Colin Brown

Bio: Colin Brown is an academic researcher from National University of Ireland, Galway. The author has contributed to research in topics: Bathymetry & Seabed. The author has an hindex of 17, co-authored 45 publications receiving 2440 citations. Previous affiliations of Colin Brown include National University of Ireland & Royal Holloway, University of London.

The magnetotelluric phase tensor

Abstract: SUMMARY The phase relationships contained in the magnetotelluric (MT) impedance tensor are shown to be a second-rank tensor. This tensor expresses how the phase relationships change with polarization in the general case where the conductivity structure is 3-D. Where galvanic effects produced by heterogeneities in near-surface conductivity distort the regional MT response the phase tensor preserves the regional phase information. Calculation of the phase tensor requires no assumption about the dimensionality of the underlying conductivity distribution and is applicable where both the heterogeneity and regional structure are 3-D. For 1-D regional conductivity structures, the phase tensor is characterized by a single coordinate invariant phase equal to the 1-D impedance tensor phase. If the regional conductivity structure is 2-D, the phase tensor is symmetric with one of its principal axes aligned parallel to the strike axis of the regional structure. In the 2-D case, the principal values (coordinate invariants) of the phase tensor are the transverse electric and magnetic polarization phases. The orientation of the phase tensor’s principal axes can be determined directly from the impedance tensor components in both 2-D and 3-D situations. In the 3-D case, the phase tensor is nonsymmetric and has a third coordinate invariant that is a distortion-free measure of the asymmetry of the regional MT response. The phase tensor can be depicted graphically as an ellipse, the major and minor axes representing the principal axes of the tensor. 3-D model studies show that the orientations of the phase tensor principal axes reflect lateral variations (gradients) in the underlying regional conductivity structure. Maps of the phase tensor ellipses provide a method of visualizing this variation.

Multiscale Terrain Analysis of Multibeam Bathymetry Data for Habitat Mapping on the Continental Slope

Abstract: Multibeam surveys can provide detailed bathymetry data for the continental slope from which quantitative descriptors of the seabed terrain (e.g., slope) may be obtained. We illustrate the value of these descriptors for benthic habitat mapping, and highlight the advantages of multiscale analysis. We examine the application of these descriptors as predictor variables for species distribution models, which are particularly valuable in the deep sea where opportunities to directly survey the benthic fauna remain limited. Our initial models are encouraging and suggest that wider adoption of these methods may assist the delivery of ecologically relevant information to marine resource managers.

Determinable and non-determinable parameters of galvanic distortion in magnetotellurics

Abstract: SUMMARY Galvanic distortion has long been recognized as an obstacle in the interpretation of magnetotelluric (MT) data. One fundamental problem for distortion removal is that the equations that describe the effects of galvanic distortion on the impedance tensor are underdetermined. We have previously shown that an explicit solution for four of the parameters of the regional (undistorted) impedance tensor can be resolved without any assumptions. These determinable parameters are the components of a tensor (the phase tensor) representing the phase information contained in the impedance. The coordinate invariants of the phase tensor provide a simple and objective guide to the dimensionality of the regional impedance tensor at each measured frequency. Where the regional structure is 2-D, one of the principal axes of the phase tensor will be aligned parallel to the strike of the regional conductivity. The distortion tensor and the parameters of the regional impedance tensor that represent the amplitude information cannot be determined without assumptions. Where the phase tensor shows the regional impedance tensor to be 1-D, the distortion tensor and the regional impedance can be determined to within a single multiplicative constant. Where a 2-D regional structure is indicated, two assumptions are necessary to determine the regional impedance tensor but the solution is not unique, and any choice of assumptions could be made with equal validity. For 3-D structures, the phase tensor provides the direction of greatest inductive response, which is the closest equivalent of a strike direction. In this case four constraints are required for a solution. In practice, a MT sounding may contain sections that display different characteristic dimension and the distortion tensor can be determined from the section of the sounding with the lowest characteristic dimension. The greatest amount of information is determined from a 1-D section. The use of the information contained in the phase tensor overcomes some of the shortcomings of traditional distortion analysis. Illustrating the tensors using an elliptical representation aids the interpretation of the tensor data involved in this analysis.

