Seismic Uncertainty Research Articles

Reducing seismic positioning uncertainty with “simple” tilted transverse isotropy earth models — Gulf of Mexico Lobster field case study

The remaining potential in many mature fields in the Gulf of Mexico consists of smaller long-reach targets that require ever-more-accurate vertical and horizontal location for drilling precision. This is particularly true at the Lobster field in Ewing Bank, where, during the previous platform drilling campaigns, some midreach prospects were interpreted to have been missed due to mispositioning. Potential future drilling at the Lobster field would target primarily long-reach prospects in areas of significant geologic dip — increasing the risk of missing the prospects. This mispositioning risk could sometimes exceed the geologic risk, and it could have significant impact on the risked reserves and on the economics. Achieving the desired positioning accuracy requires the use of 3D seismic and anisotropic prestack depth migration with accurate velocity and anisotropy models. We have developed a case study of using a long offset walkaway checkshot and a geologic frame of reference to build a tilted transverse isotropy velocity model sufficiently accurate to support potential future drilling campaigns at the Lobster field.

Non-Classical Probability Seismic Reliability Based on Pushover Analysis

In this paper, the probabilistic model is used to quantify the uncertainty of structural resistance, and the convex model is used to quantify the seismic uncertainty. The distribution parameters for the probabilistic model, together with the interval range for the convex model, are obtained through pushover analysis. Two-level function equation method is employed to calculate the seismic reliability of the structure. Case study shows that compared with the classical probability method, the proposed method is more simple and reasonable for seismic reliability analysis.

Prioritisation methodology for application of bridge monitoring systems for quick post-earthquake assessment

To facilitate quick post-earthquake assessment of bridge condition, monitoring systems can be installed onto structures. However, due to high cost it is impractical to monitor all bridges within a network. Bridges which are exposed to increased hazards, are vulnerable and have high failure consequences pose the greatest risk to network functionality should they fail in a seismic event, and would therefore benefit the most from implementation of monitoring systems and quick condition assessment methodologies. This paper outlines a methodology to prioritise bridges for monitoring and quick condition assessment based on their seismic risk. The methodology uses four factors to determine risk, i.e. seismic hazard, vulnerability, failure impact and uncertainty of available data and assessment methods. The hazard factor accounts for the seismicity levels at bridge sites and length of time of exposure to hazard. Structural and geotechnical aspects have been combined to determine the vulnerability of each bridge. Impacts quantify the consequences of bridge failure on safety and network functionality. The uncertainty premium accounts for the quality, variability and limitations of data and risk assessment methods used. The overall risk calculated for each bridge within a stock enables prioritisation of structures for monitoring and quick post-disaster assessment. The whole spectrum of approaches to bridge monitoring and condition evaluation comprises bridge specific monitoring data used for quick and accurate analyses for the most critical, high risk bridges; data sourced from wide-area strong motion arrays used for quick but less accurate assessment for medium risk structures; and traditional visual inspection based assessment of low risk bridges. A discrete scoring system was adopted and detailed tables that enable scoring the hazards, vulnerabilities, impacts and data and assessment uncertainties developed. The proposed methodology was applied to a selection of bridges from the city of Wellington, New Zealand to test its applicability and performance. A comparative study with another seismic risk assessment method was also conducted. The results showed that the methodology effectively prioritised bridges depending on seismic risk. The methodology was also able to determine if risk at a particular bridge site was predominantly related to hazard, structural vulnerability, geotechnical vulnerability or impact. The methodology is simple, quick and flexible and can be adapted based on the level of accuracy required. The uncertainty premium allows risk to be determined given variable data and assessment method quality which has the benefit of being able to tailor data collection and assessment to the needs of each network and available resources.

Assessment of seismic collapse uncertainties of steel moment-resisting frames by means of single-degree-of-freedom systems

The collapse evaluations presented to date are of various approximations, such as significant degrees of uncertainty in ground motion characterization and structural collapse modeling. Accordingly, determining the variance of collapse capacity has an important role in evaluation of the seismic risk of structures. However, its process includes time-consuming steps, for example, incremental dynamic analysis and nonlinear dynamic collapse analyses of the archetype models. In order to save time and preserve accuracy, this study attempts to assess the seismic collapse uncertainty of single-degree-of-freedom systems for evaluating the same uncertainties in steel moment frames. A combination of extended incremental dynamic analysis and Latin hypercube sampling method was utilized to study the epistemic and aleatory effects on multi-story steel moment frames (3-, 5-, 8- and 15-story frames) as well as single-degree-of-freedom models. The results indicated that due to aleatory and epistemic uncertainties, the variance of collapse capacity in three-, five- and eight-story frames was close to the results of single-degree-of-freedom systems in which P-Δ effects were assumed as small, whereas the variance of collapse capacity of a 15-story frame ( T1>2 s) was compatible with the results of single-degree-of-freedom systems when P-Δ effects were considered large.

