DEVELOPMENT OF A METHOD FOR CORRECTING THE PLACEMENT OF THE REGION OF INTEREST

Measurements of horizontal or vertical gradients of the magnetic field intensity can add a new dimension to high resolution aeromagnetic surveys of shallow basement areas. Although a number of vertical, horizontal and triaxial aeromagnetic gradiometers are in operation throughout the world, very little has been documented on the relative merits of horizontal and vertical gradiometer measurements or of measured versus calculated gradients. Our results demonstrate that there are differences in spatial resolution and bandwidth between measured and calculated gradients and that horizontal and vertical gradients provide complementary information. Finally our results indicate that better geo-referencing of horizontal gradiometer data is needed before we can derive reliable vertical gradient data from the measured longitudinal and transverse gradients Development of short-baseline aeromagnetic gradiometers began in 1975 with construction of the Geological Survey of Canada vertical gradiometer. The success of current short-baseline gradiometers depends on the use of high sensitivity optical pumping magnetometers, usually caesium vapour sensors and effective compensation for aircraft manoeuvre noise. Practical advantages of measured gradients include elimination of the diurnal problem and improved spatial resolution of small shallow sources. Transient signals have the same effect on all the magnetometer sensors so the effects are cancelled out in the gradients and diurnal-free total magnetic intensity can then be reconstructed by integrating the gradient data. The gradient data effectively remove the main field of the earth and enhance smaller scale, shallow anomalies while attenuating longer wavelength, deeper seated anomalies. In addition, measurement of the transverse horizontal gradient provides extra information between flight lines, leading to a reduction in flight line dependency of magnetic anomalies and a reduction in aliasing effects. The vertical gradient can also be derived from the longitudinal and transverse horizontal gradients with less line dependence and probably lower noise levels than the measured vertical gradient. Data from a conventional Canadian vertical gradiometer system, an Australian horizontal gradiometer system and a Southern African triaxial gradiometer system are evaluated. The measured vertical gradient data appear to provide better resolution of shallow sources than the vertical gradient calculated from the total field. The horizontal gradient data allow us to derive the vertical gradient from the horizontal gradients and provide much improved enhanced total magnetic intensity grids. The choice of a vertical or horizontal gradiometer system depends on survey objectives and both systems have their advantages. The vertical gradient appears to be marginally easier to measure and simpler to interpret. A triaxial gradiometer system provides the advantages of both at the cost of an additional sensor and retractable boom installation, compared to a horizontal gradiometer. Unfortunately, in Australia, CAA certification of a retractable tail boom may prove difficult. The viability of deriving the vertical gradient profiles from measured horizontal gradients needs more research, but if this can be done routinely, we think this would tilt the balance towards the horizontal gradiometer system. The improvement in total magnetic intensity grids using the transverse gradient information can be quite dramatic in many cases and there is always some improvement in the data. The combination of transverse gradient enhanced total magnetic intensity data and high spatial resolution 'texture' filters provides excellent definition of lower amplitude anomalies. In contrast, the calculated vertical magnetic gradient is often very similar to the measured data and the benefits of the measured vertical gradient may be small in many cases. Gradiometer performance deteriorates rapidly as the source becomes deeper and existing gradiometers are probably close to the noise level at 500m depth. Rugged topography also causes fixed wing gradiometer performance to deteriorate.

This study is carried out to investigate the role of direct measurements of vertical and horizontal magnetic gradient to detect the subsurface an engineering utilities, including electrical cable and concrete irrigation canal at a site the University of Baghdad campus, southern Baghdad, central Iraq. The measurement was achieved along three E-W parallel profiles. The length of each profile is 14 m. The spacing distance between each two adjacent profiles is 1 m, whereas the station spacing distance along each profile is 1 m. The vertical and horizontal magnetic gradient in the surveys were achieved by using two sensor levels, at 2.44 m and 1.0 m, respectively. The total magnetic intensity, vertical gradient, and horizontal gradient were measured, then an analytical signal was determined. The location of the electrical cable appears as a positive peak within the total magnetic intensity and vertical gradients curves, while the concrete canal location is shown by a negative peak in all profiles. On the other hand, the horizontal magnetic gradient curves revealed the location of the electrical cable as a negative peak, while the concert boundaries appeared as positive peaks. The analytical signal shows that the electrical cable gives a positive peak, while the concrete canal boundaries a negative peak. It is concluded that the vertical magnetic gradient is very similar in shape with the total magnetic intensity measurements of low sensor level (1 m). The vertical gradient, on the other hand, shows the best result for investigating the target in the study area, with depth not exceeding 2 m.

