Basement Topography Research Articles

Response of glacier flow and structure to proglacial lake development and climate at Fjallsjökull, south-east Iceland

ABSTRACTOver recent decades, the number of outlet glaciers terminating in lakes in Iceland has increased in line with climate warming. The mass-balance changes of these lake-terminating outlet glaciers are sensitive to rising air temperatures, due to altered glacier dynamics and increased surface melt. This study aims to better understand the relationship between proglacial lake development, climate, glacier dynamics and glacier structure at Fjallsjökull, a large, lake-terminating outlet glacier in south-east Iceland. We used satellite imagery to map glacier terminus position and lake extent between 1973 and 2016, and a combination of aerial and satellite imagery to map the structural architecture of the glacier's terminus in 1982, 1994 and 2011. The temporal evolution of ice surface velocities between 1990 and 2018 was calculated using feature tracking. Statistically significant increases in the rate of terminus retreat and lake expansion were identified in 2001, 2009 and 2011. Our surface velocity and structural datasets revealed the development of localised flow ‘corridors’ over time, which conveyed relatively faster flow towards the glacier's terminus. We attribute the overall changes in dynamics and structural architecture at Fjallsjökull to rising air temperatures, but argue that the spatial complexities are driven by glacier specific factors, such as basal topography.

Fractal Properties of Greenland Isolines

The shape of Earth’s surface topography is determined by numerous competing processes that act to either roughen or smoothen the surface. Hence, calculating topographic roughness is a useful technique for understanding the relative importance of these processes. This study analyzes the relative surface roughness of the Greenland Ice Sheet by calculating the fractal dimension of surface elevation isolines. It is shown that the fractal dimension of isolines decreases at higher elevations for nearly all the ice sheet catchments. However, the magnitude of fractality, which represents the relative complexity or roughness of the surface, is spatially variable. Catchments in the central-east of the ice sheet have the highest fractal dimension, and the north catchment has the lowest fractal dimension. Multi-fractality at lower elevations for several catchments is observed including the southeast catchment, indicating that these catchments have variable dominant forcings at different length scales. Exploring the local variation of fractal dimensions shows that the majority of isolines with high fractal dimension are clustered in the central-east region and persist in contours up to 2500 m elevation. However, it is shown that local fractal dimensions are related to surface elevation, bed elevation, and ice thickness. It is also shown that local fractal dimensions are correlated with the ruggedness of basal topography (defined as the difference between the highest and lowest elevation in a window of $$3\times 3$$ pixels on a 150 m grid). This analysis serves as a qualitative approach for investigating the processes that control the geometry of ice caps on other terrestrial planets.

Effect of shifting ice flow and basal topography in shaping three-dimensional dispersal patterns, Lac de Gras region, Northwest Territories, Canada

Tracing indicator minerals and geochemical pathfinders in glacial sediments back to their up-ice source is a common mineral exploration approach in prospective, formerly glaciated regions. In this study, we utilize surface and subsurface data from the Lac de Gras area of the Northwest Territories to develop a three-dimensional understanding of till compositional anomalies emanating from two known kimberlite pipes, DO-18 and DO-27. Specifically, this study examines the three-dimensional shape of dispersal trains as defined by geochemical pathfinder elements and kimberlite indicator minerals shed from a pair of kimberlite pipes within a till cover of variable thickness. From our ninety-four reverse circulation boreholes (n = 251 till samples), and other publicly-available geologic datasets, we have reconstructed bedrock topography, till thickness, and the subsurface geometry of two dispersal trains. Utilizing our three-dimensional dataset, we have documented the role of basal topography in creating dispersal patterns with contrasting geometries from two adjacent kimberlites, as well as in the preferential preservation of older till units. The combination of field observations of ice-flow indicators and till compositional data demonstrates that features produced by multiple ice flows are preserved in both the erosional and depositional records in this region. Three-dimensional dispersion patterns of kimberlite indicators reflect the effect of shifting ice-flow direction with respect to slope aspect of bedrock topography in governing compositional variability within glacial drift. Our findings suggest that surficial data do not capture the full extent of dispersion patterns even in areas of relatively thin and discontinuous till cover.

