On the reduction of time variations for geomagnetic repeat station measurements

For most previous geomagnetic surveys, the time variations of the observatory closest to the site of observation have been used for reduction. The accuracy of repeat station measurements depends upon the distance between the station and the observatory. Nowadays, the requirements for the accuracy of repeat station measurements are much higher, especially with respect to local secular variation anomalies. On the other hand, transportable variograph stations are available, by means of which time variations can be recorded closer to the station site. The reduction steps for this method are:

1. Reduction of measured values to the base-line of the nearby variograph station;
2. Computation of the nearest quiet night value using the baseline of the variograph;
3. Computation of the difference between this value and the corresponding one at the observatory.

The problems are:

- the stability of the base-line values of the variographs;

- the question as to what degree a quiet night value represents the normal value at that time (e.g. influence of aD _st field and/or other fields).

A repeat station survey of the Federal Republic of Germany was carried out in 1982. During the same time, eleven stations recorded the time variations in that area. We discuss the aforementioned aspects by way of an example taken from this survey.     

Die Einschichtung der Rhone im Genfersee: Ergebnisse von Strömungsmessungen im August 1983

Results obtained by means of an ultrasonic current meter in the plume of the Rhone river are summarized as follows:

1. Currents of Rhone river water entering the lake were clearly discernible up to a distance of about 1 km from the river mouth. The interflow was observed at depth of 10 to 30 m.
2. Interflow velocities decreased with increasing distance from the river mouth: from 40cm/s at a distance of 350 m to about 15 cm/s at 1 km.
3. Short-term variations of current velocities and directions documented the highly turbulent nature of the interflow.
4. The entering river water (inflow direction to the NNW) interfered with a persisting northeastward current of the lake water. At a distance of 1–2 km from the mouth the interflow gradually assumed the same direction, possibly due to deflection by Coriolis forces.
5. Current velocities showed considerable variations within a time scale in the order of hours at the same measuring position. The reasons for these fluctuations remain unclear. Possible causes may be lateral oscillations of the entering river water or its deviation by river mouth bars during periods of reduced river discharge. Variations of the discharge alone cannot explain these current fluctuations.

     

Geophysical aspects of man-made electromagnetic noise in the earth—A review

The paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows:

- EM distortion effects of man-made constructions below and over the Earth's surface defined as ‘passive-noise’,

- field observation of EM disturbances due to ‘active’ man-made sources,

- EM source mechanism of some important active sources from a geophysical point of view,

- efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways,

- application of man-made EM noise for geophysical prospecting.

The paper is based on world-wide EM noise survey studies published mainly in geophysical journals.     

Liparitic volcanism of some regional volcanic structures in Kazakhstan,Middle Asia and Caucasus

Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:

1. Geo-structural zonation of the volcanic structures.
2. Dismembered Moho surface within the volcanic structures.
3. Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
4. Ignimbritic character of the liparitic volcanism.
5. Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.

Geo-structural and petrochemical zonations are likely governed in the regions studied by a deep-seated plutonic body.     

Seismic sources and attenuation properties at the Campi Flegrei volcanic area

Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:

1. Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.
2. Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.
3. A constant Brune stress drop pattern (～4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.
4. Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.
5. The scaling of peak ground motion parameters (A _max andV _max vs seismic moment) can be explained by an ω² source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.

These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.     

Origin of rhyolites by anatectic melting of granitic crustal rocks

Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km². The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows:

-mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø.

-phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice.

-mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff.

-structurally transitional types between pumice and the granites in different state of melting.

-eutectic composition of the pumice glass for a pressure of 2 kb.

     

Gases and water isotopes in a geochemical section across the Larderello,Italy, geothermal field

Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by

1. Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;
2. Constant CO₂ (95±2 percent); near constant H₂S (1.6±0.8), N₂ (1.2±0.8), H₂ (2±1), CH₄ (1.2±1), and no O₂ in the dry gas;
3. Presence of numerous, straight chain and branched C₂ to C₆ hydrocarbons plus benzene in amounts independent of CH₄ contents with highest concentrations in the deeper wells;
4. Oxygen-18 contents of steam increasing south to north from ?5.0‰ to ?0.4‰ with little change in deuterium (?42±2‰).

