Stable clustering of offshore downhole data using a combined k-means and Gaussian mixture modelling approach

Marine Geophysical Research -     

Ocean coast effect on magnetotelluric data: a case study from Kachchh, India

The effects of the ocean coast on magnetotelluric (MT) data have been studied and results are described in this article. MT soundings from various sites along coastal plains of Kachchh were acquired where volcanic rocks overlie Mesozoic sediments having potential resource prospects. The region of our study lies in the proximity of Gulf of Kachchh and Arabian Sea. This article describes the effects the presence of a conductive body on the acquired MT data. With the help of synthetic MT modelling and induction arrows we demonstrate the influence of shallow conductive body on the MT data. The modelling results when compared to the field observations show that much of the high frequencies are unaffected by the coast, however low frequencies of the data do seem to be distorted by onshore–offshore resistivity contrasts. The least affect on high frequencies can be attributed to very shallow water depth as well as to the fact that the onshore–offshore resistivity contrasts are not large enough to influence the high frequency data badly. The results presented here report for the first time the effect of the coast on the analysis on MT data from Kachchh. Due to several academic as well as industrial onshore–offshore MT experiments presently being carried out to explore its hydrocarbon potential, our results have an important bearing on designing and acquisition of future MT surveys in this region.     

Rhenium in rivers and estuaries of India: Sources,transport and behaviour

The concentration of dissolved and particulate Re have been measured in the Narmada, Tapi and the Mandovi estuaries in the Arabian Sea and the Hooghly estuary in the Bay of Bengal. Re concentration in water and particulate matter of these estuaries is highly variable. Re in river waters analysed varies from 1 to 41 pmol/kg, the lowest in the Mandovi and the highest in the Mahi river. Re concentrations in the rivers analysed except in the Mandovi river are higher than the average global riverine Re concentration of 2.1 pmol/kg. Based on this study and the available data, the contemporary global annual flux of dissolved riverine Re is estimated to be ~ 350 × 10³ mol with an average concentration of ~ 9.2 pmol/kg, much higher than the earlier estimates. Residence time of Re in the oceans based on this estimate is 175,000 years, ~ 4 times lower compared to earlier estimates. Re behaves conservatively in all the estuaries studied. Re concentrations of seawater in the Bay of Bengal and in the Arabian Sea, estimated from the data of the Hooghly and the Mandovi estuaries respectively are ~ 40 pmol/kg, similar to the open ocean Re values of the Arabian Sea measured in this study and the values reported for in other oceanic regions. However, the dissolved Re in the Gulf of Cambay is 2 to 5 times higher, consistent with the high Re measured in the Mahi estuary and in the coastal waters of the Gulf of Cambay. The source of high Re in the Gulf of Cambay seems to be anthropogenic, measurements of Re in rivers and industrial waste waters draining into the Gulf supply amount to ~ 2300 mol of Re annually. This anthropogenic supply coupled with high residence time of water in the Gulf contribute to its high Re. Re concentration in suspended sediments of the Narmada estuary varies from 1 to 2 pmol/g, and does not show any discernible trend with salinity.The contemporary global riverine Re supply to the oceans estimated in this study is ~ 2–4 times higher compared to its removal in the reducing (anoxic/suboxic) sediments, indicating non-steady state of Re in the ocean. High dissolved riverine Re flux coupled with high Re content in the Gulf of Cambay highlights the need of a detailed study of Re in the various global rivers and in oceans including coastal regions and semi enclosed basins of the world to understand its behaviour in various reservoirs and to constrain the residence time of Re in the ocean.     

Adaptive optimal control of an autonomous underwater vehicle in the dive plane using dorsal fins

In this paper, adaptive control of low speed bio-robotic autonomous underwater vehicles (BAUVs) in the dive plane using dorsal fins is considered. It is assumed that the model parameters are completely unknown and only the depth of the vehicle is measured for feedback. Two dorsal fins are mounted in the horizontal plane on either side of the BAUV. The normal force produced by the fins, when cambered, is used for the maneuvering. The BAUV model considered here is non-minimum phase. An indirect adaptive control system is designed for the depth control using the dorsal fins. The control system consists of a gradient based identifier for online parameter estimation, an observer for state estimation, and an optimal controller. Simulation results are presented which show that the adaptive control system accomplishes precise depth control of the BAUV using dorsal fins in spite of large uncertainties in the system parameters.     

Impact of Satellite Altimetry on Simulations of Sea Level Variability by an Indian Ocean Model

This article describes the impact of satellite altimeter data on the simulations of sea level variability (SLV) by a nonlinear reduced gravity model of the entire Indian Ocean. The model has been forced by 6-hourly analyzed wind stress data containing SSM/I observations and has been able to produce realistic circulation features. However, SLV values observed by Topex/Poseidon altimeter do not fit these simulations because of imperfect initial data. Hence an attempt has been made to initialize the model using altimeter data. The initialized model-generated SLVvalues have been compared with SLV derived by altimeter for monsoon as well as nonmonsoon months of 1996. Experimental runs have been performed for 10 days, 20 days, and one month. It has been found that the initialized model results on the final day of these experiments are in very good agreement with altimeter data of the same day. It is thus possible, in principle, to hindcast and forecast sea level variations in the time scale of 10 days to one month with the availability of good quality wind data for forcing the model and altimeter observations of sea level for initializing it.     