Surface Expression of Basal and Englacial Features, Properties, and Processes of the Greenland Ice Sheet

AbstractRadar‐sounding surveys measuring ice thickness in Greenland have enabled an increasingly “complete” knowledge of basal topography and glaciological processes. Where such observations are spatially limited, bed elevation has been interpolated through mass conservation or kriging. Ordinary kriging fails to resolve anisotropy in bed geometry, however, leaving complex topography misrepresented in elevation models of the ice sheet bed. Here, we demonstrate the potential of new high‐resolution (≤5 m) surface topography data (ArcticDEM) to provide enhanced insight into basal and englacial geometry and processes. Notable surface features, quantified via residual surface elevation, are observed coincident with documented subglacial channels, and new, smaller‐scale tributaries (<2,000 m in width) and valley‐like structures are clearly identified. Residual surface elevation also allows the extent of basal ice units to be mapped, which in conjunction with radar data indicate that they act as “false bottoms,” likely due to a rheological contrast in the ice column.

Geoelectrical exploration for groundwater in crystalline basement rocks of Gbongudu community, Ibadan, Southwestern Nigeria

Gbongudu community, Ibadan underlain by crystalline rocks of Basement Complex of Southwestern Nigeria has need for alternative water supply due to inadequacies of surface water supply and common occurrences of low yield or dry boreholes coupled with rapid development within the community. Geophysical investigation involving electrical resistivity was carried out with the aim of evaluating the groundwater potential of the community. Thirty-three (33) Vertical Electrical Soundings were acquired across the community adopting Schlumberger electrode configuration with current electrode separation (AB) varying from 2 to 200 m. The acquired field data were interpreted using partial curve matching with suitable auxiliary chart and “RESIST” resistivity sounding interpretation software was also used for iteration. The results revealed two to five geo-electric layers, namely, topsoil, clayey soil, lateritic clay, weathered layer, fractured unit and fresh basement. The weathered and the fractured units form the aquiferous zones in the area investigated. Thematic maps of geoelectric and Dar-Zarrouk parameters produced from the interpreted VES data are aquifer unit (weathered layer) thickness map, aquifer unit (weathered layer) resistivity map, overburden thickness map, basement resistivity map, basement topography map, reflection coefficient map, total longitudinal conductance map, total transverse resistance map and electrical anisotropy (coefficient of anisotropy) map. The thematic maps were combined to form a single component which gives the groundwater potential map of the study area. The groundwater potential map was used to classify the study area into poor, low and good aquifer potential zones. The study reveals that the weathered and fractured zone that occur in the southeastern part of the study area constitute the water bearing zone which can be regarded as good groundwater potential. This study has provided evidence on the hydrogeologic framework and subsurface nature of main aquifer units in the area.KEY WORDS: Groundwater exploration, RESIST, Dar-Zarrouk parameters, Electrical anisotropy, Borehole drilling.

Applications of Aeromagnetic and Electrical Resistivity Data for Mapping Spatial Distribution of Groundwater Potentials of Dutse, Jigawa State, Nigeria

Optimal mapping of groundwater resource is usually accomplished using integrated geophysical survey in which two or more techniques are applied. In this work, spectral analysis of aeromagnetic data was used to map the topography of the magnetic crystalline fresh basement underlying the water bearing aquifer of Dutse, Jigawa State Nigeria. Vertical electric sounding technique was used to detect the spatial distribution of the depths to the top of the aquifer within the same area.  The overall goal of the work is to determine the groundwater potential of the area by mapping the spatial distribution of groundwater availability based on aquifer thickness and basement topography. The aquifer depth was found to be within the range of 5 to 15 m with a mean value of 10 m. The basement was found to be highly undulating having depths ranging from 6 to 69 m with a mean value of 24 m. The existence of isolated deep depressions within the basement which appears to be filled by water bearing sediments was thought to be favorable structures for groundwater accumulation. The spatial distribution of groundwater potential was therefore mapped in a reclassified image of three zones, high, moderate and low. The study therefore provides a vital tool for groundwater exploitation and management strategies.

