Magnetic survey is the most widely used shallow geophysical techniquefor the location of archaeological activity in the United Kingdom but is often discountedftom use within alluviated landscapes. Results presented in this thesis from fluxgate gradiometer surveys conducted over an alluviated flood plain near the village of Yarnton, Oxfordshire, UK, demonstrate a wide variation of magnetic response between adjacent sites. This suggests a more complicated relationship between the rock magnetic properties of underlying archaeological sediments and the resultant magnetic anomalies recorded during surface magnetometer surveys. This study aims to investigate this relationship further and determine the influence of post-depositional mineral dissolution in water logged soils; a factor that together with increased alluvial overburden, has often been cited as an explanation for disappointing magnetic results over similar sites. A wide ranging study has been conducted including geophysical surveys, environmental magnetic analysis of archaeological sediments recovered during excavation and experimental work to investigate the influence of burning. Initial, rapid determination of isothermal magnetic parameters, such as susceptibility, has been complemented, for selected samples, by more detailed hysteresis measurements and thermomagnetic variation over a temperature range from 20 to 973K. Interpretation of the resulting data has been assisted through the development of semi-quantitative numerical models to describe the complex magnetic mixtures present. The results, including over 20ha (20, OOOOm2)of geophysical survey and the analysis of more than 500 samples, demonstrate the important role of fire for magnetic enhancement and also provides evidence, under suitable conditions, for more esoteric biogenic mechanisms. The main conclusions reached suggest archaeological features magnetised before the onset of floodplain conditions may still be detected through geophysical survey, particularly if more sensitive, caesium vapour magnetometers are applied In addition, semi-quantitative unmixing models allow both the thermal history of burnt sediments to be estimated and provide a means for identifying biogenically enhanced samples.

The need to construct optimal drape surfaces arises in airborne geophysical surveys where it is necessary to fly a safe distance above the ground and within the performance limits of the aircraft used, but as close as possible to the surface. The problem is formulated as an LP with constraints at every point of a grid covering the area concerned, yielding a very large problem. The objective of the LP is to create as "good" a surface as possible. This formulation is new, as previous methods did not aim to minimise an objective function. If the desired surface has only slope limitations, the resulting drape surface must be constrained in the first derivative. Such a drape surface is readily constructed using the Lifting Algoritlun. It is shown that the Litling Algorithm is both exact and has great speed advantages. Some numerical results confinning exacmcss and speed are presented, as is the algorithm's analogy to a flow network method. An enhanced lifting method with a better order of complexity is also proposed and tested numerically. In most practical situations a drape surface is required which has both first and second derivatives constrained. If only a cut through such a surface is considered, the problem can be solved with relative ease by exploiting its nctwork~Jike structure. This method fonns the basis of one of the preferred heuristics developed later. It was not possible to generalise this method to a full two~dimensional drape surface. A commercially available LP package fares better in finding the optimal solution. Several heuristic methods were examined. first a general heuristic method based on a lifting approach was developed. This was followed by a method using repeated application of the method used for sections (the Alternating One-dimensional Dual Algorithm ["AODA"]). Three heuristics based on thimbles were also designed. Thimbles are caps whose first and second derivatives are acceptable and which are placed over local infeasibilities in the topography The work ends with a chapter comparing the efficiency of various heuristics and comparing the results obtained using a number of test datasets. It was fOLmd that heuristic methods provide acceptable drape surfaces and that the choice lies between speed and accuracy, with a previously designed smoothing method being the fastesl and the AODA the most accurate and quick enough. / Operations Research / D.Phil.

The need to construct optimal drape surfaces arises in airborne geophysical surveys where it is necessary to fly a safe distance above the ground and within the performance limits of the aircraft used, but as close as possible to the surface. The problem is formulated as an LP with constraints at every point of a grid covering the area concerned, yielding a very large problem. The objective of the LP is to create as "good" a surface as possible. This formulation is new, as previous methods did not aim to minimise an objective function. If the desired surface has only slope limitations, the resulting drape surface must be constrained in the first derivative. Such a drape surface is readily constructed using the Lifting Algoritlun. It is shown that the Litling Algorithm is both exact and has great speed advantages. Some numerical results confinning exacmcss and speed are presented, as is the algorithm's analogy to a flow network method. An enhanced lifting method with a better order of complexity is also proposed and tested numerically. In most practical situations a drape surface is required which has both first and second derivatives constrained. If only a cut through such a surface is considered, the problem can be solved with relative ease by exploiting its nctwork~Jike structure. This method fonns the basis of one of the preferred heuristics developed later. It was not possible to generalise this method to a full two~dimensional drape surface. A commercially available LP package fares better in finding the optimal solution. Several heuristic methods were examined. first a general heuristic method based on a lifting approach was developed. This was followed by a method using repeated application of the method used for sections (the Alternating One-dimensional Dual Algorithm ["AODA"]). Three heuristics based on thimbles were also designed. Thimbles are caps whose first and second derivatives are acceptable and which are placed over local infeasibilities in the topography The work ends with a chapter comparing the efficiency of various heuristics and comparing the results obtained using a number of test datasets. It was fOLmd that heuristic methods provide acceptable drape surfaces and that the choice lies between speed and accuracy, with a previously designed smoothing method being the fastesl and the AODA the most accurate and quick enough. / Operations Research / D.Phil.

Microfiche and maps (numbered 1-7) in pocket / Includes bibliography (6 unnumbered leaves) / 109, [84] leaves (some folded) : ill., maps (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Economic Geology, 1984