Anomalous Seismic Events Research Articles

False Alarm Reduction from Anomalous Seismic Events Detected by Smart Phones

Accelerometers equipped smartphones can detect seismic events in the same way as specialized seismological stations. However, the false alarm of the low-end sensors cannot meet the stringent requirement of earthquake detection applications. While a lot of work has been done to reduce the white noise, little of them focus on eliminating the adulteration caused by human motion. To that end, we first analyze the features of the sampling data for daily purpose, and employ a noise reduction strategy to extract the event features in data pre-processing to stick the p-waves out. Secondly, a new STA/LTA+DWT P-wave pickup algorithm is proposed to recognize the false alarms from the mixed events. The simulation results show that the proposed scheme can maintain the precision of positive decision to 90%, while significantly lowering the false alarm by about a third.

Hydrothermal vents and seismic anomalies: implications for the petroleum system NE of Shetland

The influence of igneous intrusions and their associated vent complexes on hydrocarbon migration is poorly understood, yet may be significant in the exploration of petroleum basins located on passive margins. A regional review of the northeasternmost frontier sector of the UK Continental Shelf (UKCS) encompassing the southern extent of the Møre Basin has revealed evidence of extensive vent formation during the Late Paleocene. Anomalous seismic amplitude events that have not been previously described in this area are identified above and clearly related to several of the vent complexes, and a number of scenarios are postulated to explain their presence. Integrating existing geological studies and regional seismic interpretation with a new 3D seismic dataset reveals a close relationship between sills, vents and seismic anomalies which may be related to the presence of gas/fluid. Recognition of the vent complexes as potential fluid conduits long after initial formation may be important in focusing migration to younger stratigraphic levels. Important observations are made regarding a link to possible hydrocarbon migration mechanisms within the study area and the associated implications for hydrocarbon exploration and risking within this petroleum basin are investigated.

A petrophysical approach to the investigation of shallow marine geology

ABSTRACTInterpretations of commercial marine shallow seismic data are generally limited to a prognosis of lithologies based on sparse ground truth data and regional stratigraphic models and a probability assessment of encountering perceived geohazards. However, an approach based on analyses of available petrophysical data can provide a more robust assessment of shallow marine lithologies and a more confident interpretation of shallow gas and overpressured formations. Generation of well‐ and borehole‐log acoustic impedance curves provides the starting point for these investigations and, although not all of the requisite data may be present in shallow section wireline suites, the Faust and Gardner equations, and other empirically derived relationships, can supply acceptable mathematical approximations. Impedance inversions of reflection seismic data facilitate more confident interpretations of lithological units, especially when combined with additional datasets such as gamma‐ray, resistivity, porosity, and S‐wave sonic, as elastic and extended elastic inversions. Anomalous seismic events interpreted to represent a probability of encountering shallow gas using traditional interpretation methods may be further investigated using amplitude‐versus‐offset cross‐plotting techniques, including fluid factor calculations, and the potential for overpressured gas accumulations or flow sands may be estimated from velocity‐derived pore pressure calculations. Geological and synthetic seismic modelling exercises provide the opportunity to test petrophysical interpretations in the absence of ground truth control data.

Breakdown of the Gutenberg‐Richter relation for microearthquakes induced by hydraulic fracturing: influence of stratabound fractures

ABSTRACTHydraulic fracturing, a powerful completion technique used to enhance oil or gas production from impermeable strata, may trigger unintended earthquake activity. The primary basis for assessment of triggered and natural seismic hazard is the classic Gutenberg‐Richter (G‐R) relation, which expresses scale‐independent behaviour of earthquake magnitudes. Here we use a stochastic approach to simulate and test magnitude‐distance trends expressed by microseismic catalogues derived from three hydraulic fracture monitoring programmes in North America. We show that a widely observed rapid fall‐off in large‐magnitude events, almost universally quantified using the G‐R b value, may in our case be an artefact of the strongly laminated character of the stimulated oil and gas reservoirs. We also show that, for the three reservoirs considered, mechanical bed thickness can be approximated by a lognormal distribution. For a stratabound fracture network, this leads asymptotically to a Gaussian decay for induced magnitudes. We show that the stratabound model provides a more significant correspondence with our observations. If applicable in general, this result has important implications for determining the energy balance of hydraulic fracture systems (i.e. radiated seismic energy versus injected energy) as well as hazard assessments based on the probability of occurrence of anomalous seismic events.