Weld kinematics of syn-rift salt during basement-involved extension and subsequent inversion: Results from analog models

Scaled analog models based on extensional basins with synrift salt show how basement topography exerts a control factor on weld kinematics during the extension and inversion phases. In the case of basement-involved extension, syn-rift salt thickness differences may lead to variable degrees of extensional decoupling between basement topography and overburden, which in turn have a strong impact on the development of salt structures. With ongoing extension and after welding, the basin kinematics evolves toward a coupled deformation style. The basin architecture of our experimental results record the halokinetic activity related to growing diapirs and the timing of weld formationduring extension. Moreover, the structures that result from anysubsequent inversion of these basins strongly depends on the inherited welds and salt structures. While those basins are uplifted,the main contractional deformation during inversion is absorbed by the pre-existing salt structures, whose are squeezed developing secondary welds that often evolve into thrust welds. The analysis of our analog models shows that shortening of diapirs is favored by: i) basement topography changes that induce reactivation of primary welds as thrust welds; ii) reactivation of the salt unit as a contractional detachment and iii) synkinematic sedimentation during basin inversion. Finally, in this article, we also compare two natural examples from the southern North Sea that highlight deformation patterns very similar to those observed in our analog models.

Ice thickness measurements of Guliya ice cap, western Kunlun Mountains (Tibetan Plateau), China

ABSTRACTDespite their high value and importance for various glaciological applications, detailed ice thickness measurements of alpine glaciers are still very limited. Knowledge of bedrock topography is essential for paleoglaciological studies. The Guliya ice cap located on the Tibetan Plateau is one of the highest and largest ice caps in mid-low latitude regions. A detailed ground-penetrating radar (GPR) survey was conducted on the Guliya ice cap in 2015 using 20 and 40 MHz frequency antennas. An empirical Bayesian kriging method was used for ice thickness interpolation and uncertainty assessment. GPR measurements revealed complex basal topography of the Guliya glacier with a maximum thickness of 371.12 ± 13 m. The internal reflections caused by changes in the dielectric properties were registered on the 40 MHz radargrams at the summit and were attributed to density variations. As a result of this fieldwork, one of the largest ice thickness datasets in High Mountain Asia was obtained. Guliya glacier elevation changes were assessed by differencing digital elevation models. The glacier gained mass from 2000 to 2015 with an average rate of 0.270 ± 0.11 m w.e. a−1at the summit and 0.279 ± 0.11 m w.e. a−1at the lower elevations.

Basal freeze-on generates complex ice-sheet stratigraphy

Large, plume-like internal ice-layer-structures have been observed in radar images from both Antarctica and Greenland, rising from the ice-sheet base to up to half of the ice thickness. Their origins are not yet understood. Here, we simulate their genesis by basal freeze-on using numerical ice-flow modelling and analyse the transient evolution of the emerging ice-plume and the surrounding ice-layer structure as a function of both freeze-on rate and ice flux. We find good agreement between radar observations, modelled ice-plume geometry and internal layer structure, and further show that plume height relates primarily to ice-flux and only secondarily to freeze-on. An in-depth analysis, performed for Northern Greenland of observed spatial plume distribution related to ice flow, basal topography and water availability supports our findings regarding ice flux and suggests freeze-on is controlled by ascending subglacial water flow. Our results imply that widespread basal freeze-on strongly affects ice stratigraphy and consequently ice-core interpretations.