Walkabove VSP with seismic-while-drilling technology: flawless operation in a highly deviated well

SWD was recently used in the North West Shelf of Australia to acquire time-depth measurements and to obtain a vertical seismic profile (VSP) while pulling out of hole. The use of SWD technology greatly enhanced the understanding of geology by acquiring a more precise geophysical picture of the subsurface, leading to better understanding of the subsurface and placement of wells in the future. A vertical incidence VSP was acquired in an offshore deviated well for a client on the Australian North West Shelf. The data was acquired using a moving-surface source, suspended from a boat, and a four-component downhole sensor in the bottom hole assembly (BHA). The downhole data was acquired using three orthogonal geophones and a single hydrophone measurement at each VSP level. This was recorded while pulling out of hole, and processed once the tool was on surface. Time picking accuracy of the downhole data is ±0.5 ms with the frequency range 6–90Hz, both comparable to Wireline. The repeatability of the hydrophone and geophone time picks was also excellent with the average difference being 0.2 ms and maximum 0.8 ms. High resolution VSP images beneath the well in addition to corridor stacks were derived from the geophone and hydrophone data, showing the geological structure of the reservoir. The quality of the data acquired allowed the client to remove the need for a wireline VSP run; it, therefore, saved significant rig time and costs associated with it, reduced the chances of getting stuck, and significantly reduced the seismic uncertainty.

On the likelihood of post-perovskite near the core–mantle boundary: A statistical interpretation of seismic observations

Recent experimental studies indicate that perovskite, the dominant lower mantle mineral, undergoes a phase change to post-perovskite at high pressures. However, it has been unclear whether this transition occurs within the Earth’s mantle, due to uncertainties in both the thermochemical state of the lowermost mantle and the pressure–temperature conditions of the phase boundary. In this study we compare the relative fit to global seismic data of mantle models which do and do not contain post-perovskite, following a statistical approach. Our data comprise more than 10,000 Pdiff and Sdiff travel-times, global in coverage, from which we extract the global distributions of dlnVS and dlnVP near the core–mantle boundary (CMB). These distributions are sensitive to the underlying lateral variations in mineralogy and temperature even after seismic uncertainties are taken into account, and are ideally suited for investigating the likelihood of the presence of post-perovskite. A post-perovskite-bearing CMB region provides a significantly closer fit to the seismic data than a post-perovskite-free CMB region on both a global and regional scale. These results complement previous local seismic reflection studies, which have shown a consistency between seismic observations and the physical properties of post-perovskite inside the deep Earth.

Geological factors effecting compartmentalization of Rotliegend gas fields in the Netherlands

Abstract Since the discovery of the Groningen field, fifty years ago, more than 250 gas fields have been discovered in the Netherlands. A study of most of these fields shows that connected volumes are often smaller than expected from volumetric evaluation. Seismic uncertainty can often be argued as an explanation for the discrepancy, but there may be also a geological explanation. Most gas fields are found in the Permian Rotliegend and good drainage is expected because the reservoirs are relatively thick and homogeneous. Minor faulting is often thought to cause drainage problems, but this study shows it is likely in a number of cases. Twenty three Rotliegend fields with connectivity problems have been studied in part of the Dutch offshore. This analysis outlines a number of regions in the Rotliegend fairway that share a risk for fault seal and therefore reduced connected volumes. Fault seal analysis is important to understand compartmentalization but does not explain all discrepancies. In a number of regions, fault seal analysis is unsuccessful because cataclastic sand-to-sand sealing strike–slip faults are difficult to detect. Stratigraphic compartmentalization is seen to play a role in some well defined areas of the basin. A better understanding of the connectivity problems can highlight areas attractive for appraisal and near field exploration.

