Rongmao Zhou

Ms:mb Discrimination Study of Mining Explosions in Wyoming, USA, and in QianAn, China

We investigate the use of intermediate-period surface-wave magnitude, M s , and high-frequency body-wave magnitude, m b , from regional mining explosions for event discrimination by using techniques originally intended for separating earthquakes from explosions with teleseismic observations. The actual values of M s and m b suggest that the surface waves generated by long-duration mining explosions can make them appear earthquakelike. The data from the single anomalous shot indicate that if a significant part of the total explosives is simultaneously detonated the event will move into the explosion population. Data for this study are taken from a portable broadband deployment in Wyoming recording mining explosions in the Powder River Basin and a broadband network currently deployed in northeast China. The magnitudes, M s (vmax) and m b were estimated for five, kiloton-size mining explosions, four in Wyoming and one in QianAn, China. The resulting network M s : m b data were compared with data from a previous study that included earthquakes and contained single-fired explosions (Stevens and Day, 1985; Bonner et al. , 2003). Although the previous studies mostly examined events larger than those in this study, the Wyoming and China mining events plot in the earthquake population. Data from the anomalous Wyoming event, a blast in which a failure of the timing system caused a large portion of the blast pattern to simultaneously detonate, plot in the explosion population with m b 4.4. The simultaneous detonation of a large portion of blast array increased the body-wave magnitude but had little effect on surface-wave magnitude.

SOURCE PHENOMENOLOGY EXPERIMENTS IN ARIZONA

The Arizona Source Phenomenology Experiments (SPE) have resulted in an important dataset for the nuclear monitoring community. The 19 dedicated single-fired explosions and multiple delay-fired mining explosions were recorded by one of the most densely instrumented accelerometer and seismometer arrays ever fielded, and the data have already proven useful in quantifying confinement and excitation effects for the sources. It is very interesting to note that we have observed differences in the phenomenology of these two series of explosions resulting from the differences between the relatively slow (limestone) and fast (granodiorite) media. We observed differences at the two SPE sites in the way the rock failed during the explosions, how the S-waves were generated, and the amplitude behavior as a function of confinement. Our consortiums goal is to use the synergy of the multiple datasets collected during this experiment to unravel the phenomenological differences between the two emplacement media. The data suggest that the main difference between single-fired chemical and delay-fired mining explosion seismograms at regional distances is the increased surface wave energy for the latter source type. The effect of the delay-firing is to decrease the high-frequency P-wave amplitudes while increasing the surface wave energy because of the longer source duration and spall components. The results suggest that the single-fired explosions are surrogates for nuclear explosions in higher frequency bands (e.g., 6-8 Hz Pg/Lg discriminants). We have shown that the SPE shots, together with the mining explosions, are efficient sources of S-wave energy, and our next research stage is to postulate the possible sources contributing to the shear-wave energy.

Rayleigh Waves Generated by Mining Explosions and Upper Crustal Structure around the Powder River Basin, Wyoming

Fundamental-mode, intermediate period Rayleigh waves generated by mining explosions are utilized to constrain the crustal structure of Wyoming. Broadband seismic stations recorded data during two regional seismic experiments conducted in Wyoming in 1996 and 1997. The stations were deployed in a ring surrounding the Powder River Basin (PRB) at ranges 100-360 km from the Black Thunder Coal Mine located in the northeast corner of Wyoming. Signals generated by four explosions with total explosive weight from 1.1 kiloton to 2.7 kiloton are analyzed with the goal of resolving the crustal structure in northeastern Wyoming. Data from the experiments show evidence that kiloton-sized millisecond-delay-fired cast blasts produce strong 1- to 15-sec period surface waves, some extending to 20-sec period. Group velocities of fundamental-mode Rayleigh waves were estimated using the Multiple Filter Analysis (MFA) technique and refined with Phase Matched Filtering (PMF). The surface waves are dependent on propagation direction and source orientation. The northwest-southeast trend of the PRB means that some propagation paths are parallel to the basin orientation and others perpendicular. Group velocities exhibit normal dispersion and range from 0.97 (1 sec) to 3.1 (20 sec) km/sec. A least square inversion technique was used to invert group velocity dispersion curves for the shallow shear-wave velocity structure. The average model consists of nine layers in the upper 20 km of the crust. The two top layers are thin (0.35 and 0.45 km) with slow velocities (1.0 to 1.6 km/sec) as a result of the basin sediments. At depths below 15 km, the shear velocity is nearly the same for paths parallel and perpendicular to the PRB axis. A possible low-velocity zone is found along the PRB axis with a velocity contrast of 0.7 km/sec at depth of 9.8 km to 14.8 km. These detailed models give a much better fit to dispersion curves than the global CRUST2.0, particularly for periods below 5 sec, which indicates the need for path-specific models when analyzing mid-period surface waves. Comparison of theoretical seismograms using the empirically derived structure with the observed seismograms provides a basis for constraining the source processes of these mining explosions. The generation of the intermediate-period surface waves is directly related to the total source time duration that is on the order of several seconds for these cast blasts. Observational data suggest that source orientation is important as well, resulting from some combination of mine free face orientation, blasting direction, and material casting. Manuscript received 24 July 2003.

