用于多频电磁波测井的圆锥螺旋式有源接收天线

对用负阻抗变换电路同圆锥螺旋天线相结合，构成的有源接收天线进行了理论分析。并给出了该有源天线稳定工作的条件和设计原理。结果表明，用该方法设计的有源接收天线具有很宽的频带、较高的灵敏度和很小的尺寸，是多频电磁波测井较为理想的接收天线。     

Predisposition to Induced Seismicity in Some Czech Coal Mines

—Mining-induced seismicity associated with longwall face operations in the Ostrava- Karviná coal mines, Czech Republic, has been investigated in order to establish the conditions leading to a focal zone generation. The study, based on macroseismic and instrumental observations, proved that seismicity is influenced by natural as well as mining conditions. The first group includes the influence of faults, washouts and red beds, while the second one is represented by shaft and/or crosscut safety pillars and various types of remnant pillars. All the cases discussed show that many focal zones are generated in overstressed strata as a consequence of interaction of natural conditions and/or old workings with the active coal face. Received June 18, 1996, accepted March 10, 1997     

Time-dependent, coupled, Ekman boundary layer solutions incorporating Stokes drift

Recent technological advances in current measuring devices has resulted in a large observational database related to wind-driven motions in the upper ocean mixed layer. This has served to highlight the fact that transient motions make up a substantial contribution of the resulting Ekman currents. At the same time, certain discrepancies have emerged between the observed angular deflections of the steady-state currents from the surface wind stress, both at the surface and at sub-surface depths, which cannot be reconciled using the classical Ekman model. This paper seeks to tackle these two issues.First a general analytical method is presented for solving the time dependent horizontal momentum Ekman equations. Analysis of the unsteady terms that arise from simple special cases shows how the evolution proceeds through three stages. At early times, the Coriolis acceleration is insignificant, and the current is unidirectional and deepens through downward diffusion of momentum. Later Coriolis acceleration deflects the current vectors in the upper layers, whilst downward diffusion of momentum continues to deepen the layer. Finally, once diffusion has penetrated down to the depth of the steady-state current, then the transients decay on the inertial or diffusive timescale, depending upon the boundary conditions of the particular problem.In the second half of the paper, a new steady-state model is developed that includes the effects of wind-generated waves, through the action of their Stokes drift on the planetary vorticity. Comparisons between observations and the theoretical predictions, demonstrate that inclusion of the Stokes drift is the key to reconciling the discrepancies in the angular deflections of the steady-state currents. This leads to the conclusion that Ekman layer currents are significantly influenced by the surface waves.     

Microseismicity and active deformation of Messinia,SW Greece

By deploying a 30 3-component digital seismic array in the Messiniakos gulf and the surrounding region, we recorded for a period of 45 days the microseismic activity. With a minimum of six records per event, we located 1121 earthquakes corresponding to an average of 20 events per day. For the hypocenter location we used a local velocity model adopted to two controlled source seismic experiments. Within the array, traveltime residuals were within ± 0.2 s and the epicentral accuracy in the order of ± 2 km, while the hypocentral one is twice this value. Correlation of the seismicity with the tectonic elements indicated that most of the NW-SE oriented faults are active with strike-slip movement along this orientation and extension perpendicular to it. The neogene basins of Messini, Meligalas and Megalopolis are seismically very active and their eastern flanks are delineated by higher seismic activity than their western ones. This indicates that the basins are asymmetric with master faults defining their eastern-northeastern flanks. This hypothesis is supported by the asymmetric structure mapped at the offshore Messiniakos basin as densely spaced high resolution reflection seismic profiles have revealed. The western margins of the basins are less deformed and the seismic activity is dispersed over several minor NW-SE faults. Since the NW-SE striking faults onshore are truncated by major NE-SW oriented ones, their overall length is shortened, reducing their seismic potential and capacity to store large stresses that could produce events above Ms6.1. Offshore western Messinia, in the Ionian Sea, the size and activity of the faults is significantly larger and prone to develop events of larger magnitudes. Subcrustal seismicity indicates a deepening of the foci to the east-northeast.Part of this work was presented at the CIESM Conference, Monaco, 2001, and in the EGS General Assembly, Nice, 2001.     

HVSR prospecting in multi-layered environments: An example from the Tyrnavos Basin (Greece)

The Tyrnavos Basin (Central Greece) is an E-W trending graben bordered by two major antithetic sets of right-stepping dip-slip normal faults that began forming during Middle Pleistocene as a consequence of the N-S extension which affected the whole Aegean Region. This Quaternary structure is superimposed on the Pliocene-Early Pleistocene NW-SE trending Larissa Basin, approximately 700–800 m deep. Seismic activity is instrumentally well documented within and surrounding the investigated area. Based on structural, morphotectonic and palaeoseismological data, several structures previously characterised as active faults have been investigated by applying the HVSR method. This method is commonly used to map the thickness of sedimentary layers in a basin based on the variation of the fundamental 1-D frequency. The polyphased structural evolution and the consequently complex geological setting do not allow for the contouring of a unique continuous surface. We therefore developed a new interpretation technique for statistically significant HVSR peaks, in order to obtain 2-D sections that can be interpreted in a seismic-like fashion. We applied this technique across the major active faults bordering the basin. The results confirm their occurrence at depth and document the important vertical component of displacement.     

The Origin and Evolution of Silica-saturated Alkalic Suites: an Experimental Study

Experimental simulation of incremental crystal fractionationof a hy-normative hawaiite indicates that the spectrum of compositionsfrom mildly alkalic hawaiite to sodic rhyolite found in silica-saturatedalkalic suites of the ocean islands and continental hotspotsand rifts can be produced by fractionation at 9·3 kbarwith bulk water contents >     

Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton     

A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia--an Ion Microprobe U-Pb Study

Sensitive high-resolution ion microprobe U–Pb dating showsthat a biotite orthogneiss from the Hercynian belt of westerncentral Iberia contains 1000–300 Ma zircon. Older, 1000–570Ma ages within this range represent inherited, detrital materialamong which four age components may be recognized: