Evaluation of rainfall and wetland water area variability at Thirlmere Lakes using Landsat time-series data

Thirlmere Lakes is a group of five freshwater wetlands in the southwest fringe of Sydney, Australia, that is subject to cyclic wetting and drying. The lakes are surrounded by activities that have led to increasing pressure on the local surface and groundwater supply including farming and mining. The mine has been operating for more than 30 years, and in recent times, there has been speculation that the surface subsidence and underground pumping may have some impact on surface water and groundwater hydrology. A study was undertaken using satellite imagery to examine the relation between water area changes and rainfall variability. The study utilised Landsat time-series data during the period 1982–2014 to calculate changes in the lake water area (LA), through the normalised difference water index (NDWI) threshold. High classification accuracy was achieved using NDWI against high-resolution data that are available for the years 2008 (88.4 %), 2010 (92.8 %), and 2013 (96.9 %). The LA measurement was correlated against 11 historic observations that occurred in 2009, 2010, and 2011 during drier wetland conditions. Correlation analysis of the LA with the residual rainfall mass spread across the past 30 years has found that rainfall variability is a major dominant factor associated with the wetland changes. The underground mining operations, if verified by independent investigations, probably play a minor or negligible contributor to variations in total wetland area during the study period. This study has demonstrated that remote sensing is a technique that can be used to augment limited historic data.     

Electromagnetic scattering due to deformed inhomogeneous bodies (Part I: Sphere)

Summary The potential technique of the perturbation has been employed to find the analytical expressions for the response of conducting bodies of irregular shapes. In Part I spherical bodies and in Part II cylindrical ones are considered. The conductivities are assumed to be finite and radially varying. The deformation of the boundaries in both the cases are considered to be of the typek _p =r ₀[1++f ₁(, )] where is the perturbation parameter. Calculations have been made for uniform as well as non-uniform excitation fields. Since the models represent the geological ore-formation more closely, the results will find application in preparing type curves for induction prospecting.N.G.R.I. Contribution No. 70-194.     

Processes of initial collision and suturing between India and Asia

The initial collision between Indian and Asian continents marked the starting point for transformation of land-sea thermal contrast, uplift of the Tibet-Himalaya orogen, and climate change in Asia. In this paper, we review the published literatures from the past 30 years in order to draw consensus on the processes of initial collision and suturing that took place between the Indian and Asian plates. Following a comparison of the different methods that have been used to constrain the initial timing of collision, we propose that the tectono-sedimentary response in the peripheral foreland basin provides the most sensitive index of this event, and that paleomagnetism presents independent evidence as an alternative, reliable, and quantitative research method. In contrast to previous studies that have suggested collision between India and Asia started in Pakistan between ca. 55 Ma and 50 Ma and progressively closed eastwards, more recent researches have indicated that this major event first occurred in the center of the Yarlung Tsangpo suture zone (YTSZ) between ca. 65 Ma and 63 Ma and then spreading both eastwards and westwards. While continental collision is a complicated process, including the processes of deformation, sedimentation, metamorphism, and magmatism, different researchers have tended to define the nature of this event based on their own understanding, an intuitive bias that has meant that its initial timing has remained controversial for decades. Here, we recommend the use of reconstructions of each geological event within the orogenic evolution sequence as this will allow interpretation of collision timing on the basis of multidisciplinary methods.     

Computing the 3D in-situ stress field from shut-in pressure data using statistical regression

The paper describes a multivariable statistical regression technique for computing the 3D stress tensor applied to the shut-in pressure data obtained on fractures during in-situ stress measurement by hydraulic fracturing in a short borehole. This method computes the mean value of components of the 3D stress tensor and their confidence intervals at a desired confidence level. This has been applied to the data of shut-in pressure from Narwapahar mine, India and Kamaishi mine, Japan. The computed in-situ stress tensor is corroborated with that obtained by other methods.     

