Evaluation of seismic events detection algorithms

Identification of seismic events from continuously recorded seismic data in real-time through a Digital Seismic Data Recording system is a difficult task. Despite the vast amount of research in this field, the signal processing and event parameters discrimination algorithms have not yet fully come of age. Presently, we have a wide spectrum of trigger algorithms, ranging from a very simple amplitude threshold type to the sophisticated ones based on pattern recognition approaches. Some of the other approaches use adaptive technique and neural network methods. Researchers are continuously making efforts for the development of algorithms using various techniques, which produce minimum false trigger. Some approaches have been reported which are accurate for detecting first phase of events and take minimum possible computational time. In this paper several approaches for detecting event signals in background noise are presented and their precision evaluation is discussed.     

Use of amplitude ratios in phase correlation of seismic waves

A discussion concerning the probability of variation in the amplitude correlation of the seismic wave extrema under the action of random interferences is presented. In the process of choosing a correlation version with the use of amplitude criteria, it was possible to pass from one phase to another in the result of variation in the amplitude relations between the signal extremes (under the action of interferences). This phenomenon was called here "the amplitude inversion. It was proved analytically that the probability of the amplitude inversion depended not only on the interference dispersion and the ratio of signal amplitudes, but also on the width of the band occupied by the energy spectrum of the interference. The probability of the inversions (especially of the near one) increased with the increase in the width of the interference spectrum. The concept of the "amplitude domination" (for the axes of the synchronous phases) is introduced, and used for the choice of the correlation variant in a set of permissible axes. -- Petroleum Abstracts.     

Linear regression models for estimating true subsurface resistivity from apparent resistivity data

Simple linear regression (SLR) models for rapid estimation of true subsurface resistivity from apparent resistivity measurements are developed and assessed in this study. The objective is to minimize the processing time and computer memory required to carry out inversion with conventional algorithms. The arrays considered are Wenner, Wenner–Schlumberger and dipole–dipole. The parameters investigated are apparent resistivity (\(\rho _a \)) and true resistivity (\(\rho _t\)) as independent and dependent variables, respectively. For the fact that subsurface resistivity is nonlinear, the datasets were first transformed into logarithmic scale to satisfy the basic regression assumptions. Three models, one each for the three array types, are thus developed based on simple linear relationships between the dependent and independent variables. The generated SLR coefficients were used to estimate \(\rho _t\) for different \(\rho _a\) datasets for validation. Accuracy of the models was assessed using coefficient of determination (\(R^{2})\), F-test, standard error (SE) and weighted mean absolute percentage error (wMAPE). The model calibration \(R^{2}\) and F-value are obtained as 0.75 and 2286, 0.63 and 1097, and 0.47 and 446 for the Wenner, Wenner–Schlumberger and dipole–dipole array models, respectively. The SE for calibration and validation are obtained as 0.12 and 0.13, 0.16 and 0.25, and 0.21 and 0.24 for the Wenner, Wenner–Schlumberger and dipole–dipole array models, respectively. Similarly, the wMAPE for calibration and validation are estimated as 3.27 and 3.49%, 3.88 and 5.72%, and 5.35 and 6.07% for the three array models, respectively. When compared with standard constraint least-squares (SCLS) inversion and Incomplete Gauss–Newton (IGN) algorithms, the SLR models were found to reduce about 80–96.5% of the processing time and memory space required to carry out the inversion with the SCLS algorithm. It is concluded that the SLR models can rapidly estimate \(\rho _t\) for the various arrays accurately.     

A simple method for estimating excess pressure over horsts from seismic sections

Observations in the North Sea Basin max indicate significant overpressure in sediments over horst blocks but not over grabens at the same submudline depth. The purpose is to show that over a horst, of top width W, with grabens on either side of top widths G₁ and G₂, respectively, the equivalent mud density. _r can be estimated approximately from the simple equation.