These observations are interpreted as showing:

1. Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;
2. Synthesis of CH₄ from H₂ and CO₂ with CO₂ and H₂ produced by thermal metamorphism and rock-water reactions;
3. Extraction of C₂ to C₆ hydrocarbons from rock organic matter;
4. Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ～220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.

     

Distribution de l’uranium,du thorium et du potassium dans les roches volcaniques de l’Afar septentrional (Ethiopie)

Concentrations in uranium, thorium and potassium were determined by gamma spectrometry in more than 100 samples of volcanic rocks from the material obtained during the French-Italian expedition in Dankalia (Afar-Ethiopia) (December 1967 – February 1968). These samples were taken from two geographically distinct areas:

1. the danakile depression (Erta Ale range, Pierre Pruvost massif, Alayta, Afdera, Borawlu).
2. Ethiopian high lands (Adigrate — Axum).

The values we obtained give the following contents:

Dash

in the Erta Ale range, for basalts, K: 0,5%, U: 1,3 ppm, Th: 3,3 ppm, Th/U: 2,6.

for alkaline rhyolites, U: 4,1 ppm, K: 2,3%, Th: 15,2 ppm, Th/U: 3,7.

in the Pierre Pruvost massif, for acid rocks, K: 3,9%, U: 3,6 ppm, Th: 13,5 ppm, Th/U: 3,8.

in the Borawlu centre, K: 2,8%, U: 4,6 ppm, Th: 14,7, Th/U: 3,4 for the rhyolites, K: 0,6%, U: 0,9 ppm, Th: 3,1 ppm for the basalts.

on the Ethiopian high lands, for the Adigrate basaltic trap rocks, K: 0,6%, U: 0,6 ppm, Th: 1,1 ppm, Th/U: 1,7 and for the Axum zone of phonolitic pinnacles, K: 3,9%, U: 2,7 ppm, Th: 14,9 ppm, Th/U: 4,9.

The Th/K and U/K ratios are particularly high compared with the usual data, except for the Pierre Pruvost massif which seems to be different from the others. The difference of Th/U ratios for basic rocks and acid rocks is meaningful. For the magma which gave the volcanism of the studied area, one can think of an evolution more especially connected with transfers of volatile elements. The strong correlations existing between potassium, uranium and thorium probably indicate the presence of groups which are distinct from a geochemical point of view in the Dankalia lavas.     

Numerical and experimental models on the formation mechanism of collapse basins during the Green Tuff orogenesis of Japan

The geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:

1. Dome-shaped uplift with a mean diameter of 30 km.
2. Collapse of the central part of the domes forming basins with a mean diameter of 10 km.
3. Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust.

In finite element analyses performed as plane strain problems, earth's crust is assumed to be an elasto-plastic homogeneous layer and to undergo sinusoidal vertical displacement at the base of the layer due to an ascending magma reservoir. These analyses reveal that the diameter of the dome is proportional to the depth of the magma reservoir rather than to its size. The magma reservoir is estimated at 12 ～ 24 km in depth. Scale model experiments using powdered material were performed in order to reproduce a collapse basin. These three-dimensional models are reduced to a scale of 1:200,000 th of the natural size. The results of experiments show that radial and concentric cracks are produced on top of the dome and a central part encircled by concentric cracks collapses to form a basin. The boundary of the collapsed portion forms a steep cliff with a height of about 2 mm. This is equivalent to 400 m in natural size and is nearly similar to field observations.     

Pétrologie de la Série Volcanique de l’île de Fayal (Açores)

Four volcanic units have been distinguished on the islanf of Fayal. In order of decreasing age, these are:

the eastern rift, and products of the activity preceding the collapse of the caldera visible at the summit of the stratovolcano, characterized by an alkaline series: basalt-hawaiite-mugearite-trachyte;

the products of the explosive and postcaldera activity where only evolved lavas occur (benmoreites and trachytes);

the recent basaltic activity of the Horta region;

the western fissural activity — recent and historical.

