Methods based on the seismic P-wave, seismic surface wave, and apparent resistivity are commonly used in the solution of several near-surface problems. However, the solution nonuniqueness and the intrinsic limitations of these methods can cause inconsistency in the final results. Dispersion curves of surface waves, P-wave traveltimes, and apparent-resistivity data were jointly inverted to obtain internally consistent and more reliable final model of P-and S-wave velocities and resistivity. A collection of 1D layered models was obtained by a deterministic jointinversion algorithm based on the laterally constrained inversion scheme. The three data sets were jointly inverted imposing the same structure and Poisson's ratio was introduced as a physical link between P-and S-wave velocities to better constrain the inversion. No physical link was imposed between the resistivity and the seismic velocities. The inversion algorithm was tested on synthetic data and then applied to a field case, where benchmark borehole data were available. The synthetic and field examples provided results in agreement with the true model and the existing geologic information, respectively.

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Fracture networks inside geological CO 2 storage reservoirs can serve as primary fluid flow conduit, particularly in low-permeability formations. While some experiments focused on the geophysical properties of brine-and CO 2 -saturated rocks during matrix flow, geophysical monitoring of fracture flow when CO 2 displaces brine inside the fracture seems to be overlooked.We have conducted laboratory geophysical monitoring of fluid flow in a naturally fractured tight sandstone during brine and liquid CO 2 injection. For the experiment, the low-porosity, lowpermeability naturally fractured core sample from the Triassic De Geerdalen Formation was acquired from the Longyearbyen CO 2 storage pilot at Svalbard, Norway. Stress-dependence, hysteresis and the influence of fluid-rock interactions on fracture permeability were investigated.The results suggest that in addition to stress level and pore pressure, mobility and fluid type can affect fracture permeability during loading and unloading cycles. Moreover, the fluid-rock interaction may impact volumetric strain and consequently fracture permeability through swelling and dry out during water and CO 2 injection, respectively. Acoustic velocity and electrical resistivity were measured continuously in the axial direction and three radial levels.Geophysical monitoring of fracture flow revealed that the axial P-wave velocity and axial electrical resistivity are more sensitive to saturation change than the axial S-wave, radial P-wave, and radial resistivity measurements when CO 2 was displacing brine, and the matrix flow was negligible. The marginal decreases of acoustic velocity (maximum 1.6% for axial V p ) compared to 11% increase in axial electrical resistivity suggest that in the case of dominant fracture flow within the fractured tight reservoirs, the use of electrical resistivity methods have a clear advantage compared to seismic methods to monitor CO 2 plume. The knowledge learned from

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Abstract. Monitoring microseismic activity provides a window through which to observe reservoir deformation during hydrocarbon and geothermal energy production, or CO2 injection and storage. Specifically, microseismic monitoring may help constrain geomechanical models through an improved understanding of the location and geometry of faults, and the stress conditions local to them. Such techniques can be assessed in the laboratory, where fault geometries and stress conditions are well constrained. We carried out a triaxial test on a sample of Red Wildmoor sandstone, an analogue to a weak North Sea reservoir sandstone. The sample was coupled with an array of piezo-transducers, to measure ultrasonic wave velocities and monitor acoustic emissions (AE) – sample-scale microseismic activity associated with micro-cracking. We calculated the rate of AE, localised the AE events, and inferred their moment tensor from P-wave first motion polarities and amplitudes. We applied a biaxial decomposition to the resulting moment tensors of the high signal-to-noise ratio events, to provide nodal planes, slip vectors, and displacement vectors for each event. These attributes were then used to infer local stress directions and their relative magnitudes. Both the AE fracture mechanisms and the inferred stress conditions correspond to the sample-scale fracturing and applied stresses. This workflow, which considers fracture models relevant to the subsurface, can be applied to large-scale geoengineering applications to obtain fracture mechanisms and in-situ stresses from recorded microseismic data.