Measurement of the signal to noise ratio in seismic profiling

The measurement of the signal to noise ratio in continuous seismic profiling systems is carried out by means of a small general purpose digital computer equipped with an analog to digital converter. The computer is programmed to compute and compare the power spectrum of a sample of noise to the power spectrum of a sample of noise plus an echo. Power spectra of some twenty to fifty shots are averaged to yield reliable estimates of the signal to noise ratio as a function of frequency. The observed spectra agree well with spectra calculated from signals measured near to an air-gun. This measurement of signal to noise ratio as a function of frequency provides unambiguous information for the adjustment of the necessary band-pass filter thus improving the operation of a continuous seismic profiler.     

A note on the seafloor coupling characteristics of the new ONR ocean-bottom seismometer

A series of transient tests were conducted to determine the seafloor coupling characteristics of a new ocean-bottom seismometer (OBS) developed for the United States Office of Naval Research (ONR). The OBS comprises a large recording package and a separate sensor package that is deployed from the recording package. In addition to the coupling characteristics of both the sensor and the recording packages, the seismic energy radiated from the main recording package as a result of motion of the recording package was measured. The observed vertical coupling resonances of both the recording package and the sensor package are in good agreement with those predicted by a simple model of soil-structure interaction. The most important result of this study is that significant energy is radiated from the recording package in response to horizontal motions of the recording package. When the sensor package is 1 m from the recording package, the amplitude of the recorded signal is similar to that recorded in the recording package. In the field, this effect will result in distortion of seismic signals and increased background noise recorded by the sensor package if the recording package is disturbed by seafloor currents or biological activity. The amplitude of this signal attenuates by approximately a factor of two as sensor/recorder separation is increased from 1 to 6 m, suggesting that an improved response can be achieved by increasing the separation between the recording package and the sensors. This effect is much less severe for vertical disturbances of the recording package.     

GPS signal acquisition using the repeatability of successive code phase measurements

This paper introduces a new method for GPS signal acquisition, which is based on the repeatability of successive code phase measurements and the M-of-N search algorithm. The performance of the proposed method in terms of probability of signal detection is similar to that of traditional methods, except that the calculation of the probability of detection does not rely on the noise distribution or the Carrier-to-Noise ratio (C/N₀). The code phase repeatability-based method is presented along with equations for probability of detection and probability of false detection. If the distribution of the noise is known, it also provides an estimate of the C/N₀. The proposed method is illustrated for coherent and non-coherent acquisition and C/N₀ estimation.     

Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh

Background  

The origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh.     

Retrograde fluid infiltration in the high-grade Modum Complex South Norway: evidence for age,source and REE mobility

The high-grade metamorphic basement of the Modum Complex, South Norway,exhibits retrogradation and alteration due to late stage fluid infiltration.Extensive alteration zones of albite-and calcite-rich veining occur especiallywithin and around numerous metagabbros. The gabbros, intruded at 1224±15Ma, are now partly altered to amphibolites due to the subsequent high-grade metamorphism.Two generations of albite-rich rocks have been recognized: (1) a fine-grained, foliated type;(2) a coarse-grained, crosscutting type. Both types show a typical greenschist facies mineralassemblage; albite ± actinolite ± chlorite ± talc. The calcite veins/dykesrepresent a younger generation of veins than both albite-rich types. U–Pb data for spheneof type (1) yielded an age of 1080±3 Ma, determining a point on the retrogradeP-T-t path of the Modum Complex. Increasing albitisation of themetagabbros leads to a decrease in(¹⁴³ND/¹⁴⁴Nd)_oand an increase in (⁸⁷Sr/⁸⁶)_o.Albite- and calcite-rich samples show negative _Nd and positive _Sr, suggesting that fluids which interacted with the metagabbros originated from a crustal reservoir. The Nd and Sr isotopic data show disequilibrium at the microscale as well as at the macroscale. Negative Sm–Nd model ages of the albite-rich rocks demonstrate that rare-earth elements (REEs) were mobile and fractionated during albitisation.     

Cu 2p absorption spectroscopy as a probe for the site occupancy of (ZnxCu1−x)WO4 solid solution

The solid solution sanmartinite (ZnWO₄)—cuproscheelite (CuWO₄) has been studied using Cu 2p X-ray absorption spectroscopy. While a single L₃ absorption peak is observed for CuWO₄, two distinct L₃ absorption peaks with a separation of ～0.8 eV are observed for the intermediate samples in the solid solution. The two peaks represent distinct Cu sites: one with all CuO₆ next nearest neighbours in the (Cu,Zn)O₆ chains, another having at least one ZnO₆ next nearest neighbour. Both sites show a linear increase in covalency as a function of increasing Cu-content. The relative intensities of the two absorption peaks is dependent upon the Cu-content and has been used to model the site occupancies. The results reveal that the local structural effects can be associated with a composition-dependent structural phase transition from P2/c (ZnWO₄) to P $\bar 1$ (CuWO₄). Deviations from a single-site model are explained in terms of the local environments, and evidence for site preferences and local clustering are explored.     