Modelling the local distribution of cold-water corals in relation to bathymetric variables: Adding spatial context to deep-sea video data

Abstract: Video data and high-resolution multibeam bathymetry were acquired using a Remotely Operated Vehicle (ROV) on the flank of a carbonate mound (∼850 m depth) in the Porcupine Seabight, SW Ireland. The ROV-mounted multibeam system revealed details of bathymetry that were not resolved by ship-borne multibeam survey, but appear to be important in structuring the distribution of the cold-water corals Lophelia pertusa and Madrepora oculata . Quantitative measures of slope, orientation, roughness and curvature were calculated from the ROV multibeam bathymetry data across a range of spatial scales. These parameters were analysed for their ecological relevance to the distribution of the corals and used in an Ecological Niche Factor Analysis (ENFA) to identify the most suitable areas for coral colonisation within the extent of our ROV multibeam data. The suitability map covers an area nine times the size of the area imaged directly by video. Cross-validation of the results with video data indicates that the predictions are reliable. This combined survey and modelling approach offers a comprehensive method for ground-truthing discrete seabed features such as mounds. It provides spatial context to high-resolution deep-water video observations and highlights the importance of bathymetric variables in influencing coral distribution.

High‐resolution habitat suitability modelling can improve conservation of vulnerable marine ecosystems in the deep sea

Abstract: Aim The distribution of vulnerable marine ecosystems in the deep sea is poorly understood. This has led to the emergence of modelling methods to predict the occurrence of suitable habitat for conservation planning in data-sparse areas. Recent global analyses for cold-water corals predict a high probability of occurrence along the slopes of continental margins, offshore banks and seamounts in the north-eastern Atlantic, but tend to overestimate the extent of the habitat and do not provide the detail needed for finer-scale assessments and protected area planning. Using Lophelia pertusa reefs as an example, this study integrates multibeam bathymetry with a wide range of environmental data to produce a regional high-resolution habitat suitability map relevant for marine spatial planning. Location Irish continental margin (extended continental shelf claim). Methods Maximum entropy modelling was used to predict L. pertusa reef distribution at a spatial resolution of 0.002°. Coral occurrences were assembled from public databases, publications and video footage, and filtered for quality. Environmental predictor variables were produced by re-sampling of global oceanographic data sets and a regional ocean circulation model. Multi-scale terrain parameters were computed from multibeam bathymetry. Results Suitable habitat was predicted on mound features and in canyon areas along a narrow band following the slopes of the Irish continental margin, the Rockall Bank and the Porcupine Bank. Standard deviation of the seabed slope (54%), temperature (28%) and bottom shear stress (9%) were the most important variables to predict coral distribution. Main conclusions This is the first regional coral habitat suitability modelling study to incorporate full coverage multibeam bathymetry in the deep sea. The use of high-resolution environmental data and quality-controlled distribution data significantly reduces habitat overestimation demonstrated by global-scale analyses and produces detailed maps to support marine protected area network design. The strong response of the corals to local-scale terrain variability highlights the need to protect the seabed from anthropogenic impacts that may reduce its complexity, such as bottom trawling.

The magnetotelluric phase tensor

Abstract: SUMMARY The phase relationships contained in the magnetotelluric (MT) impedance tensor are shown to be a second-rank tensor. This tensor expresses how the phase relationships change with polarization in the general case where the conductivity structure is 3-D. Where galvanic effects produced by heterogeneities in near-surface conductivity distort the regional MT response the phase tensor preserves the regional phase information. Calculation of the phase tensor requires no assumption about the dimensionality of the underlying conductivity distribution and is applicable where both the heterogeneity and regional structure are 3-D. For 1-D regional conductivity structures, the phase tensor is characterized by a single coordinate invariant phase equal to the 1-D impedance tensor phase. If the regional conductivity structure is 2-D, the phase tensor is symmetric with one of its principal axes aligned parallel to the strike axis of the regional structure. In the 2-D case, the principal values (coordinate invariants) of the phase tensor are the transverse electric and magnetic polarization phases. The orientation of the phase tensor’s principal axes can be determined directly from the impedance tensor components in both 2-D and 3-D situations. In the 3-D case, the phase tensor is nonsymmetric and has a third coordinate invariant that is a distortion-free measure of the asymmetry of the regional MT response. The phase tensor can be depicted graphically as an ellipse, the major and minor axes representing the principal axes of the tensor. 3-D model studies show that the orientations of the phase tensor principal axes reflect lateral variations (gradients) in the underlying regional conductivity structure. Maps of the phase tensor ellipses provide a method of visualizing this variation.