We detected the spatio-temporal variability in the timing of start (SGS) and end of growing season (EGS) in Japan from 2003 to 2012 by analyzing satellite-observed daily green-red vegetation index with a 500-m spatial resolution. We also examined the characteristics of SGS and EGS timing in deciduous broadleaf and needleleaf forests along vertical and horizontal gradients and then evaluated the relationship between their timing and daily mean air temperature. We found that for the timing of SGS and EGS, changes along the vertical gradient in deciduous broadleaf forest tended to be larger than those in deciduous needleleaf forest. For both forest types, changes along the vertical and horizontal gradients in the timing of EGS tended to be smaller than those of SGS. Finally, in both forest types, the sensitivity of the timing of EGS to air temperature was much less than that of SGS. These results suggest that the spatio-temporal variability in the timing of SGS and EGS detected by satellite data, which may be correlated with leaf traits, photosynthetic capacity, and environment conditions, provide useful ground-truthing information along vertical and horizontal gradients.

The truncated plate and geologic contact are commercially important structures which can be located by the gravity method. The interpretation can be improved if both the horizontal and vertical gradients are known. Vertical gradients are difficult to measure precisely, but with modern gravimeters the horizontal gradient can be measured conveniently and accurately. This paper shows how the vertical gradient can be obtained from the horizontal gradient by the use of a Hilbert transform. A procedure is then presented which easily enables the position, dip angle, depth, thickness, and density contrast of a postulated plate to be precisely and unambiguously derived from a plot of the horizontal gradient against the vertical gradient at each point measured. The procedure is demonstrated using theoretical data.

