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Geology topics

Donald Plouff

Publications and source records attributed to Donald Plouff.

65 records · Page 4Linked to original sources
USGS1976

Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections

Computer programs based on the exact calculations of the gravity and magnetic anomalies of polygonal prisms are faster in operation and more accurate than previous programs based on the numerical integration of polygonal laminas. The prism programs also are of more general application than existing computer programs that are based on the exact gravity and magnetic effects of rectangular prisms. There are no restrictions on the use of the exact formula for the gravitational attraction of a polygonal prism, but the formulas for the magnetic effect are restricted in that demagnetization is not considered, and a finite answer is not obtained in the unrealistic circumstance where an observation point coincides with an edge of the prism.Least-squares methods permit calculation of the gravity or magnetic effect of models without knowledge of the density or magnetization contrasts, respectively, by comparison of the observed anomalies with theoretical dimensionless values to determine contrasts as regression coefficients. The coefficient of correlation provides a goodness of fit estimate that helps model evaluation. After calculating a magnetic terrain correction for an outcrop of Quaternary dacite and andestite near Clear Lake, Calif., an improvement of the coefficient of correlation from 88 to the 92 percent level indicates that this volcanic unit probably extends at least 150 m beneath the surface. Application of a magnetic terrain correction to disconnected outcrops of Tertiary andesite, eliminates most of a prominent v-shaped magnetic anomaly south of the San Juan Mountains, Colo.

Arizona↗
USGS1966

Magnetotelluric soundings in the southwestern United States

Twelve magnetotelluric soundings were made along a 460-mile traverse in the southwestern United States. Spectral analysis of digitized graphic records was used to determine apparent resistivities and phase differences for periods in the range 10-1,000 sec. The geoelectric section to depths of the order of 300 km was found to be divided into three principal layers. The layer nearest the surface has a resistivity of 200 ohm-m or less. The next layer, which begins in Paleozoic carbonate rock or near the top of the Precambrian rocks, has a resistivity of 500 ohm-m or more. The resistivity of the lowest layer is in the range 5-100 ohm-m and the depth to the top is in the range 20-150 km.

Arizona, Utah↗