Oceanic-like axial crustal high in the central Red Sea

The Red Sea is an important example of a rifted continental shield proceeding to seafloor spreading. However, whether the crust in the central Red Sea is continental or oceanic has been controversial. Contributing to this debate, we assess the basement geometry using seismic reflection and potential field data. We find that the basement topography from seismically derived structure corrected for evaporite and other sediment loading has an axial high with a width of 70–100 km and a height of 0.8–1.6 km. Basement axial highs are commonly found at mid-ocean ridges affected by hotspots, where enhanced mantle melting results in thickened crust. We therefore interpret this axial high as oceanic-like, potentially produced by recently enhanced melting associated with the broader Afar mantle anomaly. We also find the Bouguer gravity anomalies are strongly correlated with basement reflection depths. The apparent density contrast necessary to explain the Bouguer anomaly varies from 220 kg m−3 to 580 kg m−3 with no trend with latitude. These values are too small to be caused primarily by the density contrast between evaporites and mantle across a crust of uniform thickness and density structure, further supporting a thickened crustal origin for the axial high. Complicating interpretation, only a normal to modestly thickened axial crust is predicted from fractionation-corrected sodium contents (Na8.0), and the basement reflection is rugged, more typical of ultra-slow spreading ridges that are not close to hotspots. We try to reconcile these observations with recent results from seismic tomography, which show modest mantle S-wave velocity anomalies under this part of the Red Sea.

Dynamic changes in outlet glaciers in northern Greenland from 1948 to 2015

Abstract. The Greenland Ice Sheet (GrIS) is losing mass in response to recent climatic and oceanic warming. Since the mid-1990s, tidewater outlet glaciers across the ice sheet have thinned, retreated, and accelerated, but recent changes in northern Greenland have been comparatively understudied. Consequently, the dynamic response (i.e. changes in surface elevation and velocity) of these outlet glaciers to changes at their termini, particularly calving from floating ice tongues, is poorly constrained. Here we use satellite imagery and historical maps to produce an unprecedented 68-year record of terminus change across 18 major outlet glaciers and combine this with previously published surface elevation and velocity datasets. Overall, recent (1995–2015) retreat rates were higher than at any time in the previous 47 years (since 1948). Despite increased retreat rates from the 1990s, there was distinct variability in dynamic glacier behaviour depending on whether the terminus was grounded or floating. Grounded glaciers accelerated and thinned in response to retreat over the last 2 decades, while most glaciers terminating in ice tongues appeared dynamically insensitive to recent ice tongue retreat and/or total collapse. We also identify glacier geometry (e.g. fjord width, basal topography, and ice tongue confinement) as an important influence on the dynamic adjustment of glaciers to changes at their termini. Recent grounded outlet glacier retreat and ice tongue loss across northern Greenland suggest that the region is undergoing rapid change and could soon contribute substantially to sea level rise via the loss of grounded ice.

Response of Surface Topography to Basal Variability Along Glacial Flowlines

AbstractPredicting the amplitude and distribution of surface undulations on ice sheets and glaciers is useful because of their influence on surface mass and energy balance, atmospheric boundary layer processes, and supraglacial meltwater routing. We develop an approximate method of calculating the surface elevation response due to spatial perturbations in basal topography and slipperiness, on two‐dimensional flow sections whose thickness, surface slope, and basal slip ratio vary along flow. Our main result is an integral expressing nonuniform transfer of basal variability to the surface. It uses published Fourier transfer functions derived through perturbing plane‐slab Stokes flow but circumvents the need to subwindow the spatial domain to estimate the response. We test the method on ice flow synthesized by a finite‐element model of Stokes flow with constant viscosity and known basal topography and slipperiness perturbations; in this case, it predicts the observed size and shape of the surface undulations well, capturing more than 90% of their variance. Application of the method to the central flowline of Columbia Glacier, Alaska, and a flowline on the Greenland Ice Sheet ending on Nordenskiöld Glacier, using knowledge of the approximate bed topography and ignoring the unknown slipperiness forcing, yields less faithful prediction of their surface undulations (40–50% of their variance) but demonstrates the method's potential to reproduce their qualitative features. We discuss the factors limiting the method's performance on real flowlines.

