Summary Reliable detection of weak signals is potentially a fundamental limitation for microseismic event detection and location, especially for sensors in the near-surface region, where large source distances and high noise levels affect the signal-to-noise ratio (SNR). Previously, a nonlinear stacking method was introduced that could increase the SNR for weak signals and included methods that are insensitive to changes in signal polarity. This paper introduces nonlinear filtering methods that are a generalization of the nonlinear stacking method where the stacking operation is replaced by a filtering operation. This method keeps the SNR and polarity benefits of the stacking methods, but we can now choose to modify the pass and reject bands. For instance, we can improve the SNR for weak signals, with polarity changes, and allow additional signal misalignment criteria to be included in the passband. We will show synthetic-signal and real-signal examples where the new method has clear benefits compared to conventional stacking or other nonlinear stacking methods.

Summary In the context of surface microseismic processing, a nonlinear stack method, the phase weighted nth root stack, was evaluated and benchmarked against linear stack. From a synthetic analysis, the parameter choice was evaluated to improve the detectability of small amplitude microseismic events. The choice and impact of small exponent values for the nonlinear stack method were confirmed on one stage of a multiwell, multistage hydraulic fracturing in the Marcellus shale formation. Compared to the linear stack, up to 30 % more events were detected and located for this data example.

Summary This abstract presents a case study where an anisotropic velocity model for surface microseismic monitoring is obtained from travel time inversion using P- and SV-wave arrivals of microseismic events that are observable on the surface records. Additionally, we correct for statics using cross-correlation of the P-wave arrivals of the same events. The calibration of the velocity model proves robust to uncertainties in the locations of the reference events. The comparison to event locations derived with an isotropic velocity model results in negligible differences in the epicentre and comparable depths of events. Finally, the application of static corrections shifts the event locations while increasing the maximum stacked energy, suggesting improvement in location accuracy. The findings emphasize the relative importance of near-surface effects over the importance of anisotropy and heterogeneity for this particular case study.

Summary Noise attenuation is a key challenge for surface-acquired microseismic processing. A number of data conditioning tools have been proposed and applied with various degrees of success to improve the signal-to-noise ratio prior to detection and location of microseismic events. Random noise attenuation, trace-by-trace correlation with a large magnitude event, and nonlinear stacking techniques have all been shown individually to improve microseismic event detectability in surface-acquired microseismic datasets. This paper demonstrates how the combination of these approaches significantly increases the number of detected microseismic events while keeping the number of false triggers to a minimum. In particular, random noise attenuation and trace-by-trace correlation with a large magnitude event followed by nonlinear stacking at the stage of substack generation provide a data conditioning workflow that significantly attenuates the effects of statics, anisotropy, and, to some extent, 3D velocity variations. This work is a step towards an optimized data conditioning workflow for surface-acquired microseismic data.

Summary A near-surface velocity map is derived, using data from active shots, acquired in the frame of a surface microseismic survey, targeting the Fayetteville shale formation. This near-surface velocity map shows strong velocity variations with velocities ranging from a few hundreds of metres per second to approximately 1500 m/s. The visibility of the S-wave expression on the vertical component of surface geophones was investigated. It was found that only one-third of the events showed a visible S-wave arrival, and this arrival was below the level of noise for the other two-thirds of the events. When the S-wave is observed, it is seen preferentially in areas where near-surface velocities are larger and attenuation is lower. The use of only P-waves appears to be widespread in surface microseismic processing. This study suggests, through the derivation of a near-surface velocity map, that near-surface attenuation could be one factor justifying this approach.

Summary We have identified individual waveforms from microseismic events and tracked them from reservoir to surface using a wide aperture borehole seismic array, as well as across surface seismic lines. We noted that deeper arrivals in the long borehole array contained complex triplications that could potentially pose difficulties for event detection and location techniques that rely on identifying the direct arrivals. By modelling full waveform synthetics we were able to reconstruct the principal features of these complex arrivals for the long borehole array. We have developed an extension to the CMM approach to extract the appropriate arrival times via STA/LTA processing of the full waveform synthetics. These times are then used to augment the first arrival P and S travel times in the objective function used for CMM processing, allowing the energy in complex arrivals to be identified and beam-formed in the event detection algorithm. Mode converted arrivals may also be used in the subsequent Geiger relocation step to provide greater aperture with which to refine the event location.

Building a reliable yet cost-effective monitoring plan that all interested parties have faith in is a key challenge in carbon dioxide (CO2) geological storage projects. Several objectives need to be achieved: accurate CO2 and pressure front tracking, CO2 follow up in case of migration outside the primary reservoir. Many constraints need to be taken into account: legislation, geology, risks, costs, public perception. We present the scheduled monitoring plan of a candidate site in Spain, where 42 million tons of CO2 are planned to be injected over 30 years. The reservoir consists of high porosity/high permeability sandstone with some alternating shale packages, ~200 m thick, located at ~2200 m depth. It is overlaid by a carbonate formation, the secondary reservoir, and by a shaly primary seal. The secondary seal, mainly shaly with some alternating sand beds, goes almost up to surface.