A Novel Seismic Base Isolation System Consisting of a Lead Rubber Bearing in Series with a Friction Slider. Part II: Application to a Multi-Storey RC Building and Comparison with Traditional Systems

In a parallel paper a new High Damping Hybrid Seismic Isolator (HDHSI) has been proposed and obtained by the assembly in series of a Lead Rubber Bearing (LRB) and a Friction Slider (FS) characterized by a high friction coefficient. In the present paper, within the context of seismic base isolation techniques for the earthquake resistance of Reinforced Concrete (RC) buildings, a multi-storey RC building is analyzed as base isolated by the seismic isolator HDHSI (High Damping Hybrid Seismic Isolator). The seismic response of this base isolated RC building is compared with the seismic response of the same structure isolated by a LRB (Lead Rubber Bearing) isolator. The analysis is developed by considering different seismic events in terms of intensity and in terms of frequency content with regard to a supervening collapse. The purpose of this comparative analysis is to highlight the features offered by the HDHSI system compared to the LRB system in the seismic protection of structures. Accordingly, a nonlinear dynamic analysis is performed for a RC structure base isolated by means of the proposed device. In the analysis anomalous seismic events are considered. They are the El Centro earthquake (N00W component, 1940) which is characterized by high intensity and the Erzincan earthquake (N90W component, 1992) which is characterized by anomalous frequency content. The comparison between the two base isolation systems is presented by analyzing the time history of the shear force and the time history of the displacement at the base of the superstructure. The benefits of the HDHSI system in conferring protection to the structure are shown to be significant even under extreme seismic events.

Occurrence and nature of “bottom simulating reflectors” in the northern Gulf of Mexico

Subsurface interpretation, utilizing a database of more than 450,000 km2 (175,000 mi2) of three-dimensional (3-D) seismic in the northern Gulf of Mexico (GoM), reveals 145 discrete areas, totaling 4450 km2 (1.1 million acres) where the base of gas hydrate stability (BGHS) can be confidently inferred from seismic data. Unlike many other areas of the world, the majority of these features are not Bottom Simulating Reflectors (BSRs) in the “classic” sense, meaning continuous coherent events that cross-cut primary stratigraphy. Those typical, or continuous BSRs, are noted in only 24% of the features identified within this study. In contrast, the most common seismic manifestation of the BGHS in the GoM (59%) is the discontinuous “BSR”, delineated by widely separated anomalous seismic events that align in general conformance with seafloor bathymetry. A third type of seismic feature, pluming “BSRs”, are continuous events that are not bottom-simulating, but are bowed toward the seafloor and represent areas where large, but areally-limited increases in heat flow (linked to strong vertical fluid flux), perturb the BGHS. The limited nature of continuous BSRs and the relative abundance of discontinuous and pluming forms are attributed to the strong lithologic and structural heterogeneity of the northern GoM basin. This lithologic and structural complexity has served to disrupt and localize regionally pervasive and homogeneous gas flux that is consistent with the formation of large, continuous BSRs noted across other less complex continental margins. The various BSR forms identified in this study are shown to be closely associated (125 of 145) with the occurrence of seafloor amplitude anomalies, which are in turn usually associated with the flanks and crests of salt-cored ridges. These associations are interpreted to reflect the co-dependence of BSRs and seafloor reflectivity along the migration pathways that typify this geologic setting.