Simulating migrated seismic data by filtering an earth model: A implementation

An earth model is used in a collaborative environment in which some members provide information for its construction and others utilize the result. Validating an earth model by simulating a migration image is an important step. However, the high computational cost of generating 3D synthetic data, followed by the process of migration, limits the number of scenarios that can be validated. To overcome this computational cost, a novel strategy is used where a migration image is simulated by filtering a model with a spatial resolution filter. One of the key properties of this approach is that the model that describes a target-zone is decoupled from the macro-velocity model that is used to compute the spatial resolution filters. Consequently, different models can be filtered with the same resolution filter. For a horizontally layered medium, the Gazdag phase-shift operators are used to construct a common-offset spatial resolution filter to simulate the phase of 2D primary reflection data. To approximate a spatial resolution filter in a laterally variant medium, ray trace information is used as an illumination constraint. Additionally, the influence of seismic uncertainties on the shape of a spatial resolution filter and the resulting migration image are simulated. These filters enhance an iterative earth modeling approach.

Thermochemical interpretation of 1‐D seismic data for the lower mantle: The significance of nonadiabatic thermal gradients and compositional heterogeneity

Equation‐of‐state (EOS) modeling, whereby the seismic properties of a specified thermochemical structure are constructed from mineral physics constraints, and compared with global seismic data, provides a potentially powerful tool for distinguishing between plausible mantle structures. However, previous such studies at lower mantle depths have been hampered by insufficient evaluation of mineral physics uncertainties, overestimation of seismic uncertainties, or biases in the type of seismic and/or mineral physics data used. This has led to a wide, often conflicting, variety of models being proposed for the average lower mantle structure. In this study, we perform a thorough reassessment of mineral physics and seismic data uncertainties. Uncertainties in both the type of EOS, and mineral elastic parameters, used are taken into account. From this analysis, it is evident that the seismic variability due to these uncertainties is predominantly controlled by only a small subset of the mineral parameters. Furthermore, although adiabatic pyrolite cannot be ruled out completely, it is problematic to explain seismic velocities and gradients at all depth intervals with such a structure, especially in the interval 1660–2000 km. We therefore consider a range of alternative thermal and chemical structures, and map out the sensitivity of average seismic velocities and gradients to deviations in temperature and composition. Compositional sensitivity is tested both in terms of plausible end‐member compositions (e.g., MORB, chondrite), and via changes in each of the five major mantle oxides, SiO2, MgO, FeO, CaO, and Al2O3. Fe enrichment reduces both P and S velocities significantly, while Si enrichment (and Mg depletion) increases P and S velocities, with a larger increase in P than in S. Using purely thermal deviations from adiabatic pyrolite, it remains difficult to explain simultaneously all seismic observations. A superadiabatic temperature gradient does improve the seismic fit in the lowermost mantle, but should be accompanied by concurrent bulk chemistry changes. Our results suggest that the most plausible way to alter bulk chemistry in the lowermost mantle, simultaneously fitting density, bulk velocity and shear velocity constraints, is an increasing contribution of a hot, basalt‐enriched component with depth.

Analysing exploration uncertainties by tight integration of seismic and hydrocarbon migration modelling

High success rates in the exploration for oil and gas can be achieved by incorporating best practice strategies in estimating the geological uncertainties from seismic data and geological models. A tight coupling of the seismic information and hydrocarbon migration modelling may allow for a consistent handling of uncertainties throughout the analysis. Seismic uncertainties include velocities used for depth conversion, the positioning of structural interpretations including faults, and assigning flow properties from seismic attributes. The necessary output from the seismic uncertainty analysis could be provided as uncertainty fields that may be used as input to hydrocarbon migration modelling. The nonlinearity of the hydrocarbon migration process, including flow-path selection and trap phase filtering, frequently requires that local uncertainties are addressed in more detail. These uncertainties may require high-resolution seismic analyses in order to be further resolved. The structural interpretation is of particular importance to accumulation uncertainties, and fault juxtaposition and carrier dip direction uncertainty estimations can improve phase prediction estimates in exploration prospects. It is suggested that an improved best practice for exploration for oil and gas should include a tight coupling of the handling of uncertainties from seismic data in hydrocarbon migration modelling.

