Static deformation of two welded half-spaces due to very long strike-slip dislocations

Closed form analytic expressions for displacement and stresses at any point of either of the two homogeneous, isotropic, perfectly elastic half-spaces in welded contact due to very long strike-slip dislocations are obtained. Both cases of vertical and horizontal strike-slip dislocations are discussed in detail. Variation of the displacement with horizontal distance from the fault and with vertical distance from the interface for a vertical strike-slip fault is studied numerically.     

Computation of synthetic seismograms for plane layered earth models including absorption and dispersion phenomena

A computer program package has been written in FORTRAN-IV language and tested successfully on an ICL 1904S computer. This program enables one to compute synthetic seismograms for layered earth models. The provision for studying the effect of absorption and dispersion of seismic waves has been made with Subroutines. The present program utilizes eight Subroutines and requires about 35 K core memory. A set of examples are illustrated for absorption and dispersion models. An exponential decay of amplitude has been used for the absorption model. This method is based on the plane wave propagation in a flat-layered earth system. Normal incident P-waves are used to eliminate the effect of other phases. Change in shape of reflected waves is observed in absorption model due to damping of energy of higher frequencies. Lack of resolution is found between closely spaced reflections at higher frequencies. The effect of dispersion on seismic waves decreases the time of primary reflections as well as amplitudes of the seismic waves.     

Simulation of the hydrocarbon structures using the P-SV wave solution

Numerical solution of the scalar and elastic wave equations has greatly aided geophysicists in modeling seismic wave fields in the complicated geologic structures containing hydrocarbons and hence increases the geologic interpretation. Finite-difference method offers a versatile approach to compute synthetic seismograms numerically for given subsurface complex geological structures. To avoid the spatial derivative of the elastic parameters and density, elastodynamic equation (first-order hyperbolic equation) has been solved using the Lax-Wendroff scheme. A numerical finite-difference modeling program has been developed for the P-SV wave using the above solution. A line source with a time delay of 0.015s and dominant frequency of 120 Hz has been utilized in the simulation. In order to avoid the large values of the displacement vector in the source region,Alterman andKaral's method (1968) has been utilized. Horizontal and vertical component synthetic seismograms have been computed for two different geological models with and without oil and gas bearing zones. It has been concluded from the response that a finite-difference technique not only yields the relative arrival times but also accounts for the variation in amplitude and phase according to the elastic impedance contrast across the interfaces. It should come as no surprise to learn that in spite of the limitation of this numerical method, the scheme has provided a valid response for the thin layer, high acoustic impedance contrast and the pinch out.     

A Fourier transform method for the interpretation of self-potential anomalies due to two-dimensional inclined sheets of finite depth extent

The self-potential anomaly due to a two-dimensional inclined sheets of finite depth extent has been analysed in the frequency domain using the Fourier transform. Expression for the Fourier amplitude and phase spectra are derived. The Fourier amplitude and phase spectra are analysed so as to evaluate the parameters of the sheet. Application of this method on two anomalies (synthetic and field data) has given good results.     

Chronology and petrogenesis of young achondrites,Shergotty, Zagami,and ALHA77005: late magmatism on a geologically active planet