Distribution and movement of the mangrove gastropod Littoraria angulifera

Scott Hogg, whose company Scott Hogg & Associates, based in Toronto, Canada offers services to the exploration and airborne geophysical industry, reviews the latest refinements in the revitalised use of airborne magnetic gradiometers for the mining industry. The oil and gas exploration industry spurred the development of the first airborne magnetic gradiometers. The motivation was to use Euler’s equation with measured vertical gradient to calculate depth to magnetic source. Aeroservice introduced both a helicopter system and a fixed wing towed bird system in the 1960s. Interest in the Euler theory for magnetic analysis was rekindled in the mining industry 20 years later and continues to be the foundation of many new interpretation methods. In Canada, the GSC developed a vertical magnetic gradiometer in the 1970s. A national mapping programme began in the early 1980s and in response Canadian airborne survey contractors created a variety of fixed-wing and helicopter systems. The mining sector’s interest in airborne gradient measurement was based primarily on the increased spatial resolution and detail: small anomalies on the flanks of large features could be clearly resolved. Calculated vertical gradient maps, produced by simple filtering of total field, were found to provide almost the same benefit as measured gradient, at less cost, and the commercial interest in measured vertical gradient faded. Geometrics introduced a horizontal gradiometer in 1983. This development was of particular significance since, at the same time, they incorporated a technique developed by Nabighian and Hansen to derive a pseudo total field residual from measured horizontal gradients. The demise of the Geometric survey division took horizontal gradients off the horizon for a while. A decade later, this same concept was used by Nelson of the NRC, and more recently implemented in a variety of forms by De Beers and others. Geodass in Botswana, now Fugro, introduced the first 3- axis gradiometer in the early 1990s. In conjunction with the gradient measurements it provided a variety of compilation and mapping services that made use of the information. In Canada, Terraquest was the first to provide horizontal gradient measurement and Goldak the first to provide a full 3-axis fixed-wing gradiometer. At present, almost all of the airborne contractors offer horizontal gradient systems, and several can now provide full 3-axis configurations for simultaneous vertical and horizontal gradient measurement. Considerable interest in magnetic gradient measurement has arisen over the past few years. Some of the benefits of gradient are well founded and some are overstated and many are poorly understood. Magnetic gradient measurements can be used to advantage in interpretation. Vertical gradient maps, analytic signal maps, and a host of Euler based methods all use gradient information. The gradient information for these purposes may be calculated or measured. This review addresses methods that rely specifically on measured, not calculated, gradient. At present there are three such primary applications. The first is the potential to avoid diurnal interference, the second is to correct total field for variations in aircraft altitude, and the third is to make significant improvements in the accuracy and resolution of magnetic maps.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 342:1-14 (2007) - doi:10.3354/meps342001 Spatial trends in community richness, diversity, and evenness across rocky intertidal environmental stress gradients in eastern Canada Ricardo Scrosati*, Christine Heaven Saint Francis Xavier University, Department of Biology, Antigonish, Nova Scotia B2G 2W5, Canada *Email: rscrosat@stfx.ca ABSTRACT: An environmental stress model (ESM) developed by B. A. Menge and J. P. Sutherland and improved by other researchers predicts how local-scale richness and diversity (terms often used interchangeably in previous ESM research) should vary with environmental stress. We tested model predictions by surveying all benthic producers and consumers across vertical (elevation) and horizontal (wave/ice exposure) stress gradients in rocky intertidal habitats from the Gulf of St. Lawrence (which freezes in winter) and open Atlantic (which does not freeze) coasts of Nova Scotia, Canada. Since local winter conditions are harsher than on most temperate shores studied previously, we made predictions for an intermediate-to-high range of stress: richness and diversity would be lowest in highly stressful habitats and would increase with decreasing stress. Results matched predictions across vertical gradients on both coasts (richness and diversity were negatively related to elevation) and across horizontal gradients on the Gulf coast (richness and diversity were negatively related to wave/ice exposure) but not entirely on the Atlantic coast (richness was negatively related to wave exposure, but diversity showed an opposite trend). The spatial changes in evenness explained such differing trends in richness and diversity. Richness and diversity were higher on the Atlantic than on the Gulf of St. Lawrence coast, consistent with the stronger physical stress (winter ice scour) on the Gulf coast. Our study indicates that richness and diversity may respond differently to local-scale environmental stress, contrary to common assumptions. To determine the conditions under which such differences might occur, future ESM studies should investigate both variables and also evenness, which together with richness determines diversity. KEY WORDS: Diversity · Elevation · Environmental stress gradient · Evenness · Ice scour · Richness · Rocky intertidal · Wave exposure Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 342. Online publication date: July 24, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

A pilot Coastal Observatory was established in Liverpool Bay (eastern Irish Sea) in 2002 with an anticipated lifespan of at least 10 years. In an evolving process, near realtime measurements are integrated with coupled models in a pre-operational prediction system. The aim is to understand a coastal sea's response to natural forcing and the consequences of human activity, providing scientific underpinning for coastal management. The impact of human activity on the world's oceans and the use made of them by humanity is greatest in coastal seas. The eastern Irish Sea is an excellent study area, affected by all coastal sea processes with tidal forcing dominant and near-shore physical and bio-geochemical processes influenced by estuarine discharge where both vertical and horizontal gradients are important. The sea is subject to a range of human industrial, agricultural, fishing and recreational pressures. The time series measurements enable definition of the seasonal cycle, its interannual variability and quantification of the importance of events relative to the mean. Applications include eutrophication (the region receives significantly elevated levels of nutrient inputs), shoreline management (coastal flooding and beach erosion/accretion), and understanding present conditions to predict the impact of climate change (for instance if the number and severity of storms, or of high or low river flows, change). The integrated, systematic measurement suite, which started in August 2002, covers a range of space and time scales, concentrating on horizontal and vertical gradients, and includes in situ surface waves and vertical profiles of current, temperature, salinity, turbidity, nutrients and chlorophyll; regional water column surveys nine times a year; shore-based phased array HF radar measuring waves and surface currents out to a range of 50 km; instrumented ferries measuring surface properties; coastal tide gauges; satellite data-infrared (for sea surface temperature) and visible (for chlorophyll and suspended sediment). Acoustic telemetry and the Orbcomm satellite system are the primary resource for in situ telemetry. The core measurements and infrastructure also form the basis for process studies, instrument trials, for instance a three week glider deployment, and for the incorporation of new sensors. In cooperation with the UK Met Office, and as a component of the National Centre for Ocean Forecasting, a suite of nested 3-dimensional models (the Proudman Oceanographic Laboratory Coastal Ocean Modelling System-POLCOMS) is run daily, focusing on the Observatory area by covering the ocean/shelf of northwest Europe (at 12 km resolution), the Irish Sea (at 1.8 km) and Liverpool Bay (at ~ 200m resolution). These models simulate the physical (hydrodynamic coupled with wave) environment, nutrient and plankton dynamics, and the processes influencing sediment suspension and transport. The measurements test the models against events as they happen in a truly 3-D context - most is learned when model predictions diverge from reality. The value of some of the observations is enhanced by data assimilation which benefits both the models, keeping them on track, and the observations, augmenting their spatial coverage. All measurements and model outputs are displayed on the Coastal Observatory web-site for an audience of researchers, coastal managers and the public. After nearly 4 years of operation we are beginning to build up a picture, which only continuous monitoring provides, of the interaction between a coastal sea and its major estuaries. As always more detailed measurements indicate that processes are more complicated and distributions can have large variability