Quantitative controls on the regional geometries and heterogeneities of the Rayda to Shu'aiba formations (Northern Oman) using forward stratigraphic modelling

Abstract The complex geometry of carbonate systems is influenced by a multitude of physical as well as biological processes. The Lower Cretaceous carbonates of Northern Oman are characterised by a variability of regional-scale geometries with expected vertical and lateral facies variations. The main environmental and tectonic controls acting on the depositional processes of the Lower Cretaceous ramps and platforms through space and geological time (in 4 dimensions) are only partially understood. In this study, we use a 4D DionisosFlow Forward Stratigraphic Modelling (FSM) approach to explore the role of: (i) eustasy; (ii) subsidence; (iii) initial paleobathymetry, and (iv) wave energy, to generate carbonate stacking patterns and heterogeneities. Carbonate production was maintained constant through deposition. Multi-disciplinary and multi-scale datasets were used (i.e. seismic, well and field data) to constrain the FSM input parameters and sensitivity analysis was carried out to validate or refute some depositional model hypotheses. Results show that basement topography and eustasy have the greatest influence on the progradational geometries and the lateral continuity of clinoform architectures during the Tithonian to Valanginian second-order super-sequence. In the Valanginian to Aptian super-sequence, subsidence was the primary control for the observed aggradational stacking pattern. Lateral and vertical stacking of carbonate lithologies, textures as well as facies are thus apprehended through this FSM approach, leading to a better assessment of petroleum systems elements as reservoir, seal and trap.

Active seismic studies in valley glacier settings: strategies and limitations

ABSTRACTSubglacial tills play an important role in glacier dynamics but are difficult to characterize in situ. Amplitude Variation with Angle (AVA) analysis of seismic reflection data can distinguish between stiff tills and deformable tills. However, AVA analysis in mountain glacier environments can be problematic: reflections can be obscured by Rayleigh wave energy scattered from crevasses, and complex basal topography can impede the location of reflection points in 2-D acquisitions. We use a forward model to produce challenging synthetic seismic records in order to test the efficacy of AVA in crevassed and geometrically complex environments. We find that we can distinguish subglacial till types in moderately crevassed environments, where ‘moderate’ depends on crevasse spacing and orientation. The forward model serves as a planning tool, as it can predict AVA success or failure based on characteristics of the study glacier. Applying lessons from the forward model, we perform AVA on a seismic dataset collected from Taku Glacier in Southeast Alaska in March 2016. Taku Glacier is a valley glacier thought to overlay thick sediment deposits. A near-offset polarity reversal confirms that the tills are deformable.

Greenland Ice Sheet Surface Topography and Drainage Structure Controlled by the Transfer of Basal Variability

Ice flow can transfer variations in basal topography and basal slipperiness to the ice surface. Recent developments in this theory have made it possible to conduct numerical experiments to predict mesoscale surface topographical undulations and surface relief on an ice sheet-scale. Focussing here on the contemporary Greenland Ice Sheet (GrIS), we demonstrate that the theory can be used to predict the surface relief of the ice sheet from bed topography, ice thickness and basal slip ratio datasets. In certain regions of the GrIS our approach overestimates, while in others underestimates, the observed surface relief. The magnitude and spatial pattern of these mismatches correspond with the theory’s limitations and known uncertainties in the bed topography and basal slip ratio datasets. Our prediction experiment establishes that the first-order control on GrIS surface relief is basal topography modulated by ice thickness, surface slope and basal slip ratio. Additional analyses show that the surface relief, which is controlled by the bed-to-surface transfer of basal topography, preconditions the large scale spatial structure of surface drainage, with other factors such as surface runoff modulating the actual drainage system through influencing the temporal evolution of meltwater features. It follows that the spatial structure of surface drainage depends strongly on the transfer of basal topography to the ice surface. These findings represent an important step towards investigating and understanding the net long-term (>102 years) effect of surface drainage on ice sheet mass balance and dynamics during deglaciation events.

