VERTICAL MAGNETIC DIPOLE OVER TRANSITIONAL EARTH*

Harmonic steady-state solutions for magnetic and electric vector components are obtained for a layered transitional earth in a vertical magnetic dipole field. The conduction currents dominate the displacement currents. The time factor e^iωt is implied. MKS units are used.     

RESISTIVITY SOUNDING ON A TWO-LAYER EARTH WITH TRANSITIONAL BOUNDARY*

A theoretical solution is obtained for the problem of a two-layer earth with transitional boundary. In practice, the transition layer can stand for the weathered zone in hard rock areas where the degree of weathering diminishes with depth. Master curves and tables of data are presented for the case when the lower half-space is infinitely resistive.     

TIME-DOMAIN ELECTROMAGNETIC SOUNDING—COMPUTATION OF MULTI-LAYER RESPONSE AND THE PROBLEM OF EQUIVALENCE IN INTERPRETATION*

Computations of the time-domain electromagnetic response of a multi-layered earth have been carried out for different source-receiver coil systems. The primary excitation is a train of half-sinusoidal waveforms of alternating polarity. The conversion into the time-domain involves Fourier series summation of the matched complex mutual coupling ratios of the layered earth models computed by a digital linear filter method. As an example, the response of a perpendicular coil system on the ground surface for two source-receiver separations has been presented for a five-layer earth model. This has been compared with the responses of homogeneous, two-layer, three-layer, and four-layer models. Next, the investigations have been extended to study the problems of equivalence of three-layer models, the intermediate layer of which is either conductive or resistive. For an intermediate conductive layer (H-type), the studies show that in the early portion of the signal the interpretation of a true three-layer earth is possible to some extent, whereas the ambiguity due to equivalence persists in the late samples. On the other hand, for an intermediate resistive layer (K-type), the three-layer earth and its equivalent model cannot be distinguished from each other over the entire sampling period. On the basis of a computational approach, equivalence has been empirically established as √h/ρ=constant for H-type earth-sections, and as h²ρ=constant for K-type earth sections, where h and ρ are respectively the thickness and resistivity of the intermediate layer.     

GEOPHYSICAL PROSPECTING FOR BARITE *

A case history is presented for the exploration of barite with the electrical resistivity method. Altogether eight locations were recommended out of which six correspond to barite bodies. In the present case the gravity method, which is commonly used for the location of barite because of its high density contrast with the host rocks, has failed, perhaps due to small size of the ore bodies.     

CAN TRANSITION IN THE ELECTRICAL PROPERTIES OF THE EARTH BE DETECTED?*

The interface phenomena for low periods, i.e. the initial oscillations, and the cut-off periods are two characteristic features in Magnetotelluric Sounding curves. A method is proposed to distinguish between transitional and homogeneous layered earth by means of these two features.     

Detrital and authigenic(?) baddeleyite (ZrO2) in ferromanganese nodules of Central Indian Ocean Basin

Occurrence of baddeleyite (ZrO2) which is a rare mineral has been recorded in ferromanganese nodules of Central Indian Ocean Basin (CIOB). The mineral occurs either as independent isolated sub-rounded to elliptical grains or in clusters forming fine subhedral crystals (<3 mm) within ferromanganese concretionary growth bands. The mode of occurrence, textural features and chemistry of the mineral suggest detrital and possibly an authigenic origin for baddeleyite. For authigenic origin it is proposed that zirconium might have got released either from the terrigenous sediments or the altered seafloor rocks forming halogen complexes and subsequently it has re-precipitated in the form of baddeleyite within manganese nodules under oxic to sub-oxic conditions.     