A New Passive Seismic Control Device for Protection of Structures under Anomalous Seismic Events

In this paper, the possibility offered by the Base Isolation System, designed according to the strategy of limitation seismic force, in order to ensure a significant level of robustness to the isolated superstructure, has been analyzed. In according to the strategy of limitation seismic force, a new seismic isolator called HDHSI (High Damping Hybrid Seismic Isolator) is proposed, obtained by the assembly in series of a LRB (Lead Rubber Bearing) and a FS (Friction Slider) with high friction coefficient. The HDHSI device is in contrast to the R-FBI (Resilient-Friction Base Isolation) isolator and is the optimization of the EDF (Electricité De France) system. A modeling with HDHSI devise has been presented by using nonlinear finite elements NLlink (Nonlinear link) and nonlinear dynamic analysis for a SDOF system with harmonic force, in order to characterize the cycle-hysteretic behavior of the device for different levels of shear deformation, are performed. Nonlinear dynamic analysis for a RC structure, base isolated by the proposed devices, are performed. As seismic actions, have been adopted the following registrations: an anomalous earthquakes for intensity and in particular El Centro earthquake (N00W component, 1940) with different amplification factors; an anomalous earthquake for frequency content and in particular Erzincan earthquake (N90W component, 1992).

REDUCING RESOURCE UNCERTAINTY USING SEISMIC AMPLITUDE ANALYSIS ON THE SOUTHERN RANKIN TREND, NORTHWEST AUSTRALIA

Between 1973 and 1996 West Australian Petroleum Pty Limited (WAPET) discovered five major gas fields on the southern Rankin Trend including Spar, West Tryal Rocks, Gorgon, Chrysaor, and Dionysus (collectively termed the Greater Gorgon Resource). Recent discoveries at Chrysaor and Dionysus emphasise the role of subtle 3D seismic attributes in finding hydrocarbons and defining reserves with a minimum number of wells.The Gorgon, Chrysaor, and Dionysus fields were covered by 3D seismic data shot in 1991 and 1995, which led WAPET to discover Chrysaor and later Dionysus. Subsequent to the 3D acquisitions, field reservoirs have been correlated with anomalous seismic events (seismic amplitude and amplitude versus offset) that conform to depth structure. Follow-up work has shown that combining these 3D seismic attributes improves the prediction of wet sands, gas sands, and other lithologies.The resulting understanding and confidence provided by this 3D seismic has driven an aggressive exploration program and defined field reserves at a high confidence level. Results include the recent award of permit area WA-267-P to WAPET and the ongoing studies to begin development of the Greater Gorgon Resource.

Phenomenological representation and kinematics of general seismic sources including the seismic vector modes

SUMMARY A unified mathematical description and kinematics are presented for the general indigenous seismic sources including the single force and torque, as well as the moment tensor. A new class of earthquakes associated with a release of gravitational energy and/or a reduction of pressure gradient in the Earth's interior are shown to be observed through the detection of the ‘seismic vector modes’. The Earth is divided into two subsystems: a source region spanning a finite volume and containing a finite mass, and the remainder of the Earth surrounding this volume. The phenomenological source of the seismic waves radiated outside the source volume is given by the traction on the boundary through which the momentum exchange takes place between the two subsystems. The traction is expressed in terms of the orthogonal GSH (Generalized Spherical Harmonics) components, each of which is identified with the independent radiation pattern characterized by the harmonic order l and modal type k (toroidal, spheroidal 1 or spheroidal 2). By investigating each traction mode in terms of both the excitation mechanism in the source and the efficiency of wave radiation outside of the source region, we have recognized the practical importance of the three ‘vector-type’ components with harmonic order l= 1, in addition to the the conventional ‘tensor-type’ force components (moment tensor). The three are (1)single force, produced by the motion of the gravitational centre of the source region relative to the surrounding medium; (2)torque, produced by the change of the angular momentum and the gravitational loading in the source region; (3)pressure dipole, produced by the change of the pressure gradient in the source region. We have shown the possible occurrence of a new class of earthquakes, which dominantly radiate the vector type wave modes in the Earth's interior, especially in a partially molten region beneath volcanic areas. We have proposed to detect the vector-type components in the anomalous seismic events not well characterized by the moment tensor model. Conversion formula are derived between the force system obtained as a set of present traction modes in the 2-D (surface) description, and that obtained by Backus & Mulcahy (1976) in terms of the polynomial moments representing the 3-D description of the equivalent volume force. The traction modes provide a necessary and sufficient set of source parameters or ‘seismic observables’, which are determinable only by studying the wavefield observed far outside a source volume by the angular spectral decomposition of the radiation pattern.