Structure of the Marathon basin (NE Attica, Greece) based on gravity measurements

This work presents the results of a high resolution gravity survey conducted in the area of Marathon Basin to asses the structure of the basement. A total of 120 gravity stations were measured. Standard data reduction procedures were followed, albeit based on modern, high precision techniques. The inversion of the Bouguer anomaly map provided a rather detailed image of the basement topography completing the information available from surface geological and morphological surveys. The major structural features of the Marathon Basin appear to be controlled by a system of NE-SW neotectonic faults causing stepwise NW-SE deepening of the basement to a maximum detectable depth of~450m. These faults, together with a system of NW-SE major, and a host of minor faulting features, lend to the Basin characteristics of a Riedel structure formed by a predominantly N-S extensional field effecting clockwise block rotation. This mode of deformation may also have facilitated the formation of local anticlinal/ synclinal structures, which endow the area with its rather complicated morphology. Finally, the multiple intersecting faults appear to form a permeable network, presumably responsible for the salination of the local aquifer system; the NE-SW major normal faults may the most significant contributor to this effect.

Antecedent geologic control on nearshore morphological development: The wave dominated, high sediment supply shoreface of southern Namibia

The inner continental shelf, nearshore zone and adjacent onland coastal strip of southern Namibia has been mapped using airborne electromagnetic and magnetic techniques, providing, for the first time in the area, a continuous onshore-offshore geophysical dataset. In addition, the inner continental shelf and nearshore zones of this vigorous, wave-dominated coastline were mapped, inshore of a pervasive acoustic blanketing layer, using multibeam bathymetry, singlebeam bathymetry and boomer sub-bottom profiling systems in water depths between ‐3 and ‐55 m. We use an integrated, iterative approach to the interpretation of these brand new datasets to map the structural framework of the Late Proterozoic -age bedrock of the area beneath thick Holocene sand cover and show how, despite the vigorous wave regime and high sediment supply from the Orange River, this framework acts as a recurring primary control to the area's geomorphic evolution.The bedrock morphology is divided into three main zones that are constrained by NNW-striking and coast-parallel faults. A southern zone, characterised by both relatively shallow bedrock depth and shallow bedrock slope is bounded in the north by a dextral strike-slip fault system. This basement promontory is the primary control on the shallow, low gradient seafloor in the southern zone. Strike-slip movement on the fault shifted the bedrock to the north both down and inshore relative to the shallow basement promontory to the south. However, in this central zone, both the seafloor and seafloor gradient remain shallow. Sediment accumulation in this area is explained by two processes. Firstly, the relative downward movement of the bedrock has resulted in the creation of relatively more accommodation space and sediment delivery begins to overprint the underlying bedrock framework, smoothing the general seafloor gradient in the central zone. Secondly, this central zone, directly north of the basement promontory has existed in a swell shadow as long-lived SW directed wave energy has continually been refracted around the palaeo-promontory, promoting natural accretion of sediment within the lee of this feature. A fault-related inflection point on the modern coastline marks a change in coastline orientation from NW to slightly more north striking, and the start of the northernmost bedrock zone. Here the shallow, wide, low-relief seafloor zone transitions to a wider nearshore high-relief zone that mimics the underlying uniformly steepening nearshore bedrock profile. The combination of the uniform, coast-normal basement gradient with the fault-related inflection changes the nearshore response to incoming wave energy. In the northern zone, waves approach at a slightly more oblique angle due to the fault-controlled coastline orientation, and reinforce an already vigorous longshore drift regime that is responsible for the steeper and deeper nearshore profile, underpinned by the underlying steep basement topography. The steep antecedent topography also limits the amount by which the seafloor can aggrade and subsequently adjust its slope; an imbalance between the sediment supply and the large volume of accommodation space thus exists that cannot be overcome given the context of the vigorous longshore drift. The continual inheritance of the underlying bedrock slope in the nearshore is further revealed with the unchanging slope of the reflectors that match the present-day nearshore seafloor gradient. We suggest that the geologic framework control remains a critical factor in nearshore-beach system development, even in an area of high sediment supply and high wave energy conditions.