Spatial Variation of Sea Level Trend Along the Bangladesh Coast

The Bangladesh coast is threatened by rising sea level due to various factors. The results based on the analysis of past 22 years of tidal data of the Bangladesh coast reveal that the annual mean tidal level in the eastern Bangladesh coast is rising at an alarmingly high rate of 7.8 mm/year, which is almost twice the observed rate in the western region. This type of sea level trend seems to be the result of changing local conditions like increased precipitation and land subsidence during the recent decades. It seems that the higher rate of land subsidence in the eastern Bangladesh coast is the main causative factor for the steeper sea level trends there. The differential sea level trends show that the subsidence component in the sea level rise may be as high as 4 mm/year in the eastern Bangladesh coast. However, this needs to be verified with actual geological observations.     

Concurrent Use of OSCAT and AltiKa to Characterize Antarctic Ice Surface Features

Radar altimetry provides an important geophysical parameter, backscatter coefficient (σ₀), which is useful in studying target surface characteristics. Ku-band (Oceansat-2 scatterometer- OSCAT) and Ka-band (SARAL-AltiKa altimeter) data are concurrently used to characterize polar surface features over the Antarctic region. Maximum-likelihood classification has been employed to classify combined data set (AltiKa and OSCAT) for discrimination among sea ice, open water, and ice sheet (interior and exterior). The sea ice region obtained using the current approach has been compared with sea ice boundary derived from passive microwave data.     

Late Cainozoic stratigraphy,palaeomagnetic chronology and vegetational history from Lake George,N.S.W.

The sedimentary record from Lake George provides the longest relatively continuous Quaternary continental sequence yet available from Australia, and may record one of the longest Upper Cainozoic lacustrine records in the world.

Palaeomagnetic analysis of a 36 m core from the lake floor identifies a sequence of deposition extending through the Brunhes and Matuyama, to the Gauss magnetic Chron. A longer core from the same site, but with incomplete recovery, extends to 72 m in lacustrine sediment; the age of the base of this core estimated by extrapolation is between 4.2 and 7 Ma. As there are still older and deeper sediments in the basin, extending to an estimated depth of 134 m, the age of the tectonic formation of the Lake George basin must be reckoned as Middle Miocene or older.

The pattern of facies organisation through time demonstrates a phase of deep water deposition extending from the base of the cored sequence (72 m) up to 51.5 m, at which time a major change took place. A disconformity developed at this level, associated with a period of deep weathering and a prolonged phase of slope mantle deposition (from 51.5 to 30.8 m). A gradual return to lacustrine environments, with diminishing proportion of slope wash detritus, resulted in increased rates of deposition coincident with the Jaramillo Subchron at 21.5 m. Thereafter, throughout the Brunhes magnetic Chron, lacustrine conditions dominated, varying from deep to lake dry conditions in a rhythmic fashion, and reflecting the major climatic oscillations of the past 700 000 years, becoming more regular in the past 400 000 years.

The pollen analytical record of the upper 8.6 m, covering the last 350 000 years, provides the main framework for the reconstruction of climatic history. The pollen and algal records indicate a sequence of vegetation and lake level changes, in which four major glacial/interglacial cycles are correlated with stages 1 to 10 of the ¹⁸0 marine record. This provides by far the longest continuous biostratigraphic framework for the Quaternary period in Australia.

Comparison between the palaeoclimatic record and the lake level evidence shows that there is no simple correlation between the lake level fluctuations and the glacial/ interglacial oscillations. In fact, major falls in the lake level occured both at the peak of cold glacials and during the warm interglacials. Though the falls in the lake levels during a warm period (interglacial) can be explained by high rates of evaporation, drying during maximum cold can be explained best in terms of a fall in precipitation. Permanent to deep‐lake conditions generally occurred during intermediate cool periods following warm intervals, when perhaps the seas were still warm and low rates of evaporation on land prevailed. On the other hand, short periods of shallow to deep lake levels also occurred during warm (interglacial) periods, showing that these were associated with reasonably high rates of precipitation.     

Receiver function method in reflection seismology

The receiver function method was originally developed to analyse earthquake data recorded by multicomponent (3C) sensors and consists in deconvolving the horizontal component by the vertical component. The deconvolution process removes travel path effects from the source to the base of the target as well as the earthquake source signature. In addition, it provides the possibility of separating the emergent P and PS waves based on adaptive subtraction between recorded components if plane waves of constant ray parameters are considered. The resulting receiver function signal is the local PS wave's impulse response generated at impedance contrasts below the 3C receiver.We propose to adapt this technique to the wide‐angle multi‐component reflection acquisition geometry. We focus on the simplest case of land data reflection acquisition. Our adapted version of the receiver function approach consists in a multi‐step procedure that first removes the P wavefield recorded on the horizontal component and next removes the source signature. The separation step is performed in the τ?p domain while the source designature can be achieved in either the τ?p or the t?x domain. Our technique does not require any a priori knowledge of the subsurface. The resulting receiver function is a pure PS‐wave reflectivity response, which can be used for amplitude versus slowness or offset analysis. Stack of the receiver function leads to a high‐quality S wave image.     

Wet and dry spell analysis of Global Climate Model-generated precipitation using power laws and wavelet transforms

Climate model simulations for the twenty-first century point toward changing characteristics of precipitation. This paper investigates the impact of climate change on precipitation in the Kansabati River basin in India. A downscaling method, based on Bayesian Neural Network (BNN), is applied to project precipitation generated from six Global Climate Models (GCMs) using two scenarios (A2 and B2). Wet and dry spell properties of monthly precipitation series at five meteorologic stations in the Kansabati basin are examined by plotting successive wet and dry durations (in months) against their number of occurrences on a double-logarithmic paper. Straight-line relationships on such graphs show that power laws govern the pattern of successive persistent wet and dry monthly spells. Comparison of power-law behaviors provides useful interpretation about the temporal precipitation pattern. The impact of low-frequency precipitation variability on the characteristics of wet and dry spells is also evaluated using continuous wavelet transforms. It is found that inter-annual cycles play an important role in the formation of wet and dry spells.