Zwei Diskussionsbeiträge zu: „Präkambrische Itabirite und Uran-Gold-Konglomerate als geochemische Zeitmarken in der Erdevolution“ von W. Fiebiger,Geol. Rdsch., 65, S. 1035–1055

Ohne Zusammenfassung     

Association of proportions

Simple correlation leads to misleading results when applied to the proportions of materials in mixtures, because of the constraint that the total of the proportions is less than one. A simple mainly graphical procedure to determine whether the proportions in mixtures are associated after allowing for the constraint is presented. No assumptions about the form of the underlying probability distributions are required. The technique is extended to problems where other constraints apply.     

Estimation and removal of non-tectonic effects from gravity survey data

When gravity survey accuracies of a few microgals are sought, many correction factors must be accounted for, including meter calibration constants, water-table level fluctations, solid-earth tides, ocean tides and in some cases rapid atmospheric fluctuations. Calculation of most of these correction factors is relatively straightforward. However, the effects of ocean tide loading are not as easily estimated, partly due to the lack of knowledge of the ocean tides themselves. Amplitude and phase factors for the better-known ocean tide components O₁ and M₂ have been theoretically computed for a grid in southern California in order to correct gravity survey data at arbitrary locations for these ocean tidal-loading components. The gravity data from a three-month period were recorded on a tidal gravimeter at the station PAS and then hand-digitized in order to test the ocean tide estimation program. The O₁ and M₂ ocean tidal components were effectively reduced to less than 0.5 μGal. The remaining high-frequency tidal components appear to be K₁ and S₂. If the ocean tides are not taken into account, as much as 16–20 μGal of error can occur solely due to the effect of ocean loading on the gravitational tides when comparing two surveys near Pasadena. The effect increases towards the coastline and decreases inland. Examples of reduced data from the CIT gravity survey network, which has been observed on an approximately monthly basis since 1974, will be shown.