Computational recipes for electromagnetic inverse problems

Abstract: SUMMARY The Jacobian of the non-linear mapping from model parameters to observations is a key component in all gradient-based inversion methods, including variants on Gauss–Newton and non-linear conjugate gradients. Here, we develop a general mathematical framework for Jacobian computations arising in electromagnetic (EM) geophysical inverse problems. Our analysis, which is based on the discrete formulation of the forward problem, divides computations into components (data functionals, forward and adjoint solvers, model parameter mappings), and clarifies dependencies among these elements within realistic numerical inversion codes. To be concrete, we focus much of the specific discussion on 2-D and 3-D magnetotelluric (MT) inverse problems, but our analysis is applicable to a wide range of active and passive source EM methods. The general theory developed here provides the basis for development of a modular system of computer codes for inversion of EM geophysical data, which we summarize at the end of the paper.

Benthic habitat mapping: A review of progress towards improved understanding of the spatial ecology of the seafloor using acoustic techniques

Abstract: This review examines the various strategies and methods used to produce benthic habitat maps using acoustic remote sensing techniques, coupled with in situ sampling. The applications of three acoustic survey techniques are examined in detail: single-beam acoustic ground discrimination systems, sidescan sonar systems, and multi-beam echo sounders. Over the past decade we have witnessed the nascence of the field of benthic habitat mapping and, on the evidence of the literature reviewed in this paper, have seen a rapid evolution in the level of sophistication in our ability to image and thus map seafloor habitats. As acoustic survey tools have become ever more complex, new methods have been tested to segment, classify and combine these data with biological ground truth sample data. Although the specific methods used to derive habitat maps vary considerably, the review indicates that studies can generally be categorized into one of three over-arching strategies; 1) Abiotic surrogate mapping; 2) Assemble first, predict later (unsupervised classification); 3) Predict first, assemble later (supervised classification). Whilst there is still no widely accepted agreement on the best way to produce benthic habitat maps, all three strategies provide valuable map resources to support management objectives. Whilst there is still considerable work to be done before we can answer many of the outstanding technological, methodological, ecological and theoretical questions that have been raised here, the review concludes that the advent of spatial ecological studies founded on high-resolution environmental data sets will undoubtedly help us to examine patterns in community and species distributions. This is a vital first step in unraveling ecological complexities and thus providing improved spatial information for management of marine systems.

Multisite, multifrequency tensor decomposition of magnetotelluric data

Abstract: Accurate interpretation of magnetotelluric data requires an understanding of the directionality and dimensionality inherent in the data, and valid implementation of an appropriate method for removing the effects of shallow, small-scale galvanic scatterers on the data to yield responses representative of regionalscale structures. The galvanic distortion analysis approach advocated by Groom and Bailey has become the most adopted method, rightly so given that the approach decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and Bailey, one must determine the appropriate frequency-independent telluric distortion parameters and geoelectric strike by fitting the seven-parameter model on a frequencyby-frequency and site-by-site basis independently. Although this approach has the attraction that one gains a more intimate understanding of the data set, it is rather time-consuming and requires repetitive application. We propose an extension to Groom-Bailey decomposition in which a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. Also, we show how an analytically-derived approximate Hessian of the objective function can reduce the required computing time. We illustrate application of the analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be extended to cover the case of frequency-dependent distortion caused by the magnetic effects of the galvanic charges.