3D ACOUSTIC REVERSE-TIME MIGRATION1

Acoustic reverse-time finite-difference migration for zero-offset data is extended from two- to three-dimensional media. The formulation is based on the full three-dimensional acoustic wave equation and so has no dip restrictions and it involves extrapolation in a velocity distribution variable in three dimensions. The algorithm is demonstrated by successful migration of synthetic data sets for three models: a point diffractor, an oblique pinch-out, and a dome overlying a planar reflector.     

AUTOMATIC EDITING OF NOISY SEISMIC DATA1

Seismic data often contain traces that are dominated by noise; these traces should be removed (edited) before multichannel filtering or stacking. Noise bursts and spikes should be edited before single channel filtering. Spikes can be edited using a running median filter with a threshold; noise bursts can be edited by comparing the amplitudes of each trace to those of traces that are nearby in offset-common midpoint space. Relative amplitude decay rates of traces are diagnostic of their signal-to-noise (S/N) ratios and can be used to define trace editing criteria. The relative amplitude decay rate is calculated by comparing the time-gated trace amplitudes to a control function that is the median trace amplitude as a function of time, offset, and common midpoint. The editing threshold is set using a data-adaptive procedure that analyses a histogram of the amplitude decay rates. A performance evaluation shows that the algorithm makes slightly fewer incorrect trace editing decisions than human editors. The procedure for threshold setting achieves a good balance between preserving the fold of the data and removing the noisiest traces. Tests using a synthetic seismic line show that the relative amplitude decay rates are diagnostic of the traces’S/N ratios. However, the S/N ratios cannot be accurately usefully estimated at the start of processing, where noisy-trace editing is most needed; this is the fundamental limit to the accuracy of noisy trace editing. When trace equalization is omitted from the processing flow (as in amplitude-versus-offset analysis), precise noisy-trace editing is critical. The S/N ratio of the stack is more sensitive to type 2 errors (failing to reject noisy traces) than it is to type 1 errors (rejecting good traces). However, as the fold of the data decreases, the S/N ratio of the stack becomes increasingly sensitive to type 1 errors.     

MELTWATER FEATURES THAT SUGGEST MIOCENE ICE-SHEET OVERRIDING OF THE TRANSANTARCTIC MOUNTAINS IN VICTORIA LAND, ANTARCTICA

ABSTRACT. We illustrate here spectacular meltwater features associated with outburst floods beneath an ice sheet that overrode the Transantarctic Mountains in southern Victoria Land. Because of long-term hyperarid polar climate, these features are part of an ancient landscape preserved for about 14 million years. Some channels are associated with areal scouring of basement rocks extending from sea level to as much as 1200–2100 m elevation in coastal regions. Scablands with scallops, potholes and plunge pools are cut in Beacon Super group sandstones and Ferrar Dolerite and cover wide areas of high western plateaus near the mountain crest. Subglacial channel systems commonly originate near divides and converge downhill toward the northeast. We argue that the landforms were created beneath a major Antarctic Ice Sheet that submerged the whole area, with the possible exception of the high peaks of the Royal Society Range, as it flowed northeastward toward the outer Antarctic continental shelf. Areal scouring, associated with warm-based regimes, is restricted to the lower slopes close to the coast. In the higher terra in, meltwater channels and scabland alongside preserved patches of regolith are best explained by the breaching of cold-based ice on the mountain rim by subglacial melt water outbursts. Melt from warm-based ice, along with subglacial lakes trapped upstream of the mountain rim, are possible sources of the meltwater necessary to form the channel systems and scablands.     

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Emplacement of mantle-derived magma (magmatic accretion) isoften presumed or inferred to be an important cause of regionalgranulite facies metamorphism and crustal anatexis. The juxtapositionof mafic cumulates and regionally distributed granulite faciesrocks has led some to consider the Ivrea zone (northern Italy,Southern Alps) as an important exposure that demonstrates thiscausal relationship. However, regional PTt paths indicated bymetamorphic reaction textures and PT conditions inferred fromgeothermobarometry indicate that the emplacement of mafic plutonicrocks (Mafic Complex) at the Ivrea zone occurred during decompressionfrom ambient pressures at the regional thermal maximum. Fieldand petrographic observations, supported by PT estimates, indicatethat regional retrograde decompression and emplacement of theupper parts of the Mafic Complex probably accompanied extensionduring the Late Carboniferous–Early Permian. A spatiallyrestricted decompression-melting event accompanied final emplacement,depleting supracrustal rocks enclosed by an     

Fades of subglacial channel sedimentation in late-Pleistocene drumlins, Northern Ireland

The sedimentology of three sand-cored drumlins in central Ulster. Northern Ireland is described. The sand-cores of the drumlins consist of complex sequences of cohesive sediment gravity forms, grain-flow deposits, turbiditic sands and deltaic sands and gravels, showing complex fades relationships. The bulk of the sediments formed in water-filled cavities associated with a major subglacial meltwater escape route excavated in bedrock. The sand-cores are overlain by a streamlined carapace of basal melt-out till. A model is presented to account for spatial variations in faeies arrangement within the sand-cores. Three specific meltwater depositional subenvironments were recognised: (1) main channel axis, (2) main channel flank, and (3) minor tributary channel. The sand-cores formed prior to the main phase of basal melt-out till deposition and before drumlin streamlining.     

Finding common ground: relational concepts of land tenure and economy in the oil palm frontier of Papua New Guinea

In the oil palm frontier regions of West New Britain and Oro provinces, Papua New Guinea, customary land tenure arrangements are changing in response to the growing demand for land for agricultural development. This paper examines one aspect of these changes, namely the gifting and selling of customary land for oil palm development to people who have no customary birthrights to the land. By analysing how access rights are maintained over the relatively long cultivation cycle of oil palm (approximately 25 years), and in the context of the rapidly changing socio-economic and demographic environments of the oil palm frontiers, the paper demonstrates that while land transactions seemingly entail the commodification of land, land rights and security of land tenure remain embedded in social relationships. For customary landowners, the moral basis of land rights is contingent on 'outsiders' maintaining particular kinds of social and economic relationships with their customary landowning 'hosts'. In exploring how these social relationships are constituted through the performance of particular kinds of exchange relationships, the paper provides insights into relational concepts of land rights and how these are able to persist in Papua New Guinea's oil palm frontier regions where resource struggles are often intense and where large migrant populations are seeking land for agricultural development.