No abstract

No abstract

Two techniques for leveling aeromagnetic survey data are compared. The first estimates a set of values for the magnetic intensity at each flight‐line/tie‐line intersection point. These are obtained after an intersection‐moving procedure to reduce loop closures and are subject to the constraint that the datums calculated from the differences between the raw data and the corrected data have minimum along‐line variation. The procedure (a least‐squares method) is equivalent to the older manual loop closure methods used for magnetic, gravimetric, and geodetic surveying. The second method involves the fitting of polynomials to the observed flight‐line/tie‐line intersection errors along each line in the survey. These are then subtracted from the raw data and intersection locations are adjusted to give minimum intersection errors. In the analysis of two surveys the least‐squares method was found to be superior as a leveling procedure. It produced datums which were similar in amplitude and had a high correlation with the base station magnetometer recordings. The polynomial method produced datums which had greater amplitudes and showed a tendency to fluctuate at the ends of lines.

Minimum air temperature measurements and airborne thermal scanner measurements of apparent surface temperature were obtained in a regional frost risk study in gently undulating grazing country in the Southern Tablelands of New South Wales, Australia.The minimum air temperature data obtained at 7 sites on 100 nights have been used to derive a predictive relationship between temperature lapse rate and night-time wind speed and net radiation loss. An extended network was used on 30 nights using an additional 24 sites. Excellent agreement was observed between the lapse rates collected from both the 7-station and 31-station networks on 24 nights.Maps of minimum air temperature across the region have been obtained for selected individual nights using Laplacian smoothing spline functions based on elevation and map co-ordinates.Thermal scanner data were obtained with an aircraft on two nights and the paper presents detailed comparisons between apparent surface temperature data and minimum air temperatures obtained from the regional maps. These comparisons have made it possible to successfully distinguish between broad topographic controls and the effect of local surface characteristics, especially in the case of high-resolution thermal scanner data. Such local controls include the effects of trees, surface water and various man-made features, as well as very local topographic features such as narrow depressions, which are only noticeable from site surveys and detailed aerial photography. It is concluded that thermal imagery is an important aid in understanding spatial distribution patterns of night-time air temperatures and hence in regional frost risk assessment.

A procedure for estimating background‐correction terms for the uranium channel of an airborne gamma‐ray survey has been developed. The residuals obtained from a multiple linear regression of flight‐line means for the uranium channel on the means for thorium and potassium are used to correct the uranium channel for each line. The procedure assumes that, were it not for these background errors, the uranium flight‐line means would be a linear function of the means for potassium and thorium. It also assumes that the background correction is the same for the whole of each line. In spite of these limitations, the method produces good background estimates consistent with those found by more sophisticated methods.