73rd EAGE Conference and Exhibition incorporating SPE EUROPEC 2011

We present here a joint inversion method to build P- and S-wave velocity models from surface-wave and P-wave refraction data, specifically designed to deal with laterally varying layered environments which can present strong velocity contrasts with depth. In this case, a smooth minimum-structure inversion produces smooth models even for geological models, which are overall layered. The proposed algorithm is also able to incorporate a-priori information available over the site and any physical law to link model parameters. This method presents advantages with respect to individual surface wave analysis and refraction tomography since it imposes internal consistency for all the model parameters, reducing the required a-priori assumptions and the ill-ness of the two methods. We describe the algorithm and we show its application to synthetic and field datasets.

There are often difficulties in adequately mapping spatial distribution of soil properties at small scale due to soil variations. Gamma-ray spectrometry is a new technique that can potentially address this while improving the mapping of soil texture and available potassium (avK) and minimising huge soil survey cost and maximizing economic returns. To achieve this, secondary spatial information is incorporated into the study. Using portable Exploranium GR 320 gamma ray spectrometer, concentrations of 40K, 238U and 232Th in sandstone washed materials were measured in three plots while representative soil samples were analysed. The result showed homogeneity in on-ground γ-rays concentrations derived from γK based on the geo-lithological nature of soils. The recorded γK values and total gamma ray intensities were related to the soil texture and avK properties. Strong linear relationships were identified between γK and avK (r2 = 0.78, P < 0.001); and soil texture (r2 = 0.48 to 0.67). The success of γ-ray spectrometry in predicting avK and soil texture classes relied on this strong relationship that transformed γK map into a 0.2m resolution soil property map by regression analysis, associated with 78% variance in avK and 51% clay.

The GPR was applied on areas with laterite deposits and bauxite occurrences to see its capability of discriminate the different weathered horizons and detect the presence of bauxite horizon and also measure its width. The GPR was tested on two bauxite occurrences in Pará State, Brazil: Paragominas Region and Trombetas River. The good quality of the radargrams was expected since the electric resistivity of the soil, evaluated by Vertical Electrical Sounding which reveals a resistivity varying from 450 to 3000 Ohm.m. The GPR data acquisition with constant-offset results in clear GPR sections where it was possible to separate four zones or horizons as well as its thicknesses. Each zone shows a different reflection pattern or radarfacies, which helps to differentiate them. The first horizon corresponds to the Belterra Clay, which is the most superficial layer, (AT); below, there is the laterite sequence composed by more three horizons, where the second one is a diffractions zone, which corresponds to a clay with concretionary laterites (ND); the third zone shows a homogeneous pattern and it corresponds to a massive ferruginous horizon (LT); an fourth zone shows discontinuous and small reflectors and/or chaotic pattern which corresponds to the main bauxite horizon (BX).

ated the ability to handle high-resolution simulations of seismic wave propagation in 3D complex domains. However, the exponential accuracy of the method and the reduction of the computational effort rely on the use of conformal hexahedral meshes. Generating an all-hexahedral mesh based upon the available meshing can be difficult. We propose to use a 3D unstructured tetrahedral mesh generator and then split the resulting mesh into hexahedral elements. This approach allows using the same modeling SEM program without any modifications, and it remains faster than dealing with tetrahedral spectral element due to the tensorisation property for spatial derivatives. With this approach, the SEM method doesn’t have restrictions to solving complex problems in sonic or seismic data with respect to methods using tetrahedral meshes. Two examples dealing, respectively, with sonic modeling and the other with surface seismic modeling illustrate the feasibility of the proposed method.

We studied and developed an efficient method to handle the wave simulation in presence of topography based on curvilinear finite difference. Foremost, we derived the modified equations on the continuous problem. We used afterwards optimized stencils and optimized selective filters adapted from aeroacoustics.