The two last units are characterized by exclusively basaltic, frequently picritic, eruptions. The lava groups cannot be distinguished by chemical criteria and have thus been treated as a single suite. Ninety samples have been analysed by X-ray fluorescence, and the mineralogy of 6 representative specimens has been determined by microprobe. The data were used to work out the evolution of the lava. The series is shown to have been produced by crystal fractionation under moderate water pressure from an alkali basalt. Moderate fractionation of amphibole during the last stages allow the liquids to remain weakly undersaturated from initial basalts until final trachytes. Mineralogical and chemical diversity between the most evolved lavas, benmoreites and trachytes, is an evidence of the strong influence ofpH₂O and/orfO₂ on the composition of such residua.     

Research on the principle and methodology of seismic zonation

Based on the cognizance of the temporal-spatial inhomogeneity of seismicity in North China, adopting the results of earthquake prediction in the past two decades and the currently used methods of seismic hazard analysis, and after some zonation trials in North China, some improvements on the zonation principle and methodology were made:

1. Seismic zones were taken as statistic units where seismicity parameters were obtained. Tendency analysis was introduced. Earthquake annual average occurrence rates were estimated corresponding to the seismicity level in the future period;
2. Average annual earthquake occurrence rates for a given magnitude interval of a specific seismic zone were assigned to potential sources considering the relative risk level among these sources. Thus, the risk of great earthquakes can be estimated.
3. The probabilistic spatial distribution function under the condition of magnitude interval was suggested to reflect the temporal and spatial inhomogeneity of seismicity.
4. An orientation function in the seismic hazard analysis model was adopted, which reflects the real condition of earthquake foci in China.

     

Perspectives on seismic and rockburst research in the South African gold mining industry: 1983–1987

This paper summarizes seismic and rockburst research activities related to South African deep-level gold mines over the period 1983 to 1987. It covers continued research in directions that were considered in the Seismicity in Mines Symposium in 1982 as well as in several new areas of research. Five broad areas are identified:

1. Seismic data acquisition and processing. Improved seismic systems are being developed. Velocity models related to known stratigraphy are being used to provide more accurate estimates of seismic locations.
2. Source mechanisms and near-source effects on seismic wave transmission. This work provides fundamental insights into seismicity and rock behaviour and is being applied in rockburst prediction research.
3. Mine layouts. Excess shear stress is being investigated as a design parameter by analyzing mining configurations and resultant seismicity. In addition, better understanding of the behaviour of highly stressed remnants and pillars is also being obtained from seismic studies.
4. Strong ground motion studies. Evaluation of the performance of support elements, including recently developed backfill materials, requires better knowledge of ground motion around underground excavations during seismic events and rockbursts.
5. Rockburst prediction and control. Rockburst prediction research continues with some reported success. In addition, the feasibility of actively triggering fault slip or conditioning the rock ahead of the stope face to ameliorate the rockburst hazard is currently being investigated.

     

Methodology for the construction of earth barriers to divert lava flows: the Mt. Etna 1983 eruption

This paper gives a critical analysis of the technical solutions adopted for erecting earthworks to divert lava flows on the basis of the experience obtained recently during the 1983 eruption of Mount Etna. The purpose of the paper is to explain questions of general nature and throw light on problems that must still be solved, which render it difficult to extrapolate the adopted solutions to other volcanic eruptions having radically different characteristics. An account of the earthworks made at the various sites to the east and the west of the lava flow is given, and an analysis is made of the planning choices, the decisions taken during the work, the daily output and thence the relative costs. The works carried out are summarized in Table 1 giving characteristics and costs of the works and times and sizes of the equipments used. The following works were carried out:

4 barriers for a total length of ca. 1700 m with a height variable from 8 to 20 m.

2 channels for a total length of 1000 m.

many emergency measures during night and day-time also on fresh lava.