Using the coda‐wave interferometry method and time‐lapse VSP data to estimate velocity changes from geological carbon sequestration in a brine aquifer

Injection and movement/saturation of CO2 in a geological formation can cause changes in seismic velocities and attenuation, resulting in changes in seismic-wave scattering and propagation. Accurately estimating seismic-velocity changes from time-lapse seismograms can provide valuable information about where CO2 moves. We investigate the capability of the coda-wave interferometry method for monitoring geological carbon sequestration using field time-lapse VSP data. The coda-wave interferometry method can estimate relative temporal changes in seismic velocities. Pre-injection and post-injection field VSP data sets were acquired for monitoring of injected CO2 in a brine aquifer. We estimate the temporal velocity changes at the centers of a moving time window using the coda-wave interferometry method, and then obtain the mean velocity change by averaging the temporal velocity changes over time. The time-lapse VSP data along three azimuthal directions with different offset ranges from the observation well are used in this study. Generally, the estimated mean velocity changes from field VSP data from different shots show similar changing pattern with receiver depth. Mid offset shots (shot 5, 6, and 8) give largest estimation, while closest offset shots (shot 1, 2, and 4) give mid estimated changes and shot 9 with offset 1.5km give smallest estimated velocity changes. The estimated velocity changes are as large as 0.5%. This demonstrates that the coda-wave interferometry method can be used for monitoring of CO2 injection using time-lapse VSP data. The different changes along different azimuths may be caused by the local geology heterogeneity.

A High-resolution Vector-imaging Condition for Elastic Reverse-time Migration

We introduce a new vector-imaging condition for elastic reverse-time migration to produce high-resolution migration images. This new imaging condition is developed by the use of polarization vectors of wave fields along with the conventional imaging condition. The conventional imaging condition as that used for scalar-wave migration, is established on the principle that incident and reflected waves coincide in phase at a reflection point. Our vector-imaging condition also makes use of directional coincidence of the forward and backward propagating elastic waves. The polarization vectors of elastic wave fields are obtained by spatial derivatives of the corresponding waves in the frequency domain. We demonstrate using synthetic examples that the use of polarization vectors in the imaging condition significantly improves migration images. Elastic-wave reverse-time migration with the new vector-imaging condition will enable us to reliably image complex subsurface structures, particularly for true-amplitude migration.

Double-difference tomography of microseismic data for monitoring carbon sequestration

Summary More than 2000 microseismic events have been recorded since early 2008 using a cemented geophone string for monitoring CO2 injection at the Aneth oil field in Utah. The double-difference seismic tomography method has recently been developed for improving earthquake locations and imaging the seismic velocity structure. The method uses both the absolute and differential arrival times to invert the velocity distribution and the source locations simultaneously, leading to an improved precision for microseismic event location. We explore the use of doubledifference tomography for obtaining high-precision locations of microseismic events and improved resolution of velocity structure at the Aneth oil field.

Monitoring of Geological Carbon Sequestration Using the Coda-wave Interferometry Method and Time-lapse VSP Data

Injection and movement/saturation of CO2 in a geological formation can cause changes in seismic velocities and attenuation, resulting in changes in seismic-wave scattering and propagation. Accurately estimating seismic-velocity changes from time-lapse seismograms can provide valuable information about where CO2 moves. We investigate the capability of the coda-wave interferometry method for monitoring geological carbon sequestration using synthetic and field time-lapse VSP data. The coda-wave interferometry method can estimate relative temporal changes in seismic velocities. Synthetic time-lapse VSP data sets were generated using a finite-difference elastic-wave equation scheme for a CO2 injection scenario and a possible leakage scenario. Pre-injection and post-injection field VSP data sets were acquired for monitoring of injected CO2 in a brine aquifer. We estimate the temporal velocity changes at the centers of a moving time window using the coda-wave interferometry method, and then obtain the mean velocity change by averaging the temporal velocity changes over time. The estimated mean velocity changes, from both synthetic and field VSP data, increase significantly for receiver positions approaching the top of a CO2 reservoir. This demonstrates that the coda-wave interferometry method can be used for reliable monitoring of CO2 injection using time-lapse VSP data.

New evidence for the quasi-steady structure, phase reversal and drift in the solar-terrestrial corotating disturbances☆

Abstract The quasi-steady structures in the corotating solar-terrestrial disturbances and their phase reversal reappear in the solar cycle 21 and 22, which confirms that the above phenomenon found by the authors is a regular feature occurring on a time scale of one century. Moreover, the geomagnetic disturbance data covering the past one hundred years up to 1991–1992 indicated that the quasi-steady corotating structure in the even cycles tends to drift linearly in peak against solar rotation. A tentative prediction is made for the corotating disturbances which may appear in the forthcoming pre-minimum years.