fluid as was injected in the hydraulic fracturing experiments. Cylindrical cores (with 40 mm diam and 80 mm length) were hydrostatically pressurized, and at one of the axial rock faces fluid pressure was applied. The opposite face was connected to free air. The fluid pressure at the axial face of the core was kept constant, while the flow rate into the core was measured. In another type of leak-off experiment, the pressurization history of a fracture face was imitated. At one axial face of the core fluid pressure was applied for about the propagation time of the hydraulic fracture, after which the pump was stopped and the pressure dropped. From the pressure drop rate the leak-off rate can be calculated using the system compressibility. By making the decompression volume small, a similar initial pressure decline rate as in the hydraulic fracturing experiments was achieved.