During 50 days of 13 working hours per day a total volume of ca. 750,000 m³ of material was moved and ca. 10 km of service roads were built. The total cost of the above mentioned works was 3,678 millions of Italian liras. The final result fulfilled the planning forecasts: all the earthworks erected carried out their task of guiding and diverting the lava flows. Amongst other things, the study of the evolution of eruption has shown that preventive and emergency works can successfully be programmed. Finally, the technological aspects of the works are discussed with particular reference to their planning, site organization, determination of type and number of vehicles to be employed, output to expect and thus, for a correcta priori estimate of the advantage-cost ratio, a forecast of the expenses involved. The paper aims at giving information that may be of use during emergency situations and at stimulating the study of an operations program for mitigating the effects of lava flows in areas of volcanic risk.     

Sur l'éruption du Nyamulagira de décembre 1981 à janvier 1982: cône et coulée du Rugarambiro (Kivu,Zaïre)

The December 1981 — January 1982 eruption which started in the Christmas night on the SE side of Nyamulagira, gave the longest historical flow (26 km) representing the highest production rate of this volcano in this century (280×10⁶m³ of erupted magmas in 19 days). This eruption built Rugarambiro, a composite spatter cinder-cone. The ejected lava is a K-hawaiite (kivite) whose basicity decreased during the eruption (first emission: D.I. = 40; last products: D.I. = 35). This chemical evolution is reflected by:

--the modal composition of lavas. The first emissions are poor in ferromagnesian phenocrysts (olivine + clinopyroxene: 3%) and rich in plagioclase (12%); the contrary is observed in the last ejected lavas (livine + Cpx: 16%; plagioclase: 1%);

--the nature of the crystallizing minerals in the groundmass. In fact, only the first ejections include alkaline feldspars, nepheline and Tiphlogopite;

--the glassy phase composition which is more differentiated in the first lavas (D.I. 68–84) than in the last ones (D.I. 42–61).

A stratification of the Nyamulagira magmatic chamber is proposed where magmatic differentiation has probably occurred for fractional crystallization. Mineralogical thermobarometers lead to locate this magmatic reservoir at the depth of 6–7 km that we had already hypothesized. The presence of phenocrysts of bytownite, basic chrysolite, diopside and salite indicates a basaltic paragenesis which marks a hawaiitic magma chamber feeding.     

Characterization of and correction for cultural noise

Surveys of time varying electromagnetic fields result in time series consisting of signals and noise, the latter defined as that part of the data which cannot be explained by a theory. Man-made contributions to noise can be subdivided into active and passive sources and are complex in character. As Szarka has treated this topic extensively in a recent review paper (Szarka, 1988), only a few further examples are presented here. Following discussion of noise correction in transient electromagnetic investigations which consists mainly of sophisticated stacking and filter procedures, several aspects of its correction in magnetotelluric and geomagnetic depth sounding data are considered. These include:

1. The methods of treatment of single time series in the presence of visible noise—its detection, removal and sometimes replacement by data predicted from undisturbed intervals.
2. The investigation of time series interrelations. This is mainly coherence based and—if possible—takes advantage of remote reference techniques.
3. The examination of the statistical properties of the time series by regression analysis. This leads to the weighting of time segments of data in order to achieve unbiased and minimum variance estimates based on identically and independently Gaussian distributed residuals.
4. The application of constraints. These can further improve the estimates' quality.
5. The use of simultaneously recorded multistation data. This can contribute remarkably to noise suppression as well as to the treatment of non-uniform source fields.
6. Leveraging and confidence limits. Problems relating to the former have not yet been solved satisfactorily while the Jacknife method seems to be an easy way of determining the latter.

Thanks to the modern processing techniques reviewed in this paper it should be possible to obtain a rather dense net of high quality data in spite of the world-wide increasing noise level. As most processing codes are widely accessible current problems are more related to availability of instruments, carrying out the measurements and reserving enough time for thorough data processing.     