Seismic Modeling for Gas Field Development in Offshore Niger Delta: A Case History: ABSTRACT

In practice, seismic data has been used to map the geometry of events in the subsurface, mainly from reflection continuity and the character of reflection packages. Seismic modeling techniques are attempts to mathematically and geometrically represent the geology of the subsurface and to depict the interaction of the geology with a propagating seismic wave front. In the Niger delta, recent development in stratigraphic exploration has induced the examination of more subtle features of reflection, mainly polarity, amplitude, and waveform, to define the limits of seismic resolution and hence predict the geometry of the subsurface fluid and solid interfaces. The field being appraised for development in this study is located approximately 35 km offshore southeast of the Niger delta in a water depth ranging from 20 to 25 m, and the project involves a modeling evaluation of five sand units encountered at different levels in the subsurface. The sands being modeled occur within the Agbada Formation (Miocene age), an alternation sequence of transitional sands and shales characterized by high-amplitude, anomalous seismic events over the Nkop field. Structurally, the field is characterized by seaward facing, contemporaneous growth faults consisting mainly of a rollover anticline located in a regional fault couple, bounded to more » the north and south by a growth fault. « less

Predictive Seismic Modeling Case History from the Niger Delta

Seismic modeling techniques provide the mechanics for simulating the geology of the subsurface by depicting the impact of a propagating seismic wavefront on subsurface structures. In practice, seismic data have been used to map the geometry of events in the subsurface, mainly from reflection continuity and the character of reflection packages. In the Niger delta, recent developments in stratigraphic exploration has induced the examination of more subtle features of reflection, mainly polarity, amplitude, and waveform to define the limits of seismic resolution and hence predict the geometry of subsurface fluid and solid interfaces. The case history discussed here involved interpretative study for defining the fluid contents of prospective oil and gas leads as indicated by anomalous seismic events on a Niger delta field located in a water depth of 25 m. An appropriate source signal (5-35 Hertz minimum phase) is selected, and the wavelet is convolved with a practical geologic model to obtain a synthetic seismogram. By an interactive process involving slight modifications in the geologic model, a synthetic seismogram is ultimately derived that matches a field signal, thus providing a more accurate prediction of the geological formation under study. The technique was effect (as confirmed by later drilling) inmore » appraising the fluid contents of the targeted pay zones encountered at gas/water, oil/water, and gas/oil/water contacts in the O field, located in the eastern offshore area of the Niger delta. The method further demonstrated that structural and stratigraphic modeling are effective tools for testing the mapability of a geologic concept and are able to evaluate the significance of reflectivity changes or anomalies on uncalibrated seismic data.« less

Anomalous seismic events observed at the CSM HDR project, U.K. : Baria, R; Green, A S P; Jones, R H Int J Rock Mech Min SciV26, N3/4, July 1989, P257–269

Anomalous seismic events observed at the CSM HDR project, U.K. : Baria, R; Green, A S P; Jones, R H Int J Rock Mech Min SciV26, N3/4, July 1989, P257–269

Anomalous seismic events observed at the CSM HDR project, U.K.

Abstract Seismic events detected during fluid injection into a crystalline jointed rock (granite) with an anisotropic stress regime are predominantly due to shear dislocation at the joints. Although seismic signals from shear dislocation are the most commonly recognised, it is becoming evident that there are other processes at work. Data from the Camborne School of Mines (CSM) hot dry rock (HDR) geothermal project site are presented in this paper, to show that there are microseismic events which are not caused by shear dislocation. These events vary from long period events detected during a gas lift test to high-frequency events detected during a massive viscous stimulation at a depth of 2300 m. These events do not show the clear P and S waves associated with shear failure. The long period events look very similar to the events detected during boulder splitting experiments in the laboratory and those detected at the Los Alamos HDR site during hydraulic injection. Tube waves detected durng the circulation of water between two wells indicates that water was squirted in and out of the well. Mechanisms for the generation of these events are not clear, but their presence does suggest that the modelling of rocko joint behaviour during fluid injection will have to take these anomalous events into consideration in order to produce a representative model.