Simulating migrated and inverted seismic data by filtering a geologic model

The simulation of migrated and inverted data is hampered by the high computational cost of generating 3D synthetic data, followed by processes of migration and inversion. For example, simulating the migrated seismic signature of subtle stratigraphic traps demands the expensive exercise of 3D forward modeling, followed by 3D migration of the synthetic seismograms. This computational cost can be overcome using a strategy for simulating migrated and inverted data by filtering a geologic model with 3D spatial-resolution and angle filters, respectively. A key property of the approach is this: The geologic model that describes a target zone is decoupled from the macrovelocity model used to compute the filters. The process enables a target-orientedapproach, by which a geologically detailed earth model describing a reservoir is adjusted without having to recalculate the filters. Because a spatial-resolution filter combines the results of the modeling and migration operators, the simulated images can be compared directly to a real migration image. We decompose the spatial-resolution filter into two parts and show that applying one of those parts produces output directly comparable to 1D inverted real data. Two-dimensional synthetic examples that include seismic uncertainties demonstrate the usefulness of the approach. Results from a real data example show that horizontal smearing, which is not simulated by the 1D convolution model result, is essential to understand the seismic expression of the deformation related to sulfate dissolution and karst collapse.

Bayesian time-lapse inversion

A new, fast inversion approach for time-lapse seismic data is developed where the uncertainty of the inversion results is an integral part of the solution. The inversion method estimates changes in the elastic material properties of a reservoir because of production of hydrocarbons, including uncertainty bounds on these estimates. The changes in elastic properties then can be related to changes in hydrocarbon saturation and reservoir pressure by using rock-physics relations. The inversion operates directly on the difference between a repeat survey and a baseline survey. This is advantageous with respect to the uncertainty calculation, because an estimate of the seismic uncertainty can be obtained directly from the difference data in zones not affected by production. The method is formulated in a Bayesian setting, and the solution is represented by explicit expressions for the posterior expectation and the covariance of the elastic parameter changes. The explicit analytical form of the posterior distribution provides a computationally fast inversion method. Results of the applied approach to a real data set from the Norne field are consistent with the expected effects of water flushing because of water injection.

Assessing and using seismic amplitude and acoustic impedance uncertainty in Saudi Arabia

In Saudi Arabia, seismic data have played an indispensable role in exploration and production. The most significant impact has been structure mapping via traveltime information. However, there has also been successful exploitation of amplitude information; e.g., determination of lithology, fractures, and porosity. A desirable goal is to make more use of all of the seismic information, realizing that this goal is complicated by the fact that the seismic data and reservoirs in Saudi Arabia are primarily in challenging land environments.

Reply to "Comment on 'Improving Sparse Network Seismic Location with Bayesian Kriging and Teleseismically Constrained Calibration Events,' by Stephen C. Myers and Craig A. Schultz," by A. Douglas

We thank Alan Douglas (Douglas, 2004) for his thoughtful comments on Myers and Schultz (2000). In Myers and Schultz (2000) we improve seismic location accuracy and uncertainty estimates for a sparse, regional network by improving travel-time predictions and better characterizing residual uncertainties. Station-specific, travel-time corrections and associated uncertainty estimates are calculated using the Bayesian kriging methodology of Schultz et al. (1998). The Bayesian kriging methodology is an empirical approach that exploits the spatial correlation of travel-time residuals to interpolate an empirical (calibration) set of travel-time residuals into a minimum-variance, geographically continuous surface of travel-time corrections and an associated uncertainty surface. In Myers and Schultz (2000) we test the Bayesian kriging approach by relocating events in the 1991 Racha, Georgia earthquake sequence (∼ 42°N, 43°E). We compare uncalibrated locations (based on P -wave travel times for the ak135 model; Kennett et al. , 1995) and calibrated locations (calibrated travel-times and uncertainties) to benchmark locations that are constrained with a dense, local-distance network. The calibration data set consists of event locations that are constrained by a teleseismic network. One goal of the study was to assess the utility of teleseismically constrained events for calibration. We find that better travel-time prediction for regional paths can be achieved by using the teleseismically constrained calibration events, and the improved travel-time prediction improves regional-network location accuracy. A major advantage of Bayesian kriging over many other interpolation techniques is the propagation of uncertainty from the calibration data set into a travel-time-prediction uncertainty surface. Sources of data set uncertainty include calibration-event locations, phase observations for calibration events, and calibration-event spacing. We find that coverage ellipses (Evernden, 1969) are representative of observed epicenter accuracy (i.e., confidence level of the coverage ellipse is indicative of the number of occurrences of the benchmark location within the ellipse) when the …

