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Armin Freundt 《Bulletin of Volcanology》1998,59(6):414-435
High-grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature
or even solidus temperature. Corresponding pyroclastic-flow particles range from plastic to partially liquid and are able
to aggregate or coalesce. This contrasts with particles in pyroclastic flows producing unwelded ignimbrite, which are capable
of elastic grain interactions. The low aspect ratio and great areal extent of high-grade ignimbrites requires transport in
a particulate state either by (a) high-concentration mass flow facilitated by fluidizing gas reducing internal friction, or
by (b) expanded turbulent flow of low but downward increasing concentration. This paper presents experiments designed to investigate
the effects of plastic to liquid particles on these two contrasting transport mechanisms. Gas fluidization experiments using
polyethyleneglycole (PEG) powders heated above minimum sintering (Tms) and melting (Tm) temperatures cover a wide range of fluidization velocities (Umf>Ua>0.6·Ut) but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash, where particle volume concentration
outside the bubbles is high (≈10–1). When the powders reach a critical temperature Tm≥T≥Tms, defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized
beds as well as in experiments using mixtures of high- and low-Tm (≥30 wt.%) PEG powders, when T≥Tms of the lower-Tm powder. This indicates that extended particulate transport at T≥Tms is not possible at such high particle concentrations. In the turbulent flow experiments, liquid sprays of molten PEG or water,
vertically injected into a high-Re (>104) horizontal air flow, form a low-concentration (10–5 to 10–4) turbulent suspension current. Proximal formation of partially coalesced aggregates, which settle faster than individual
particles, causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid-particle
sedimentation from turbulent suspensions. As particles become finer downstream and coalescence efficiency decreases in response
to cooling, more distally formed aggregates become too small and rare to modify sedimentation-rate decay from that of suspension
flows containing solid particles. The key difference between the two transport systems is particle concentration, C. Since
particle collision rate Rcoll∝C2, collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency (1≥Ecoal≥0) is larger than 10-3. Low-concentration suspensions, on the other hand, require much higher values of Ecoal for significant aggregation to occur. Dilute pyroclastic flows will have higher particle volume fractions (≈10–3) than the experimental currents, but then viscous pyroclasts should have lower coalescence efficiencies than PEG droplets.
Experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high-grade
ignimbrite sheets.
Received: 28 August 1996 / Accepted: 3 December 1997 相似文献
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Conventional seismic data processing methods based on post‐stack time migration have been playing an important role in coal exploration for decades. However, post‐stack time migration processing often results in low‐quality images in complex geological environments. In order to obtain high‐quality images, we present a strategy that applies the Kirchhoff prestack time migration (PSTM) method to coal seismic data. In this paper, we describe the implementation of Kirchhoff PSTM to a 3D coal seam. Meanwhile we derive the workflow of 3D Kirchhoff PSTM processing based on coal seismic data. The processing sequence of 3D Kirchhoff PSTM includes two major steps: 1) the estimation of the 3D root‐mean‐square (RMS) velocity field; 2) Kirchhoff prestack time migration processing. During the construction of a 3D velocity model, dip moveout velocity is served as an initial migration velocity field. We combine 3D Kirchhoff PSTM with the continuous adjustment of a 3D RMS velocity field by the criteria of flattened common reflection point gathers. In comparison with post‐stack time migration, the application of 3D Kirchhoff PSTM to coal seismic data produces better images of the coal seam reflections. 相似文献
13.
Formation of high-grade ignimbrites Part II. A pyroclastic suspension current model with implications also for low-grade ignimbrites 总被引：2，自引：2，他引：0
Armin Freundt 《Bulletin of Volcanology》1999,60(7):545-567
Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as
dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic
flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents
are derived from column-collapse modeling for magmas with an initial H2O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or
radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature,
bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents
to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80%
of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 107-1012 kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining
currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the
fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the
prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent
if eruption temperature is To>1.3.Tmw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation
of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle
aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout.
As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic
currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially
leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate
occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite
masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite
mass.
Received: 10 May 1998 / Accepted: 21 October 1998 相似文献
14.
焊接产品工业CT检测方法研究 总被引：1，自引：0，他引：1
目的：研究工业CT对钎焊焊接试样内缺陷的检测灵敏度，方法：运用工业CT对发动机钎焊产品质量的检测方法。结果：工业CT方法也适用于焊缝内部质量的检测。结论：可通过提高工业CT的空间分辨率以增强系统对缺陷的检测灵敏度。 相似文献
15.
影响煤层气解吸扩散运移的地质因素 总被引：1，自引：0，他引：1
基于大量基础研究，从不同角度探讨影响煤层气解吸扩散运移因素。分析解吸扩散运移过程中气体组分、储层介质、温度、压力、地应力等内、外因素对煤层气扩散运移的制约关系及程度，获得规律性的认识。 相似文献
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Ranking welding intensity in pyroclastic deposits 总被引：1，自引：1，他引：1
Welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Progressive welding is recorded by changes in the petrographic (e.g., fabric) and physical (e.g., density) properties of the deposits. Mapping the intensity of welding can be integral to studies of pyroclastic deposits, but making systematic comparisons between deposits can be problematical. Here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations (e.g., oblateness of pumice lapilli and micro-fabric orientation) and measurements of physical properties, including density, porosity, point load strength and uniaxial compressive strength. Our dataset comprises measurements on 100 samples collected from a single cooling unit of the Bandelier Tuff and parallel measurements on 8 samples of more densely welded deposits. The proposed classification comprises six ranks of welding intensity ranging from unconsolidated (Rank I) to obsidian-like vitrophyre (Rank VI) and should allow for reproducible mapping of subtle variations in welding intensity between different deposits. The application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre-welding thicknesses.Editorial Responsibility: T. Druitt 相似文献