A pilot Coastal Observatory has been established in Liverpool Bay, which integrates (near) real-time measurements with coupled models in a pre-operational coastal prediction system and displays the results on the web. The aim is to understand a coastal sea's response to natural forcing and the consequences of human activity. The eastern Irish Sea is an excellent study area, affected by all coastal sea processes, including near-shore physical and biogeochemical processes influenced by estuarine inflow where both vertical and horizontal gradients are important. The time series enable definition of the seasonal cycle, its inter-annual variability and quantification of the importance of events relative to the mean. Applications include eutrophication (the region receives significantly elevated levels of nutrient inputs), shoreline management (coastal flooding and beach erosion / accretion), and understanding present conditions to predict the impact of climate change (for instance if the number and severity of storms, or of high or low river flows, change). The integrated measurement suite, which started in August 2002, covers a range of space and time scales, concentrating on horizontal and vertical gradients and includes in situ time series, 4–6 weekly regional water column surveys, an instrumented ferry, a shore-based h.f. radar system measuring surface currents and waves, coastal tide gauges, visible and infra-red satellite data. A suite of nested 3-D hydrodynamic and ecosystem models is run daily, focusing on the Observatory area by covering the ocean / shelf of northwest Europe (at 12 km resolution), the Irish Sea (at 1.8 km) and Liverpool Bay (200–300 m resolution). The measurements will test the models against events as they happen in a truly 3-D context. All measurements and model outputs are displayed on the Coastal Observatory web-site (http://coastobs.pol.ac.uk) for an audience of researchers, coastal managers and the public.

In the Dauphinoise zone of the French Western Alps, a posttectonic thermoremanent magnetization, carried by pyrrhotite, has been isolated. The paleomagnetic signature is interpreted as the record of successive partial thermoremanent magnetizations of opposite directions acquired during the early Miocene slow cooling of the epimetamorphic marly limestones of the studied area. A local vertical thermal gradient of 22 ± 2 °C/km was derived from the unblocking temperature of a reversal recorded at different altitudes. Using this vertical gradient, we have produced a paleotemperature map at the time of the reversal (about 20 Ma). The map shows a horizontal regional thermal gradient of 7 °C/km; the higher temperatures were toward the crystalline basement. Paleomagnetic directions demonstrate that this horizontal gradient was present during cooling. Its origin can be explained by large-volume fluid circulation along the major fault that forms the boundary between the crystalline basement and its Liassic cover.

The spatial distributions of bacterial communities may be driven by multiple environmental factors. Thus, understanding the relationships between bacterial distribution and environmental factors is critical for understanding wetland stability and the functioning of freshwater lakes. However, little research on the bacterial communities in deep sediment layers exists. In this study, thirty clone libraries of 16S rRNA were constructed from a beach wetland of the Poyang Lake along both horizontal (distance to the water-land junction) and vertical (sediment depth) gradients to assess the effects of sediment properties on bacterial community structure and diversity. Our results showed that bacterial diversity increased along the horizontal gradient and decreased along the vertical gradient. The heterogeneous sediment properties along gradients substantially affected the dominant bacterial groups at the phylum and species levels. For example, the NH+4 concentration decreased with increasing depth, which was positively correlated with the relative abundance of Alphaproteobacteria. The changes in bacterial diversity and dominant bacterial groups showed that the top layer had a different bacterial community structure than the deeper layers. Principal component analysis revealed that both gradients, not each gradient independently, contributed to the shift in the bacterial community structure. A multiple linear regression model explained the changes in bacterial diversity and richness along the depth and distance gradients. Overall, our results suggest that spatial gradients associated with sediment properties shaped the bacterial communities in the Poyang Lake beach wetland.