A single‐force model for the 1975 Kalapana, Hawaii, Earthquake

A single force mechanism is investigated as the source of long‐period seismic radiation from the 1975 Kalapana, Hawaii, earthquake (MS = 7.1). The observed Love wave radiation pattern determined from the spectra of World‐Wide Standard Seismograph Network and High Gain Long Period records at 100 s is two‐lobed with azimuth, consistent with a near‐horizontal single force acting opposite (strike ∼330°) to the observed displacement direction of the earthquake; this pattern is inconsistent with the expected double‐couple pattern. Assuming a form of the force time history of a one‐cycle sinusoid, the total duration of the event estimated from Rayleigh waves at two International Deployment of Accelerometers stations is approximately 180 s. The peak amplitude fo of the time function is 1 × 1015 N from amplitudes of Love and Rayleigh waves. The interpretation is that the bulk of the seismic radiation was produced by large‐scale slumping of a large area of the south flank of Kilauea volcano. The single force is a crude representation of the effect on the earth of the motion of a partially decoupled large slide mass. Using the mass estimated from the tsunami generation area (∼ 1015–1016 kg), the peak acceleration of the slide block (0.1–1 m s−2) inferred from the seismic force is comparable with the acceleration due to gravity on a gently inclined plane. The slump model for the Kalapana earthquake is also more qualitatively consistent with the large horizontal deformation (8 m on land) and tsunami associated with the earthquake, which are difficult to explain with the conventional double‐couple source model. The single‐force source has been used previously to model the long‐period seismic waves from the landslide accompanying the eruption of Mount St. Helens volcano, and may explain other anomalous seismic events as being due to massive slumping of sediments or unconsolidated material and not to elastic dislocation.

The Occurrence of Anomalous Seismic Events in Eastern Tibet

Sirmmary Comparison of body-wave and surface-wave magnitudes has become one of the principal methods of seismic discrimination between natural earthquakes and underground nuclear explosions. For some earthquakes, called ' anomalous events ', this criterion of discrimination appears to fail. Most of the reported anomalous events (about 25) are located in Tibet near the intersection of the eastern end of the Himalayas with the northward projection of the Andaman-Burman arc. The majority of the events were originally classified as anomalous because either the reported body-wave magnitude, mbr was inaccurate, or the surface-wave magnitude, M,, did not adequately represent the long-period character of the source. M, is based on the amplitude of 20 s period, fundamental-mode Rayleigh waves, which are not excited efficiently by some earthquake sources of particular depth and mechanism. Some of the earthquakes may occur deep within the thick crust of the Tibetan plateau at depths at which the fundamental mode is poorly excited. The thick crust provides a waw guide for the efficient generation of Love waves and higher-mode Rayleigh waves, and it is found that observations of these waves provide a discriminant for some of the anomalous events. A surface-wave magnitude scale based on the amplitudes of higher modes offers an especially attractive discriminant. These higher-mode observations, combined with more reliable mb determinations, identify all of the reported anomalous events in Tibet-with thc exception of a single sequence of events-as natural earthquakes. The remaining sequence of anomalous events occurred in a very limited zrea where a concentration of tectonic stress may be anticipated. Thus, these remaining anomalous earthquakes may be related to high tectonic stress, perhaps in association with the formation of a new fault. Observations of dilatational first motions allow the events in the anomalous sequence to be discriminated. Thus, all of the reported anomalous events in Tibet are identified as natural earthquakes. To our knowledge, all events from Tibet or elsewhere that have been discussed in the literature as being anomalous, can in fact, be identified as earthquakes using one or more of the above-mentioned techniques.