Seismic observation has key advantages that, in the right geological conditions, make it extremely valuable for CO2 storage monitoring. Among surface techniques allowing the observation of the entire storage complex, it has by far the greatest vertical and lateral resolution and it can provide a full 3D view of the reservoir and overburden. However, in our experience, theoretical applicability of seismic acquisition for CO2 plume tracking has often been challenged by the geology: high rock stiffness, heterogeneity, large depths, or low porosity in a storage site are not favorable factors. Moreover, there is uncertainty on the minimum levels of detectability. CO2 might mix either homogeneously or create patches of variable saturation, and this can result in a large variability on the expected seismic signal, especially for saturations lower than 50%. We investigate the seismic signal expected for a location in the Gippsland Basin, Victoria, Australia. Using classical rock physics equations, CO2 detectability at the reservoir level will present some challenges, but the possibility for detection above the reservoir is quite favorable. An understanding of the relative heterogeneity of storage formations is critical to establishing uncertainty and detection limits of time-lapse seismic technology.

We describe here the interpretation done for the appraisal of a CO2 geologic storage site in a saline formation in the Duero basin, Spain. This interpretation was done in two phases: (1) using 2D data only, (2) using 2D data plus newly acquired 3D data. As a result, this project was ideal to observe what the added value of 3D data was: fault and horizon mapping were improved, formation thickness was better constrained, geologic prognosis was more accurate and small faults could be mapped in the overburden. We also discuss how our seismic interpretation had to be used in the frame of CO2 storage: risks on containment, compartmentalization, and structural uncertainty were better evaluated.

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The 174 km diameter Terby impact crater (28.0°S–74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6–10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km3 (a large amount considering classical martian fan deltas such as Eberswalde (6 km3)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential "reference geologic cross-section" for Mars stratigraphy.

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently undertaking the Otway Project, which involves the injection and storage of 100,000 tones of carbon dioxide within the subsurface. CO2 injection will be into Naylor onshore depleted gas reservoir and, therefore, the project will provide important experience for monitoring and verification under these conditions. The overall complexities of the field, its deep, small size and in particular the presence of both free and residual gas zones present a serious challenge for time-lapse seismic monitoring. Borehole seismic has a strong advantage over surface seismic: energy crosses surface layers only once, and hence is much less sensitive to the variations in the weathered layer properties than surface seismic. Consequently a comprehensive borehole seismic observational program was designed at the Otway project. In the initial phase a Zero Offset VSP, an Offset VSP, and walkaway VSP data were acquired with a minivibroseis (6000 lb) seismic source in the Naylor-1 well in May 2006. In 2007, in the newly drilled injection well (CRC-1) a series of wireline logs, a Zero Offset, an Offset VSP and the first 3DVSP was acquired with a weight drop source simultaneously with 3D surface seismic. The results of VSP data analysis will be shown and discussed in light of its improved repeatability and image resolution in comparison to surface seismic data. We will also discuss evolving workflows developed to overcome inherently poor land seismic repeatability which is mainly related to changes in the near surface layer conditions.

The Otway Project conducted under the Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is the first of its kind, where CO2 is injected into a depleted gas reservoir. The use of depleted fields for CO2 storage is likely to become widely adopted globally and, therefore, the project will provide important experience for monitoring under these conditions. However, such scenario is not favorable for the application of geophysical techniques for the purpose of CO2 monitoring and verification (M&V) because the injection of CO2 into a CH4 depleted reservoir is modeled to produce very subtle changes in elastic properties of the reservoir rock which may be very difficult to measure. Consequently geophysical program for the Otway site was design according to the expected time-lapse effects. It combines both surface and borehole seismic methods. Surface seismic should provide a global vision of the underground and an indirect confirmation of the CO2 containment by recording no differences between the successive time-lapse experiments. Vertical Seismic Profile (VSP) surveys are expected to provide an improved characterization of the reservoir and hopefully a direct indication of the fluid distribution and/or its potential upward migration along the reservoir bounding fault pattern. Indeed the results of the current analysis of both pre-base line (test) and base-line 2D and 3D VSP data are encouraging. The availability of vector wave field (three-component) data recorded in VSP surveys should significantly improve the outcomes of M&V program at Naylor site.

Abstract The CO2CRC Otway Project, in Victoria, Australia, is one of the first projects of CO2 storage in a depleted gas reservoir. CO2 injection in the sandstone reservoir, at a depth of 2,000 mSS, started in March 2008 with the objective to inject up to 100,000 tonnes of CO2 over two years. This study compares the level of predictability obtained with different cases depending on the initial data, using the same numerical compositional simulation package. We use recorded data (production and injection) to build a new numerical reservoir model. A dynamic model had already been built before the injection well started (Xu et al., 2006) and was validated by history matching using the gas production data reported. In this paper, we used the same updated static model (Dance et al. 2007) as used for the pre-injection model, which is based on the production data and the data obtained from the injection well (CRC-1). With this updated static model, a different dynamic model is built using injection data and through a newly developed simulator option, which better simulates the CO2-water behavior. The injection rate and pressure data from CRC-1 are now available and the actual breakthrough time – at which the CO2 plume reached the monitoring well (Naylor-1) located 300 m away from CRC-1 – can be history matched. Various relative permeability curves including new laboratory measurements performed on a core taken from the reservoir formation (Waarre C) were used. The results from the updated dynamic modeling using this measured relative permeability data are compared to results using data from literature. In general, experimental measurements for drainage and imbibition processes are not available This study gives a better understanding of the parameters which strongly influence simulated CO2 behavior. It shows the relation between the data availability and prediction reliability.