Two large earthquakes occurred during the last decade on Sakhalin Island, the M w 7.6 Neftegorskoe earthquake of 27 May 1995 and the M w 6.8 Uglegorskoe earthquake of 4 August 2000, in the north and south of the island, respectively. Only about five seismograph stations record earthquakes along the 1000 km, mostly strike-slip plate boundary that transects the island from north to south. In spite of that, it was possible to investigate seismicity patterns of the last two to three decades quantitatively. We found that in, and surrounding, their source volumes, both of these main shocks were preceded by periods of pronounced seismic quiescence, which lasted 2.5 ± 0.5 years. The distances to which the production of earthquakes was reduced reached several hundred kilometers. The probability that these periods of anomalously low seismicity occurred by chance is estimated to be about 1% to 2%. These conclusions were reached independently by the application of two methods, which are based on different approaches. The RTL-algorithm measures the level of seismic activity in moving time windows by counting the number of earthquakes, weighted by their size, and inversely weighted by their distance, in time and space from the point of observation. The Z -mapping approach measures the difference of the seismicity rate, within moving time windows, to the background rate by the standard deviate Z . This generates an array of comparisons that cover all of the available time and space, and that can be searched for all anomalous departures from the normal seismicity rate. The RTL-analysis was based on the original catalog with K -classes measuring the earthquake sizes; the Z -mapping was based on the catalog with K transformed into magnitudes. The RTL-analysis started with data from 1980, the Z -mapping technique used the data from 1974 on. In both methods, cylindrical volumes, centered at the respective epicenters, were sampled. The Z-mapping technique additionally investigated the seismicity in about 1000 volumes centered at the nodes of a randomly placed regular grid with node spacing of 20 km. The fact that the two methods yield almost identical results strongly suggests that the observed precursory quiescence anomalies are robust and real. If the seismicity on Sakhalin Island is monitored at a completeness-level an order of magnitude below the present one, then it may be possible to detect future episodes of quiescence in real time.
Crosshole tomography requires solution of a mixed‐determined inverse problem and addition of a priori information in the form of auxiliary constraints to achieve a stable solution. Composite distribution inversion (CDI) constraints are developed by assuming parameters are drawn from a composite distribution consisting of both normally and uniformly distributed parameters. Nonanomalous parameter estimates are assumed to be Gaussian while anomalous parameters are assumed uniform. The resulting constraints are sensitive to anomaly volume and are an alternative to the usual constraints of minimizing [Formula: see text] solution length or some measure of roughness. Damped least‐squares inversion, which minimizes solution length, distributes anomalous signal through poorly resolved areas to produce in attenuated and smoothed anomalies. Similar regularization methods, such as smoothness or flatness constraints, also degrade small spatial wavelength features and produce diffuse images of distinct anomalies. CDI constraints preserve small spatial wavelength features by encouraging small amplitude anomalies to assume the value of the reference model and by allowing truly anomalous parameter estimates to assume whatever value minimizes prediction error without incurring additional penalty. CDI tomograms are characterized by nearly ideal point‐spread functions, suggesting the possibility of better quantitative parameter estimates than are produced using most existing methods. CDI tomograms of both synthetic and field data are shown to produce less diffuse images with more accurate anomaly amplitude estimates than damped least‐squares methods. The CDI algorithm is potentially applicable to nontomographic inversion problems.
Recent advances in logging while drilling have enabled the development of a seismic tool that acquires and transmits seismic data in real time during the drilling process. Examples of this technology demonstrate the use of this service to significantly reduce risk and uncertainty, enabling better decision making during well construction operations. Operationally, the service runs with no interference with normal rig operations. This technology can have a significant impact on the cost of exploration and development drilling, particularly in the deepwater environment and other areas with significant seismic uncertainties. The service was first used to drill a wildcat well in the South Caspian Sea. The well was deviated to intersect the reservoir section within a steeply dipping structure at about 4500 m true vertical depth (TVD) while avoiding faulting, high pore pressures, and areas of poor seismic data quality in the overburden at the crest of the structure. The key product of the new seismic tool, real-time check-shot data, was used to position the drill bit and the borehole on the seismic section used to plan the well. Furthermore, an analysis of the seismic waveforms recorded in the tool memory demonstrates the capability of this technology to produce data of sufficient quality to allow imaging of the formation ahead and to the side of the bit. Images from the tool have higher resolution and good ties with the migrated surface seismic images. Future advances in technology offer the exciting possibility of imaging significant distances ahead of the bit in relevant real time. Introduction Check-shot surveys provide direct measurements of seismic travel times from the surface to survey positions along the well length, calibrating the depth scale of a surface-seismic image at these positions. Seismic travel times position the borehole on the seismic image used to plan the well. In-time access to such calibration data is critical where large uncertainties in the time-depth relationship exist, or in wells where it is essential to set casing in a particular interval identified by the seismic data. Until now, drill-bit seismic data surveys have been the only option for while-drilling check-shot surveys. Drill-bit seismic surveys use the noise generated by the bit as the acoustic signal, measuring the time it takes for this signal to travel to the surface (e.g., Armstrong et al.1). This technology works well in some environments, but it is unreliable where formations are soft, and it cannot be used in highly deviated holes or in general with polycrystalline diamond compact (PDC) bits. With a few exceptions, e.g. Haldorsen et al.,2 it also does not provide full-waveform data of sufficiently good quality to be used for meaningful look-ahead imaging. The introduction of a while-drilling seismic tool (Esmersoy et al.,3 Underhill et al.4) in the drilling assembly provides an alternative solution. In the following, we will use the abbreviation SMWD to represent "seismic measurement while drilling" for the technique, the tool, and the data. SMWD Technique The operation of SMWD, illustrated in Fig. 1, is similar to a conventional wireline operation in that it uses a surface source and a downhole receiver. The technique employs receivers in the bottomhole assembly (BHA) and a seismic source deployed from a boat or the rig. The source is fired during quiet times when both direct and reflected seismic signals can be collected by the SMWD tool. Check-shot times are automatically detected downhole and data are sent uphole via measurement-while-drilling (MWD) telemetry for processing, visualization, and interpretation. Because data is recorded while the drillers are making a drill string connection, the measurement does not interrupt the drilling. Compared to a conventional wireline operation, the use of this while-drilling tool significantly reduces the rig time, as well as the risk of borehole damage and stuck tools, associated with running a wireline survey.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.