More than 50000 tons of CO 2 have been injected at Ketzin into the Stuttgart Formation, a saline aquifer, at approximately 620 m depth, as of the summer 2011. We present here results from the 1 st repeat 3D seismic survey that was performed at the site in the autumn 2009, after about 22000 tons of CO 2 had been injected. We show here that rather complex time-lapse signatures of this CO 2 can be clearly observed within a radius of about 300 m from the injection well. The highly irregular amplitude response within this radius is attributed to the heterogeneity of the injection reservoir. Time delays to a reflection below the injection level are also observed. Petrophysical measurements on core samples and geophysical logging of CO 2 saturation levels allow an estimate of the total amount of CO 2 visible in the seismic data to be made. These estimates are somewhat lower than the actual amount of CO 2 injected at the time of the survey and they are dependent upon the choice of a number of parameters. In spite of some uncertainty, the close agreement between the amount injected and the amount observed is encouraging for quantitative monitoring of CO 2 storage site using seismic methods.

A 3D 25-fold seismic survey with a bin size of 12 by [Formula: see text] and about [Formula: see text] of subsurface coverage was acquired in 2005 near a former natural gas storage site west of Berlin, as part of the five-year EU funded [Formula: see text] project. Main objectives of the seismic survey were to verify earlier geologic interpretations of structure based on vintage 2D seismic and borehole data and to map, if possible, the reservoir pathways in which the [Formula: see text] will be injected at [Formula: see text] depth, as well as providing a baseline for future seismic surveys and planning of drilling operations. The uppermost [Formula: see text] are well imaged and show an anticlinal structure with an east-west striking central graben on its top that extendsdown to the target horizon. About [Formula: see text] of throwis seen on the bounding faults. No faults are imaged near the planned drill sites. Remnant gas, cushion and residual gas, is present near the top of the anticline in the depth interval of about [Formula: see text] and has a clear seismic signature; both higher amplitudes in the reservoir horizons and velocity pulldown are observed. Amplitude mapping of these remnant gas horizons shows that they do not extend as far south as the injection site, which is located on the southern flank of the anticline. Amplitude anomalies, gas chimneys along an east-west striking fault, show that the stored or remnant gas either has been or is presently migrating out of the reservoir formations. Summed amplitude mapping of the planned injection horizon indicates that this lithologically heterogeneous formation may be more porous at the injection site.

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A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of $ 300-1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and elect...

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We developed a method for joint inversion of seismic refraction and resistivity data, using sharp-boundary models with few layers (typically three). We demonstrated the usefulness of the approach via examples from near-surface case studies involving shallow groundwater exploration and geotechnical investigations, although it should also be applicable to other types of layered environments, e.g., sedimentary basins. In our model parameterization, the layer boundaries were common for the resistivity and velocity distributions. Within the layers, only lateral variations in the material parameters (resistivity and velocity) were allowed, and we assumed no correlation between these. The inversion was performed using a nonlinear least-squares algorithm, using lateral smoothing to the layer boundaries and to the materialparameters. Depending on the subsurface conditions, the smoothing can be applied either to the depth of the layer boundaries or to the layer thicknesses. The forward responses and Jacobian for refraction seismics were calculated through ray tracing. The resistivity computations were performed with finite differences and a cell-to-layer transform for the Fréchet derivatives. Our method performed well in synthetic tests, and in the case studies, the layer boundaries were in good agreement with in situ tests and seismic reflection data, although minimum-structure inversion generally has a better data fit due to more freedom to introduce model heterogeneity. We further found that our joint inversion approach can provide more accurate thickness estimates for seismic hidden layers.

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the ∼6.5 km 2 survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wideangle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial.The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

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