A plan for a new generation 2M-class telescope in Indonesia

One of the prime astrophysical interests of the Observatorium Bosscha is, and has always been, double star research: visual double star research with the double-60 cm Zeiss telescope (dedicated in 1928), and theoretical research of evolved massive spectroscopic binaries (since 1972). For one thing, this is the very reason that this IAU Colloquium No. 80, celebrating the 60th anniversary of the Observatorium Bosscha in Lembang, is devoted to binary astrophysics.Up to now, visual, photographic, and photometric tools have been used for binary research at the Observatorium Bosscha. An important, essential additional tool for binary research is spectrographic equipment, in order to measure radial velocities of binary components.Therefore, we suggest to make a plan for a new modern telescope, a reflector with a primary mirror of about 2 m in diameter and with a modern spectrograph/detector combination for radial velocity measurements.At a number of major astronomical observatories scientists have been considerating to erect new telescopes devoted primarily to radial velocity measurements. The reason for this is that at the end of this decade the parallax and proper motion measurements to be made by the ESA astrometric satellite Hipparcos will become available of more than 100 000 single stars and double stars. At that time there will be a compelling need for radial velocity measurements of all these stars to complement the parallax and proper motion measurements. With the combination of this data enormous progress will be made in double star research, and in the study of galactic dynamics, another topic of interest of the Observatorium Bosscha. If it could be realized to build such a dedicated radial velocity telescope in Indonesia, Indonesian astronomers could take a leading role in this field of research.Without going into technical details, we would like to emphasize here that such a new instrument should be a trueNew Generation Telescope, and that the Institut Teknologi Bandung should participate from the very beginning in its design, construction and assembling, and the subsequent servicing; ITB could participate in the field of optics, mechanics, and electronics. Modern astronomy offers tremendous challenges to technology, which are of great interest to technological institutes. The new telescope should be computer controlled, and the spectrograph should have a modern digital read-out (Reticon, IPCS, or CCD). The telescope should have one of those recently becoming availablethin mirrors, allowing more mechanical freedom. It could be a telescope with a siderostat which feeds the light into a fixed telescope, thus improving both the stability of the telescope and that of the spectrograph. In this way the staff and students of ITB, as well as the technical staff of the Observatorium Bosscha will be drawn into modern techniques of many varieties. And for ITB such an enterprise may even have a spin-off into other fields than astronomy.One aspect which is of great importance for the new telescope is the selection of its site. The present site of the Observatorium Bosscha in Lembang is a good one, but for a new modern telescope one wants to make sure that it is going to be located at the most ideal site.Therefore an Indonesian site-survey should be initiated promptly. Site survey equipment is available at many big observatories and could be borrowed. The site survey should extend over at least 4–5 years to monitor the meteorological and environmental situation at many sites.In the meantime the design and fund rainsing can be considered. Modern day astronomy depends on financial support from governments and inter-governmental organizations. Therefore it is urged that a proposal for a new telescope as indicated above clearly describes the advantages of such a new telescope both for astrophysical research in Indonesia, and for the introduction of new technologies in Indonesian technological institutes.The recently formed Steering Committee for Indonesian-Netherlands Astrophysics (INA) is willing to explore the possibilities for this plan. We hope that after investigating the interest of ITB in this matter, a proposal could be made before the end of this year.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia 3–7 June, 1983.     

Jupiter: New retrieved thermal profiles and ammonia distributions

New thermal profiles of Jupiter are retrieved from recent far infrared spectral measurements and for H₂ mixing ratios varying from 0.8 to 0.94. The effective temperature corresponding to the inferred thermal profile is 123.15 ± 0.35°K. Far-infrared brightness temperature spectra computed from these profiles are compared to experimental data including measurements made at high spectral resolution in the NH₃ν₂ band at 10 μm and in NH₃ pure rotational bands between 40 and 110 μm. It is found that a strong depletion of NH₃ does occur in the Jovian stratosphere and that ammonia seems to be undersaturated in the upper troposphere.     

High-density CO2−N2 inclusions in eclogite-facies metasediments of the Münchberg gneiss complex,SE Germany

The eclogite-facies metasedimentary rocks in the Münchberg gneiss complex (T=630±30° C/P17–24 kbar) locally contain CO₂–N₂-rich fluid inclusions of extremely low molar volumes (32 cm³/mol) in quartz. These fluid compositions are mainly found in rocks intercalated with calcsilicate bands. Densities were determined from low-temperature phase transitions like stable or metastable homogenization (L+VL), partial homogenization (S+L+VS+L) and the transition S+LL (L = liquid, V = vapour, S = solid). The high fluid densities are in agreement with eclogite-facies pressure and temperature and subsequent amphibolite facies. CO₂–N₂ inclusions were not observed in adjacent eclogites nor in non-calcareous metasediments. These rock types contain predominantly H₂O-rich inclusions correlating with amphibolite-facies conditions. The variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite-facies metamorphism.     

Organic geochemistry of silicified wood,Petrified Forest National Park,Arizona

Lignin derivatives have been isolated for the first time from carbonaceous sections of the silicified (~90% SiO₂) conifer Araucarioxylon arizonicum. The products released by sequential high vacuum pyrolysis and identified by combined gas chromatography-mass spectrometry (GC-MS) include a wide variety of alkyl-substituted, phenolic and condensed aromatic compounds.Brauns spruce lignin was pyrolized and analyzed by GC-MS as a comparison for the fossil wood data. The primary pyrolyzates at the 300°C step were CO₂, H₂O, ethanol and propanol. The main product at 450°C was 4-methyl-2-methoxyphenol (methyl guaiacol), but at 600°C the pyrolyzates were similar both in product composition and in relative abundance to those from silicified wood. The results suggest that the fossil wood experienced a mild thermal event during which the ether bonds were ruptured and loss of oxygen occurred along with the rearrangement of the original wood into a highly stable polymer.