Numerical simulation is an essential tool for investigation of seawater intrusion in coastal aquifer. For most groundwater modeling software, boundary condition along the beach is required. But it was normally assumed due to the uncertainty in the seawater – freshwater interface. Using FEFLOW, a groundwater simulation software based on finite element method, we investigated the intrusion scope and the exiting point of groundwater outflow under various boundary conditions. Seven cases were designed, among which three cases with boundary conditions of a freshwater layer over seawater, three cases with a triangle freshwater zone between seawater and the beach, and the last one without freshwater at the seawater boundary. Results showed that the last case has the longest intrusion scope. The scope of seawater intrusion is determined by both the horizontal water head gradient along the bottom of the aquifer and the vertical water head gradient along the beach. Both higher horizontal gradient and lower vertical gradient result in larger intrusion scope. In some circumstances, the vertical gradient has greater impact on seawater intrusion than the horizontal gradient, and act as the main power inhibiting seawater intrusion.

A coordinated survey between a subsurface Lagrangian float and a ship-towed Triaxus profiler obtained detailed measurements of a restratifying surface intensified front (above 30 m) within the California Current System. The survey began as downfront winds incited mixing in the boundary layer. As winds relaxed and mixing subsided, the system entered a different dynamical regime as the front developed an overturning circulation with large vertical velocities that tilted isopycnals and stratified the upper ocean within a day. The horizontal buoyancy gradient was 1.5 × 10−6 s−2 and associated with vorticity, divergence, and strain that approached the Coriolis frequency. Estimates of vertical velocity from the Lagrangian float reached 1.2 × 10−3 m s−1. These horizontal gradients and vertical velocities were consistent with submesoscale dynamics that are distinct from the classic quasigeostrophic framework used to describe larger-scale flows. Vertical and horizontal gradients of velocity and buoyancy in the vicinity of the float revealed that sheared currents differentially advected the horizontal buoyancy gradient to increase vertical stratification. This was supported by analyses of temperature and salinity gradients that composed the horizontal and vertical stratification. Potential vorticity was conserved during restratification at 16 m, consistent with adiabatic processes. Conversely, potential vorticity near the surface (8 m) increased, highlighting the role of friction in modulating near-surface stratification. The observed increase in stratification due to these submesoscale processes was equivalent to a heat flux of 2000 W m−2, which is an order-of-magnitude larger than the average observed surface heat flux of 100 W m−2.

In some scenarios, we need to combine multiple video streams and pictures into a single video. In this paper, we develop an application to synthesize two channel video stream and two channel pictures. Our application can be used for synthetic display of multiple network video streams and offline video synthetic storage. As the single thread decoding of multiple video streams takes a lot of time, we propose the use of a multithreading producer-consumer pattern. It makes full use of the advantages of multi-core CPU and improves the programming efficiency. In our experiments, we compare video synthesis through single thread and multithreading. Multithreading is much faster than single threading.

The vast and growing literature on the search for stratigraphic traps for petroleum ignores gravity gradients, for which the theory has been available since the heyday of the Eötvös torsion balance decades ago. These are discussed in this paper. The horizontal and vertical gradients can be measured with available gravimeters and (to a limited extent) with the Eötvös torsion balance. A major advantage of the gradient method is that surveying to determine position and elevation of the station is not required. Both theory and practice have been reported in the geophysical literature, but the important application to stratigraphic traps has not been mentioned. We evaluate here the method for locating “pinchouts”, a term which embraces “stratigraphic” and “unconformity” traps in Halbouty’s (1972) classification. Both position and depth of the assumed pinchout are determined by the gradient anomaly. The magnitudes of anomalies of horizontal and vertical gradients are about equal. However, pending new instrumentation, only the horizontal gradient is practically useful for field surveys. The gradient method is quantitatively promising and, used in conjunction with other methods, should significantly advance the search for stratigraphic traps for petroleum.