An efficient non-linear least-squares 1D inversion scheme for resistivity and IP sounding data

Non-linear least-squares inversion operates iteratively by updating the model parameters in each step by a correction vector which is the solution of a set of normal equations. Inversion of geoelectrical data is an ill-posed problem. This and the ensuing suboptimality restrict the initial model to being in the near vicinity of the true model. The problem may be reduced by introducing damping into the system of equations. It is shown that an appropriate choice of the damping parameter obtained adaptively and the use of a conjugate-gradient algorithm to solve the normal equations make the 1D inversion scheme efficient and robust. The scheme uses an optimal damping parameter that is dependent on the noise in the data, in each iterative step. The changes in the damping and relative residual error with iteration number are illustrated. A comparison of its efficacy over the conventional Marquardt and simulated annealing methods, tested on Inman's model, is made. Inversion of induced polarization (IP) sounding is obtained by inverting twice (true and modified) DC apparent resistivity data. The inversion of IP data presented here is generic and can be applied to any of the IP observables, such as chargeability, frequency effect, phase, etc., as long as these observables are explicitly related to the DC apparent resistivity. The scheme is used successfully in inverting noise-free and noisy synthetic data and field data taken from the published literature.     

Tropical rain forest mapping and community analysis of South Andaman Islands (India)

Present study deals with the vegetation type mapping, structure and composition analysis of the tropical forests, spread over 1,294 km² area in South Andaman Islands. Seventeen vegetation classes spreading over 89.92% forested area of the islands were mapped with the overall accuracy of 88.89%. Evergreen, semi-evergreen and mangrove forests were reasonably well distributed forests, while moist deciduous and littoral evergreen were narrowly restricted. The stocking was quite variable across the forest types. 60.04% of forested area was under medium to high canopy density. Secondary and degraded forest types were mapped. Information on floristic composition, structure and diversity of various forest types were obtained from 84 field sample plots. An inventory of 423 species of plants from 101 families included 155 trees, 84 shrubs, 150 herbs and 84 climbers. Tree density and mean basal area ranged from 517 to 900 stems ha⁻¹ and 36.15 to 53.58 m² ha⁻¹ respectively. Evergreen forests accounted for highest diversity followed almost equally by semi-evergreen and moist deciduous forests.     

Application of landscape ecology and remote sensing for assessment,monitoring and conservation of biodiversity

Landscape ecology, inter alia, addresses the question as to how altered landscape patterns affect the distribution, persistence, and abundance of a species. Landscape ecology plays an important role in integrating the different scales of biodiversity from habitat patch to biome level. Satellite remote sensing technology with multi-sensor capabilities offers multi-scale information on landscape composition and configuration. Advances in geospatial analytical tools and spatial statistics have improved the capability to quantify spatial heterogeneity. Globally, landscape level characterization of biodiversity has become an important discipline of science. Considering the vast extent, heterogeneity, and ecological and economic importance of forest landscapes, significant efforts have been made in India during the past decade to strengthen landscape level biodiversity characterization. The generic frame work of studies comprises preparation of national databases providing information on composition and configuration of different landscapes using multi-scale remote sensing techniques, understanding the landscape patterns using geospatial models to elicit disturbance and diversity patterns and application of this information for bioprospecting and conservation purposes. Studies on hierarchical linkage of multi-scale information to study the processes of change, landscape function, dynamics of habitat fragmentation, invasion, development of network of conservation areas based on the understanding of multi-species responses to landscape mosaics, macro-ecological studies to understand environment and species richness, habitat and species transitions and losses, landscape level solutions to adaptation and mitigation strategies to climate change are a few of the future challenges. The paper presents the current experiences and, analyses in conjunction with international scenario and identifies future challenges of Indian landscape level biodiversity studies.     

Time-averaged flow structure in the central region of a stream confluence: a discussion

This paper is a discussion of Rhoads and Kenworthy (1998) ‘Time-averaged flow structure in the central region of a stream confluence’ Earth Surface Processes and Landforms, 23 , 171–191, that focuses upon the methods used to identify secondary circulation in river channel confluences. It argues that the Rozovskii method that Rhoads and Kenworthy use to rotate their field data to allow identification of secondary circulation cells is flawed, and can result in misleading conclusions about the nature of flow processes in confluences. It recommends that there is a re-emphasis upon helical as opposed to secondary circulation, and that recent developments in both field monitoring and numerical modelling may help significantly in this respect. Copyright © 1999 John Wiley & Sons, Ltd.     