Application of EM algorithms for seismic facices classification

Identification of the geological facies and their distribution from seismic and other available geological information is important during the early stage of reservoir development (e.g. decision on initial well locations). Traditionally, this is done by manually inspecting the signatures of the seismic attribute maps, which is very time-consuming. This paper proposes an application of the Expectation-Maximization (EM) algorithm to automatically identify geological facies from seismic data. While the properties within a certain geological facies are relatively homogeneous, the properties between geological facies can be rather different. Assuming that noisy seismic data of a geological facies, which reflect rock properties, can be approximated with a Gaussian distribution, the seismic data of a reservoir composed of several geological facies are samples from a Gaussian mixture model. The mean of each Gaussian model represents the average value of the seismic data within each facies while the variance gives the variation of the seismic data within a facies. The proportions in the Gaussian mixture model represent the relative volumes of different facies in the reservoir. In this setting, the facies classification problem becomes a problem of estimating the parameters defining the Gaussian mixture model. The EM algorithm has long been used to estimate Gaussian mixture model parameters. As the standard EM algorithm does not consider spatial relationship among data, it can generate spatially scattered seismic facies which is physically unrealistic. We improve the standard EM algorithm by adding a spatial constraint to enhance spatial continuity of the estimated geological facies. By applying the EM algorithms to acoustic impedance and Poisson’s ratio data for two synthetic examples, we are able to identify the facies distribution.     

Robust algorithms for polyhedral modelling of fractured rock mass structure

This paper presents robust algorithms for application in polyhedral modelling of rock mass structure based on discrete fracture networks. The algorithms improve the robustness of current polyhedral modellers to challenging or degenerate geometries and geometrical errors. These issues are inevitably encountered when the fracture network realisations incorporate large numbers of fractures, or regions of densely packed fractures are modelled, or when the number of fracture network realisations interrogated is large. Several stages in the polyhedral modelling process which are sensitive to geometrical errors are reviewed and algorithms which are robust and capable of handling topological errors in the input fracture network geometry are presented.     

Image analysis algorithms for estimating porous media multiphase flow variables from computed microtomography data: a validation study

Image analysis of three-dimensional microtomographic image data has become an integral component of pore scale investigations of multiphase flow through porous media. This study focuses on the validation of image analysis algorithms for identifying phases and estimating porosity, saturation, solid surface area, and interfacial area between fluid phases from gray-scale X-ray microtomographic image data. The data used in this study consisted of (1) a two-phase high precision bead pack from which porosity and solid surface area estimates were obtained and (2) three-phase cylindrical capillary tubes of three different radii, each containing an air–water interface, from which interfacial area was estimated. The image analysis algorithm employed here combines an anisotropic diffusion filter to remove noise from the original gray-scale image data, a k-means cluster analysis to obtain segmented data, and the construction of isosurfaces to estimate solid surface area and interfacial area. Our method was compared with laboratory measurements, as well as estimates obtained from a number of other image analysis algorithms presented in the literature. Porosity estimates for the two-phase bead pack were within 1.5% error of laboratory measurements and agreed well with estimates obtained using an indicator kriging segmentation algorithm. Additionally, our method estimated the solid surface area of the high precision beads within 10% of the laboratory measurements, whereas solid surface area estimates obtained from voxel counting and two-point correlation functions overestimated the surface area by 20–40%. Interfacial area estimates for the air–water menisci contained within the capillary tubes were obtained using our image analysis algorithm, and using other image analysis algorithms, including voxel counting, two-point correlation functions, and the porous media marching cubes. Our image analysis algorithm, and other algorithms based on marching cubes, resulted in errors ranging from 1% to 20% of the analytical interfacial area estimates, whereas voxel counting and two-point correlation functions overestimated the analytical interfacial area by 20–40%. In addition, the sensitivity of the image analysis algorithms on the resolution of the microtomographic image data was investigated, and the results indicated that there was little or no improvement in the comparison with laboratory estimates for the resolutions and conditions tested.     