Electrical resistivity investigation of sub- surface topography of Rafin Bareda Drainage Basin as a tool for cassiterite-columbite exploration in Dutsen-Wai, Nigeria

Geophysical investigation has been undertaken at Rafin-Bareda drainage basin with the aim of identifying suitable points of high Cassiterite potential for the study area. The people around such drainage basin exploit these cassiterite and columbite deposits by trial and error method.The trial and error process of mining is dangerous to the lives of the miners.Hence, a Schlumberger array with half current electrode spacing ranging 1-100m was used. A total of 36 vertical electrical sounding (VES) stations were equally spaced at 20 m interval along eight profiles in the area using Ohmega terrameter. The field data were interpreted using IPI2WINsoftware. The results obtained from the obtained VES data were applied to produce the geoelectric sections of the study area, which indicate a 2-4 range of geoelectrical layers. The corresponding geologic section comprises of sandy clay soil, weathered and fresh laterite, weathered and fresh basement as the main rock types. The 3D surface of the basement topography of the study area shows the undulating nature of the bedrock subsurface with the position of the various VES points. These enabled us to determine zones with high cassiterite potential and the best location for drilling productive cassiterite in the area. The depth to the basement rock, resistivity and thickness values ranged from 4.50-36.40m, 59-1724Ωm and 1.12-36.40m, respectively. Thus, the geoelectric sections suggest that the point of high depression on the basement rock represents a promising area for Cassiterite-Columbite. The Cassiterite mineralization map identifies the VES points with high cassiterite potential. Keyword: Geological profiles, Vertical electric sounding (VES), Cassiterite-Columbite,Dutsen-wai drainage

Relevance of Detail in Basal Topography for Basal Slipperiness Inversions: A Case Study on Pine Island Glacier, Antarctica

Given high-resolution satellite-derived surface elevation and velocity data, ice-sheet models generally estimate mechanical basal boundary conditions using surface-to-bed inversion methods. In this work, we address the sensitivity of results from inversion methods to the accuracy of the bed elevation data on Pine Island Glacier. We show that misfit between observations and model output is reduced when high-resolution bed topography is used in the inverse model. By looking at results with a range of detail included in the bed elevation, we consider the separation of basal drag due to the bed topography (form drag) and that due to inherent bed properties (skin drag). The mean value of basal shear stress is reduced when more detailed topography is included in the model. This suggests that without a fully resolved bed a significant amount of the basal shear stress recovered from inversion methods may be due to the unresolved bed topography. However, the spatial structure of the retrieved fields is robust as the bed accuracy is varied; the fields are instead sensitive to the degree of regularisation applied to the inversion. While the implications for the future temporal evolution of PIG are not quantified here directly, our work raises the possibility that skin drag may be overestimated in the current generation of numerical ice-sheet models of this area. These shortcomings could be overcome by inverting simultaneously for both bed topography and basal slipperiness.

English

Qualitative and quantitative interpretations of aeromagnetic anomalies over Ogoja and Bansara areas of Anambra Basin, Lower Benue Trough of Nigeria were carried out using spectral analysis and source parameter imaging methods. The study area which covers an area of approximately 6050 km2 lies within latitude 60° 0' to 7° 0' North and longitude 8° 30' to 9° 0' East. The regional anomaly was separated from the total magnetic intensity map to obtain the residual anomaly using first order polynomial fitting technique. The residual data was analyzed spectrally to obtain 18 spectral blocks for sedimentary depth estimates (deep and shallow depths). The edges and causative bodies of the residual anomaly were also sharpened to reduce anomaly complexity as well as fault trend amplification using first, second and horizontal derivatives. The 3-D basement topography map of the study area shows linear depression with deepest sedimentary thickness at the southeastern region of the study area, which implies that the feasibility of hydrocarbon potential will be higher in Bansara area than in Ogoja. The deepest depths obtained from spectral analysis and source parameter imaging are 5437.0 and 5059.9 m, respectively.    Key words:  Spectral analysis, source parameter imaging, magnetic anomaly, sedimentary thickness, intrusive bodies, hydrocarbon potentials.