Acoustic finite-difference modelling is playing an increasingly important role in seismic imaging (e.g. in reverse time migration) but the additional cost of elastic finite-difference modelling restricts its use in commercial imaging technology. The cost of full elastic finite-difference modelling can exceed the cost of acoustic modelling in the same velocity model by two orders of magnitude or more. A technique is discussed that corrects an acoustic finite-difference simulation for elastic effects. It is based on estimating the error incurred when using the acoustic wave equation as an approximate solution to the elastic wave equation. The errors are used to generate an effective source field for an additional acoustic simulation that calculates a correction to the wavefield produced in the original acoustic simulation. The cost of this approach is roughly twice that of an acoustic simulation and therefore much less than that of a full elastic simulation. Plane wave theory is used to examine the properties of the method and results from finite-difference simulations are presented to illustrate its accuracy.

Blast damage to coal seams due to incorrect blast stand-off distances costs the equivalent of one unnecessary open cut mine for every 10 existing Australian mines. The current approach for mapping coal seam tops is through drilling and pierce-point logging. To provide appropriate depth control with ±0.2 m precision for blast hole drilling, a 50m x 50m grid is required in advance of overburden removal. Pierce-point mapping can be inaccurate and expensive as the seam is not flat. To date, there are no effective and economic techniques to map coal seam structures in the open cut environment where coal seams are shallow. In this paper, we propose two Seismic-While-Drilling (SWD)-based approaches to see the top of the coal seam ahead of the drill-bit: 1) reverse VSP recording, in which the geophones are planted on the surface near the drill rig; 2) in-seam seismic recording, in which the geophones are locked in the coal seam in a borehole. Numerical modeling demonstrates the feasibility of these two approaches for predicting the approach of coal seams during blasthole drilling. If these methods can be reliably realized in practice, significant coal resources can be saved and the mines can become more productive and profitable.

We compare time-lapse sonic and neutron porosity logs of the Nagaoka CO2 sequestration experiment against the uniform and patchy saturation models, which represent two end-members of the P-wave velocity and CO2 saturation relationship. Most of the data points fall between the two limits, suggesting that the relationship is somewhere between uniform and patchy saturation. The behaviour between these limits can be explained by the mechanism of wave-induced-fluid on mesoscopic fluid heterogeneities (porescale << mesoscale << P-wavelength), which causes wave attenuation and velocity dispersion. We model these fluid effects using the 1D and 3D continuous random media model (CRM). The log data approximately follow the predictions of the CRM models for fluid patch sizes of 1 to 5 mm. This heterogeneity scale is much larger than the porescale features of a reservoir thin section, indicating that the mechanism of wave-induced fluid flow on the mesoscale can occur in sonic log data and therefore controls the velocity-saturation relation.

The electrical resistivity of sedimentary rocks depends strongly on pore fluid content and pore connectivity, and bulk resistivity increases where carbon dioxide displaces conductive saline pore fluid. Therefore, controlled-source electromagnetic (CSEM) methods are potentially a key geophysical technique for exploring and monitoring carbon sequestration sites. We investigate the feasibility of CSEM characterization of the CO2 sequestration pilot site at Ketzin, Germany. Simulation studies using newly developed, accurate 1D and 3D modelling tools indicate that surface-based CSEM surveys may well be able to image the background resistivity structure, but will probably not detect the small CO2 reservoir. Borehole-to-surface configurations exhibit much higher sensitivity to the reservoir and may thus be particularly well-suited for monitoring applications. A recently conducted large-scale CSEM field survey has demonstrated the feasibility of CSEM measurements at the site, and provided data that contain subsurface responses down to the target reservoir depth.

Within the European project CO2ReMoVe traveltime and attenuation tomography have been applied on one of the high resolution seismic line acquired in 2006 on the Sleipner CO2 geological storage site. The aim is to gain an accurate model in depth, of both velocities and attenuation, to constrain better the quantification studies. The results obtained from the tomographyc inversion of traveltimes and spectral-centroid frequency shift, respectively, confirm the cumulation of CO2 in the uppermost layer of the reservoir, whereas within the plume, lateral variations of both velocity and quality factor values are observed. The presence of fine shale lenses, and their impact on the CO2 distribution within the plume may be the reason for these lateral variations. Through the comparison of the velocity and Q tomographic values with the theoretical curves from petrophysics studies, it will be possible to interpretate them in terms of effective variations in CO2 saturation and heterogeneities within the plume.