Structure of the upper mantle in the Circum-Arctic region from regional seismic tomography

We present a new three-dimensional model of P-velocity anomalies in the upper mantle beneath the Circum-Arctic region based on tomographic inversion of global data from the catalogues of the International Seismological Center (ISC, 2007). We used travel times of seismic waves from events located in the study area which were recorded by the worldwide network, as well as data from remote events registered by stations in the study region. The obtained mantle seismic anomalies clearly correlate with the main lithosphere structures in the Circum-Arctic region. High-velocity anomalies down to 250–300 km depth correspond to Precambrian thick lithosphere plates, such as the East European Platform with the adjacent shelf areas, Siberian Plate, Canadian Shield, and Greenland. It should be noted that lithosphere beneath the central part of Greenland appears to be strongly thinned, which can be explained by the effect of the Iceland plume which passed under Greenland 50–60 million years ago. Beneath Chukotka, Yakutia, and Alaska we observe low-velocity anomalies which represent weak and relatively thin actively deformed lithosphere. Some of these low-velocity areas coincide with manifestations of Cenozoic volcanism. A high-velocity anomaly at 500–700 km depth beneath Chukotka may be a relic of the subduction zone which occurred here about 100 million years ago. In the oceanic areas, the tomography results are strongly inhomogeneous. Beneath the North Atlantic, we observe very strong low-velocity anomalies which indicate an important role of the Iceland plume and active rifting in the opening of the oceanic basin. On the contrary, beneath the central part of the Arctic Ocean, no significant anomalies are observed, which implies a passive character of rifting.     

The timing of regional Lateglacial events and post-glacial sedimentation rates from Lake Superior

We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand.The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170 cal. BP (calibrated years before 1950, ca 7950–8250 ¹⁴C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.     

Analytical solution for longitudinal seismic response of long tunnels subjected to Rayleigh waves

The longitudinal seismic response of a long tunnel subjected to Rayleigh waves is investigated in this paper. The tunnel is assumed to be infinitely long, has a uniform cross section, and rests on a viscoelastic foundation. The free-field deformation under Rayleigh waves traveling parallel to the tunnel axis is decomposed into two directions, namely, the axial motion and the vertical motion, and transformed into dynamic loads imposed on the tunnel. Based on the Fourier and Laplace integral transform techniques, the governing equations of tunnels are simplified into algebraic equations, and the analytical solutions are obtained with the convolution theorem. The final solutions of the tunnel responses in terms of deflection, velocity, acceleration, axial force, bending moment, and shear force are investigated. The proposed solution is verified by comparison of its results and those from the finite element program ABAQUS. Further parametric analysis is carried out to investigate the influence of soil-structure relative stiffness ratio and wave frequency on dynamic longitudinal responses of the tunnel.     

Debris flow hazards for mountain regions of Russia: regional features and key events

The total area of debris flow territories of the Russian Federation accounts for about 10% of the area of the country. The highest debris flow activity areas located in Kamchatka-Kuril, North Caucasus and Baikal debris flow provinces. The largest debris flow events connected with volcano eruptions. Maximum volume of debris flow deposits per one event reached 500 × 10⁶ m³ (lahar formed during the eruption of Bezymyanny volcano in Kamchatka in 1956). In the mountains of the Greater Caucasus, the maximum volume of transported debris material reached 3 × 10⁶ m³; the largest debris flows here had glacial reasons. In the Baikal debris flow province, the highest debris flow activity located in the ridges of the Baikal rift zone (the East Sayan Mountains, the Khamar-Daban Ridge and the ridges of the Stanovoye Highland). Spatial features of debris flow processes within the territory of Russia are analyzed, and the map of Debris Flow Hazard in Russia is presented. We classified the debris flow hazard areas into 2 zones, 6 regions and 15 provinces. Warm and cold zones are distinguished. The warm zone covers mountainous areas within the southern part of Russia with temperate climate; rain-induced debris flows are predominant there. The cold zone includes mountainous areas with subarctic and arctic climate; they are characterized by a short warm period, the occurrence of permafrost, as well as the predominance of slush flows. Debris flow events are described for each province. We collected a list of remarkable debris flow events with some parameters of their magnitude and impact. Due to climate change, the characteristics of debris flows will change in the future. Availability of maps and information from previous events will allow to analyze the new cases of debris flows.     

Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau,Northeast India

In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18–20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.