ANALYSIS OF ROCKBOLT REINFORCEMENT USING BEAM-COLUMN THEORY

A simple analytical procedure that applies classical beam-column theory for evaluating passive rockbolt roof reinforcement is presented in this paper. The analytical model is derived from first principles and is capable of modelling any number of reinforcing bolts. Each rockbolt is modelled as a linear spring and the model allows for non-uniform bolt spacing. In this study the rock beam is assumed to be isotropic and linearly elastic for the sake of simplicity. However, the analytical model can be extended to include anisotropic rockmass as well as inelastic material behaviour. The solution to the coupled set of governing equations is obtained by using a simple numerical solution procedure. The results from the analytical model indicate that the critical buckling load of a rock beam is strongly influenced by the ambient rock modulus. For salt-rock excavations the rock modulus typically declines with time due to various phenomena, and a diminished modulus could seriously compromise roof stability. The other main conclusion of this study is that rockbolts loose their effectiveness in restraining a roof beam once its critical buckling load is approached. In such a situation, increasing bolt stiffness does not improve its reinforcing action on a roof beam but it enhances the possibility of bolt failure due to anchor pull-out. © 1997 by John Wiley & Sons, Ltd. Int. J. Numer. Anal. Meth. Geomech., vol. 21, 241–253 (1997)     

Sensitivity of bed shear stress estimated from vertical velocity profiles: the problem of sampling resolution

Bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile. However, most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed. The lack of near-bed measurements is a critical loss of information which may affect bed shear stress estimates. Detailed velocity profiles obtained from a field acoustic Doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near-bed points is critical for the estimation of bed shear stress in a shallow river environment. Moreover, the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform. © 1998 John Wiley & Sons, Ltd.     

Linking hydrology and sediment dynamics of large alluvial rivers to landscape diversity in the Ganga dispersal system,India

Discharge and sediment load data for several stations along the Ganga River and its major tributaries in the western Ganga plains (WGP) for a period of ~30 years have been analysed to understand the hydrological characteristics and sediment dynamics. In terms of hydrology, the rivers are less flood‐prone than believed, exceeding bankfull discharges less frequently than the expected 1.5 year return interval. This has been attributed to the rivers of this region occupying incised valleys formed in the Late Quaternary period. Rivers draining the WGP are supply‐limited systems compared to those draining the eastern Ganga plains (EGP) which have been characterized as transport‐limited systems. We suggest that such geomorphic diversity as a function of spatial variability in precipitation regime and hinterland geology has existed for at least the Late Quaternary period and they in turn influence the modern day hydrology of the river systems in a significant way. Copyright © 2016 John Wiley & Sons, Ltd.     

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high-resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex-shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small-scale sedimentary features generated by internal waves are superimposed on large-scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

An optimal Savitzky–Golay derivative filter with geophysical applications: an example of self-potential data

We propose a strategy in designing an optimal set of filter parameters, such as the order of interpolating polynomial and the filter length for a Savitzky–Golay derivative filter. The proposed strategy is based on the ‘principle of parsimony’ while satisfying the optimality conditions. The optimality conditions are based on the Durbin–Watson lag-1 test statistic and the Derringer–Suich desirability function. While the former checks for an appropriate data fitting, the latter, on the other hand, ensures minimal shape distortion of the reconstructed response. The proposed strategy of designing filter parameters is developed and validated through numerical experiments using Gaussian pulse as a test function which is contaminated with additive white Gaussian noise. In the numerical tests, the polynomial orders used were 3, 5 and 7, but the filter length for each polynomial was varying until the optimality conditions were satisfied. The Savitzky–Golay derivative filtering is used in obtaining the robust reconstruction of noisy geophysical anomaly and the robust estimation of its first- and second-order derivatives. We validated the proposed technique on the published self-potential anomaly data using a data-based interpretation technique where the reconstructed anomaly and its first- and second-order derivatives were used in estimating model parameters. The data-based interpretation using the proposed technique of Savitzky–Golay derivative filtering provides a close agreement with the published results.     

Supermagnetic enhancement,superparamagnetism, and archaeological soils

A range of mineral magnetic measurements have been carried out on archaeological sediments from Orkney and Cyprus. In a soil profile from Orkney, a magnetic enhancement factor of over 200 is observed in susceptibility data between the bedrock and a Norse sediment. The magnetic enhancement is associated with an increase in superparamagnetic grains probably caused by burning. Until now it has proved difficult to confirm the presence of superparamagnetic grains in natural samples using room temperature magnetic measurements. However, clear differences are to be found between the hysteresis loops of various magnetic domain states, including superparamagnetism. An algorithm has been developed to unmix hysteresis loops in terms of constituent domain states of ferrimagnetic iron oxides. Unmixing 128 hysteresis loops of archaeological sediments has shown that the dominant domain state in all sediments is superparamagnetic. Remarkably uniform superparamagnetic grain sizes of between 80 and 95 Å were found for all 128 sediments. © 1999 John Wiley & Sons, Inc.