A study was undertaken to determine the chronology, petrogenesis and relationships among the shergottites, Shergotty and Zagami and the unique achondrite ALHA77005. These meteorites are the product of a variety of complex processes.Petrogenesis: Chondrite-normalized abundance patterns of Shergotty and Zagami are very similar and show pronounced depletions of both the light REE (La-Nd) and heavy REE (Dy-Lu) relative to Sm-Gd. These characteristic depletions are even more pronounced for ALHA77005. The light REE depletion is qualitatively consistent with the presence of cumulus pyroxene and/or olivine in these meteorites, but trace element models show that the parental magmas of all three meteorites were probably also light REE depleted. Both trace element model calculations and combined Rb-Sr and Sm-Nd isotopic systematics show that the meteorites could not have been co-magmatic nor can ALHA77005 be representative of the source material of the shergottites. Light REE depletion of the parental magmas also implies light REE depletion of the source material. The Sm-Nd systematics of the shergottites require a time-averaged sub-chondritic (light REE enriched) Sm-Nd ratio since 4.6 AE ago. The Sm-Nd systematics of ALHA77005 permit a time-averaged super-chondritic (light REE depleted) Sm/Nd ratio if its crystallization age is less than T_ICE = 0.72 AE.Chronology. Rb-Sr internal isochrons for all three meteorites and a Sm-Nd internal isochron for Zagami are concordant at ~ 180 Myr. ³⁹Ar-⁴⁰Ar plateau ages of Shergotty and Zagami maskelynite are ~250–260 Myr. These ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite. The concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity. The time of this event is most precisely given by the Rb-Sr age of 180 ± 4 Myr for Zagami.The crystallization ages of the meteorites were not precisely determined. Extreme upper limits are determined by Sm-Nd model ages relative to an eucrite initial ${}^{143}\text{Nd}{}^{144}\text{Nd = 0.505835}$ at 4.6 AE ago. These model ages for Shergotty, Zagami and ALHA77005 are 3600, 3500 and 2850 Myr, respectively. The Sm-Nd whole rock age of 1340 ± 60 Myr for the three meteorites gives the crystallization age if the Sm/Nd ratios of the precursor materials were always the same. We consider this 1340 Myr age as a “best estimate” upper limit. “Best estimate” lower limits for Shergotty and Zagami are taken from the average ³⁹Ar-⁴⁰Ar ages of 1200 and 900 Myr of pyroxene separates. The average ³⁹Ar-⁴⁰Ar age of a whole rock sample of ALHA77005 was 1600 Myr and can be partitioned between a low temperature (feldspar) phase and a high temperature (olivine + pyroxene + inclusions) “phase”. The average apparent ³⁹Ar-⁴⁰Ar age of the low temperature phase is ~1050 Myr, which is chosen as the “best estimate” lower limit to the age. The crystallization ages of Shergotty, Zagami and ALHA77005 probably lie within the ranges of 1200–1300, 900–1300 and 1000–1300 Myr, respectively. The Rb-Sr whole rock age of 4400 ± 400 Myr and single-stage BABI model ages of ~4800–5100 Myr are interpreted as reflecting differentiation of the parent body at ~4600 Myr ago.The complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history.     

A method for rapid evaluation of magnetic anomalies over thin sheets

A method for rapid evaluation of magnetic anomalies over thin sheets is presented. This method is based on characteristic distancesX _3/4,X _1/2,X _1/4 at which the anomaly falls off to 3/4, 1/2, and 1/4 of the total (peak to peak) amplitude, respectively. Simple mathematical relations using these characteristic distances are presented to estimate the death and dip of the sheet. This method is fast and does not require prior knowledge of the base level and the origin. A few field examples are included to show the applicability and efficacy of the method.     

A study of the vertical magnetic anomaly caused by magnetite deposits in Sua area of Palamau district,Bihar (India)

Summary A magnetic survey carried out in Sua area of the Palamau district, Bihar (India) has shown very interesting anomalies. The survey has been conducted specially to obtain the extension of the magnetite deposits which are being quarried at Sua. The interpretation of magnetic anomalies have clearly brought out the following facts: a) the magnetite is extending both sides of the quarry to a considerable distance, b) the nature of the anomaly indicates that the magnetite occurs in the form of veins. It has also been possible to find out the thickness of the veins and the depth at which they occur.     

The organic analysis of the murchison meteorite

The organic compounds released from the Murchison carbonaceous chondrite following vaporization-pyrolysis at 150, 300 and 430°C were investigated. The total organic yield was 272 ppm and consisted of n-alkanes, alkenes, aromatic hydrocarbons and thioaromatics. The composition and yield at all three temperatures are compared with those obtained by an identical analysis on another carbonaceous chondrite, Allende, and two terrestrial rocks.Comparison of compounds released by 150°C volatilization of Murchison and Allende reveals remarkable similarities, suggesting commonality in the processes responsible for their formation. The origin of the organic compounds found can be explained in terms of the Fischer-Tropsch synthesis followed by partial equilibration of the primary products. However, problems concerning the relationship between types of carbonaceous chondrites remain unresolved.     

Circulation and fertility of waters in Concepcion Bay

The spatial and temporal distributions of oxygen, nutrients and pigments in the waters of Concepcion Bay, Chile (36°40′S, 73°01′W) are described for 1978–1979. Analysis of the seasonal fluctuations shows the upwelling of water poor in oxygen and rich in nutrients inside the bay during summer. The upwelled water fertilizes the bay and produces progressive eutrophication, causing mass mortalities, discoloured water and mineralization of organic matter.