Seismicity of the Koyna region and regional tectonomagmatism of the Western Margin (India)

Recent findings on the Meso-Cenozoic tectonomagmatism and deep-seated anomalous geophysical structures suggest a close linkage between the seismicity of the Koyna region, the Westernghat uplift (WG-U) and associated thermomechanical and fluid activities. The WG-U seems to be the result of late Cretaceous thermal mobilization, erosion of the Deccan trap cover and superposition of compressional stress. The association of seismicity with uplift seems to result from movement of deep-seated heat and fluids/volatiles along the edges (or boundary faults) of the uplift; because the force required for crustal deformation depends on the relief. Observed gradients in relief may be attributed to the differential erosion-rates and heat inputs, due to the time gap of 50 Ma in the break-ups and plume activities on the eastern and western sides and consequence magmatism. Further, the geology and tectonics strongly indicate that the western margin (WM) is a relic of a mobile arm (MA), that included Madagascar, and which formed a part of the Proterozoic mobile belt of greater India (fort>85 Ma). The mobile nature of the WM facilitates mantle upwellings and transient elevation of isotherms at depth, raising the possibility of intermittent metamorphism and greater deformation.Superposition of the ongoing compression and uplift-induced forces make local permeability and pore-fluid pressure vital in triggering the seismic slip over the Peninsular shield. Certain representative model calculations have been carried out to estimate change in the e.m. induction characteristics caused by an intermittent hydraulic connectivity. The results show a drop in the resistivity which could be a useful monitoring index. The close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the South Indian Shield (SIS) also.     

Quasi-stationary electromagnetic response of covered permeable conductors to some pulses of spatially uniform magnetic field

Summary The quasi-stationary electromagnetic response of permeable conductors surrounded by conducting halo and excited by a uniform magnetic field having a step function time dependence has been obtained theoretically for both spherical and cylindrical conductors. For the magnetic conductors the results give the long period limit of the response and define a critical time complementary to critical frequency (Ward [1]²)) which may be used to estimate the magnetic content. Further utilizing the principle of superposition the responses to ramp, half-sine, saw tooth and triangular pulse excitations are also given.National Geophysical Research Institute, Hyderabad-7, (A.P.), India. NGRI Contribution No. 72-363.     

Ambiguity in the electromagnetic interpretation due to a conducting halo surrounding the target

Summary Using the electromagnetic response of a two-layer spherical model it is shown that the estimate of the depth, radius, conductivity and magnetic contrast of a spherical conductor through the multifrequency or multi-time measurements is dependent upon the electrical and geometrical parameters of the conducting halo surrounding the target. The results identify the corrections or ambiguity in the interpretation depending upon whether some knowledge about the halo is available or not.National Geophysical Research Institute, Hyderabad-7, A.P., India.-NGRI Contribution No. 72–364.     

Turam response of a permeable inhomogeneous cylindrical conductor surrounded by a thin shell

Summary Electromagnetic response characteristics of an inhomogeneous permeable cylinder surrounded by a thin insulated concentric dissipative shell is obtained in presence of a line-source of current. The 3-layer boundary value problem is solved by assuming the thickness of the shell to be much smaller than the skin-depth in the shell. Numerically computed results are presented in the form of argand diagrams to examine the influence of the shell parameters (conductivity and its proximity with the core) and magnetic contrast. The multi-frequency response of the system is also given which resolves the two conducting layers and analyzes the frequency dependence of screening behavior of the shell.N.G.R.I. Contribution No. 282.     

Quasi-static transient response of a covered permeable inhomogeneous cylinder to a line current source

Summary Transient electromagnetic response of a cylindrical conductor covered by (a) an insulated and (b) a galvanically connected coaxial shell has been obtained. The exciting pulse is produced by switching off a long alternating current source. The study incorporates the influence of magnetic permeability contrast of the cylinder with the surrounding medium and also that of the inhomogeneity in the conductivity. The generalised initial value problem is significant in the study of certain geomagnetic anomalies. The results will also aid to the interpretation of induction prospecting data for elongated porphyry conducting mineral deposits.N.G.R.I. Contribution No. 71-280.