Inferring upper-mantle temperatures from seismic velocities

Constraints on mantle temperatures are fundamental for a better understanding of the dynamics of the Earth. We evaluate how well upper-mantle thermal structure can be inferred from seismic velocities. Our forward calculation of seismic velocities and density includes uncertainties in anharmonic and anelastic parameters for various compositional models. The sensitivity of V P ( V S) to temperature along a 1300 °C adiabat decreases with depth from −0.75±0.15% (1.3±0.30%) per 100 °C at 200 km, to −0.23±0.05% (−0.45±0.10%) per 100 °C at 800 km, with an additional reduction in the wadsleyite stability zone. Shear anelasticity introduces a significant non-linear dependence of seismic velocities on temperature in the upper-mantle. This means that seismic anomalies with similar amplitudes but different signs correspond to temperature anomalies of a different magnitude, and absolute seismic velocities are required for making a thermal interpretation. With depth, the importance of anelasticity decreases. Temperature derivatives of bulk sound velocity ( V φ ) and density ( ρ) are a factor two to five lower than those of V P,S and do not depend strongly on temperature. Common mantle compositions cause velocity anomalies of about 1% throughout the upper-mantle. Above 400 km depth, the effect of composition on V P and V S is secondary to the effect of temperature, but it gains importance with increasing depth. For most upper-mantle compositions and depths, the relative sensitivity of V P, V S and V φ to temperature and composition is not different enough to distinguish the two factors from combined velocity models. But ∂ln ρ/ ∂ln V S allows a separation of thermal anomalies from compositional anomalies for iron-depleted and subducted material. Spherically symmetric reference profiles underlie most seismic velocity models. However, small differences between common one-dimensional velocity models translate into disparate thermal interpretations. A better understanding of seismic uncertainties and the physical interpretation of reference models is necessary to interpret absolute velocities. Assuming seismic structure is well resolved and composition known, uncertainties in inferred thermal structure are ±100 °C above 400 km depth and ±250 °C in the shallow lower-mantle. This makes an inversion of seismic velocity models for temperature feasible, above 400 km depth. At larger depths, testing seismic velocities calculated for a proposed thermal and compositional structure against observations is a more promising interpretation approach.

Blenheim Field: the Appraisal of a Small Oil Field With a Horizontal Well

Blenheim is a small Palaeocene oil field ( c. 53 × 10 6 BBL oil in-place), in which significant seismic uncertainty complicated a development decision. Initial appraisal plans required a conventional deviated well sited to minimize the uncertainty, gather further reservoir and fluid data, and, if successful, provide a production well. Later studies indicated improved economics if horizontal wells were successful. Several well designs, with and without pilot holes, were analysed to discover the optimum balance of data gathering, risk reduction and development cost reduction. The optimum well required accurate well steering in an area of seismic uncertainty. A high-resolution biostratigraphic study was made and provided detailed correlation of intra-reservoir mudstones, enabling wellsite analysis to aid steering decisions. The pilot hole proved the most likely seismic interpretation to be correct and permitted the horizontal well to be drilled as planned. Wellsite biostratigraphic analysis and real-time MWD aided steering decisions and contributed to a successful horizontal well being drilled. The well flowed in excess of the minimum economic rates required and allowed the field to be successfully developed.

Re-Engineering Simulation: Managing Complexity and Complexification in Reservoir Projects

Summary Managing complexity and technological complexification is a necessity in today's business environment. This paper outlines a method to increase value addition significantly by multidisciplinary reservoir studies. In this context, value addition refers to a positive impact on a business decision. The approach ensures a level of complexification in line both with business questions at hand and the realities of reservoirs. Sparse well control, seismic uncertainties, imperfect geologic models, time constraints, software viruses, and computing hardware limitations represent some common reservoir realities. The process model detailed in the paper uses these apparent shortcomings to moderate (i.e., guide) the level of complexification. Several project examples illustrate the implementation of the process model. The paper is an extension of three previous investigations1–3 that deal with issues of method and uncertainty in reservoir-performance forecasting.

Interactive Visual Identification of a Reservoir Boundary Integrating Seismic and Well-Test Data

Summary We describe an interactive system designed to allow users to identify a range of possible reservoir boundaries given a seismic subsurface image and well-test information. Traditionally, data from these two sources are considered separately. This system was developed to interpret both forms of information simultaneously. This is achieved by displaying the seismic image, allowing the user to characterize the seismic uncertainty resulting from the reservoir delineation (i.e., by selecting inner and outer reservoir boundary limits), choosing an intermediate reservoir boundary, running a fluid-flow simulation, and comparing the results with the well-test data. In addition, the system incorporates a multiwindowing interface and has been developed for use by noncomputer specialists.