Turbidity and mass concentration of suspended matter in lake water: A comparison of two calibration methods

Turbidity measurements by foreward scattering nephelometer have been regularly conducted on the north basin of Lake Lugano (Switzerland/Italy) since September 1992. In order to determine mass concentrations from these data, both, in-situ and laboratory calibration experiments have been performed in November and December 1992 on suspended matter of the hypolimnion. Results show:

1. Mass concentration/turbidity ratios are different in the two turbidity zones examined due to variation in the optical signature of the suspended matter.
2. Laboratory calibrations using suspended matter originating from the water depths of interest give representative results provided that physical and chemical alterations occuring during the particle transfer process remain negligible.

     

The effect of underlying geological structures on the propagation of seismic waves

Viewing from the energy angle and taking the Beijing depression as an example, this paper studies the effects of underlying geological structures, mainly bedrock topography and bedrock faults, on the propagation of seismic waves and discusses the effects of the overlying soil layer on seismic waves. From the study, some conclusions are drawn as follows:

1. Underlying bedrock faults affect the duration, frequency spectra and characteristics of energy distribution of seismic waves.
2. Underlying bedrock topography changes the field of ground motion not only because the bedrock at different places receives different amounts of energy from the same source but also because its asperities diverge or converge seismic waves.
3. Overlying soil layer is able both to absorb and to amplify seismic waves.

In the paper, the idea of expressing the intensity of seismic waves in terms of energy is put forward. Comparison between the expressions of the seismic wave intensity in terms of energy and the maximum amplitude shows that the former is better than the latter in reflecting the effects of underlying geological structures on seismic wave propagation.     

Characteristics of afferent activity of ampullary electroreceptors in catfishParasilurus asotus

The spontaneous and evoked activity of electroreceptors were electrophysiologically studied. The results are:

1. The spontaneous discharge rate ranged from 15 to 85 imp/s with a mode of 50 imp/s for 126 organs of 18 animals.
2. By analysis of periodic histograms and interspike interval histograms of responses to sinusoidal electric stimulations, the frequency response characteristic was determined. The frequency response curve shows a band pass type, the band width ranged from 5 Hz to 30 Hz and the best frequency is around 15 Hz.
3. The thresholds of responses for 47 organs were measured by injection of sinusoidal current into the organs. The threshold values were less than 0.1nA (61µV/cm) for 35 organs (74%), and less than 0.01nA (6.1 µV/cm) for the rest of 9 organs (19%).

     

Lake restoration. Why oxygenation and artificial mixing cannot substitute for a decrease in the external phosphorus loading

Data obtained from Lakes Baldegg and Sempach, two artificially mixed and oxygenated lakes, show that it is technically possible to significantly improve redox conditions, even in large eutrophic lakes. It is discussed why

- the well-known release of phosphorus from anoxic sediments does not necessarily result from an abiotic reduction of phosphorus containing iron compounds,

- net phosphorus retention by sediments is not simply linearly related to the phosphorus content of a lake,

- artificial oxygenation of a previously anoxic hypolimnion does not permanently increase phosphorus retention capacity of lake sediments.

From this it is concluded that improvement of hypolimnic redox conditions by lake internal measures such as aeration or oxygenation may accelerate the rate of recovery induced by a reduction of the external phosphorus loading, but that oxygenation per se will hardly be able to cause a reduction of trophic state.     

On gradings for one — dimensional seismologic predictions

This paper gives a complete commentary on some popular gradings which based on the 2×2 contingency table, for one—dimensional seismologic prediction. It contains:

1. In view of the statistical correlation, the Wallen gradingV=√QS is the most equitable one amomg all gradings.
2. Each one of the three gradings: Zhenchao Gu gradingS, Obuhov gradingQ and Shaoxie Xu gradingR, has its own inadequacy although they are so easy for use.
3. A set of gradings can be derived fromV. The harmonic mean ofS andQ, H(S,Q) and the Hedike gradingS _H are the members of this set. The order of some popular gradings are given in Theorem 5. 3.
4. In view of the measure of the overlapping part a gradingW is introduced. Another set of gradings may be derived from it. The index of efficiencyh is the severest one among this set.