Geology and Genesis of Barite Deposits of Lasbela and Khuzdar Districts, Balochistan, Pakistan

Abstract: Geology and genesis of the sediment-hosted barite deposits in the Lasbela and Khuzdar districts of Balochistan, Pakistan have been studied and described. Hot solutions generated in rifting environments during the initial stage of separation of the continental crustal block related to the Gondwanaland fragmentation subsequently reposed in Jurassic rocks, most probably played an important role on barite formation.
Paragenetic relationships indicate that the period of barite mineralization was distinctly later than that of zinc-lead sulphide mineralization in the Lasbela and Khuzdar districts. The barite deposits occur on top and in peripheral to the zinc-lead sulphides. The earliest barite mineralization took place in the clastic rock sequence of the lower Spingwar Formation as cross cutting veins. Younger mineralization as stratabound-replacement type occurred within the platform limestones of Loralai Formation, just above the Spingwar Formation. The youngest mineralization as stratiform is hosted within black shales, mudstones and argillaceous limestones of Anjira Formation at the top of Loralai Formation. In the light of the evidences gathered from field and laboratory investigations a genetic model has been deduced.     

Lateritisation of gabbro-granophyre rock units of the Ezhimala Complex of north Kerala, India

Laterites are ubiquitous in Kerala and are developed over different rock types. The laterite profiles developed over gabbro, granophyre of the Ezhimala Complex are discussed here.

The principal mineral assemblages of the laterites are kaolinite, gibbsite, goethite, hematite and quartz with minor amounts of zircon. Amorphous ferric hydroxide, Al-hydroxide and Al-silicate are also present. Samples from various units of the weathering profile have been analysed for major and trace elements. Major-element data show a steady depletion of Si and enrichment of sesquioxides and TiO₂ during weathering. It is also observed that the more the content of the mobile elements (Si, Mg, Ca, K, Na) in the parent rock, the greater is its depletion during lateritisation. The trace elements Co, Cr, V, Cu and Ni show an enrichment while Zn and Ba show depletion over both the rock types. Pb shows almost a consistent behaviour.     

Leaching behavior of lignite fly ash with shake and column tests

The maximum concentration of the majority of the trace metals in the leachates from shake and column test of lignite fly ash (LFA) was within the prescribed limits; however, total dissolved solids, total hardness, cations and anions (except K⁺), being above the prescribed limits, may lead to the increase in the hardness and salinity in the soil on the disposal of LFA. Present generation of huge amount of fly ash from thermal power plants (TPPs) is a big challenge concerning contamination of soil, crop produce and surface and ground water bodies due to the presence of some of the toxic trace metals in it. The leaching behavior of alkaline LFA (pH, 10.94), from TPP of Neyveli Lignite Corporation (NLC), India, was investigated by shake and column tests using water and sodium acetate buffer. The leaching of trace metals from LFA was governed by their concentrations, association with the ash particles, leaching duration and pH of the leachate (most influencing parameter). The leaching of metals followed the order: buffer column > aqueous column > aqueous shake > buffer shake test.     

Cenozoic development of southwestern Malay Basin: new insights from subsidence analysis and thermal history

Subsidence and thermal history analysis are carried out in order to investigate the Cenozoic basin development of the southwestern (Tenggol Arch and basinal side) part of the Malay basin. Structurally, the southwestern part consists of normal faults and horst and graben geometry. Tectonic subsidence curves show that the basinal side is more active than the Tenggol Arch due to movement along normal faults. Cenozoic development initiated with the deposition of sedimentary Units M & L (Oligocene) and the activation of the Tenggol fault on the basinal side. Several periods of accelerated and slow subsidence are observed during the Oligocene to Middle Miocene that could be associated with changes in regional stresses caused by pulsating plate movement. The Malay Basin experienced inversion throughout the Middle to Late Miocene related to mantle induced slab avalanche effect, causing relatively higher tectonic subsidence rates on the Tenggol Arch compared to the basinal side, suggesting that the Tenggol Arch is less affected by inversion than the basinal side. After a period (Late Miocene) of non-deposition, the basin was reactivated (Pliocene to recent) due to thermal relaxation with thick sedimentation. Paleo heat flows estimated utilizing a novel technique introduced in this study and present day heat flow calibrated using BHT data further supports our results, in that increase in heat flow is related to rapid tectonic subsidence. An anomalously high heat pulse affected the basin during inversion and could be the cause of meta-sediment formation whereas present heat flows, although high compared to average basins, shows decreasing trend from the inversion period.