The North American gravity database as well as databases from Canada, Mexico, and the United Stat... more The North American gravity database as well as databases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising procedures for calculating gravity anomalies, taking into account our enhanced computational power, improved terrain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences between previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve regional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conventionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and previous procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduction because of the widespread use of the global positioning system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjective " ellipsoidal " to differentiate anomalies calculated using heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid.
Eos, Transactions American Geophysical Union, 2002
Doing Science---that is what the majority of the readers of Eos do, day in and day out, and most ... more Doing Science---that is what the majority of the readers of Eos do, day in and day out, and most of us consider ourselves reasonably accomplished at it. Thus, why should we be interested in a book on Doing Science? I will try to convince you in the following few paragraphs that you will find this book enjoyable and filled with
A 70-km-long seismic reflection profile in western Michigan provides new insight into the nature,... more A 70-km-long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5-s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ~2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault-bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic-filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate-to high-angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region. ß ß ß ß ß ß ß ß ß ß o o %ø ß oø .... øøøø• ? ß o ß ß 'øø% Pap. 81-10, edited by F.H.
The exact solution for the oblate spheroidal layer model of the earth is applied to recent gravit... more The exact solution for the oblate spheroidal layer model of the earth is applied to recent gravity data measured at the Hilton mine, Mount Isa, Queensland. We find that our extracted values of the gravitational constant from the Hilton mine data are consistent with the laboratory value within the accuracy of the mass density profile determination made at the Hilton mine and the surrounding area.
Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within th... more Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within the Precambrian sedimentary rocks to a depth of 4 km in the McClure-Sparks No. 1-8 drill hole. The inplace densities of these units were calculated from the corrected observations. The densities of the Phanerozoic rocks vary considerably owing to the presence of evaporites contrasting with the clastic and carbonate rocks, and the mean density of the Phanerozoic rock column is 2.61 g/cm 3. The borehole gravimeter density of the Precambrian clastic rocks is 2.77 g/cm 3, 0.05 g/cm s greater than the density obtained from sample cuttings. The source of this discrepancy is unknown, but one possible explanation is the gravimetric effect of mafic igneous rocks, which may be more common within the Precambrian sedimentary rocks than is indicated by intersection of these rocks in the drill hole.
Two igneous bodies were encountered during the drilling of the Michigan Basin deep drill hole. Th... more Two igneous bodies were encountered during the drilling of the Michigan Basin deep drill hole. The upper body (unit 1) extended from 4970 to 4998 m, while the lower body (unit 2) was observed from 5252 m to the bottom of the hole at 5324 m. Samples used in this investigation were taken at 1.5-m intervals in the core section from 5306 to 5322 m. In addition, one sample from the lower contact of unit I was examined. The assemblage observed in unit 2 consists of clinopyroxene-chlorite-albite-epidote-actinolitesphene, while the one sample from unit 1 contains albite-chlorite-carbonate-epidote. Both of these assemblages can be characterized as spilitic and are representative of conditions which prevail during greenschist facies metamorphism. Comparison of the compositions of the various phases coexisting in the two assemblages with published experimental studies suggests conditions of equilibration of approximately 450øC and a fluid pressure of 1-2 kbar.
The North American gravity database as well as databases from Canada, Mexico, and the United Stat... more The North American gravity database as well as databases from Canada, Mexico, and the United States are being revised to improve their coverage, versatility, and accuracy. An important part of this effort is revising procedures for calculating gravity anomalies, taking into account our enhanced computational power, improved terrain databases and datums, and increased interest in more accurately defining long-wavelength anomaly components. Users of the databases may note minor differences between previous and revised database values as a result of these procedures. Generally, the differences do not impact the interpretation of local anomalies but do improve regional anomaly studies. The most striking revision is the use of the internationally accepted terrestrial ellipsoid for the height datum of gravity stations rather than the conventionally used geoid or sea level. Principal facts of gravity observations and anomalies based on both revised and previous procedures together with germane metadata will be available on an interactive Web-based data system as well as from national agencies and data centers. The use of the revised procedures is encouraged for gravity data reduction because of the widespread use of the global positioning system in gravity fieldwork and the need for increased accuracy and precision of anomalies and consistency with North American and national databases. Anomalies based on the revised standards should be preceded by the adjective " ellipsoidal " to differentiate anomalies calculated using heights with respect to the ellipsoid from those based on conventional elevations referenced to the geoid.
Eos, Transactions American Geophysical Union, 2002
Doing Science---that is what the majority of the readers of Eos do, day in and day out, and most ... more Doing Science---that is what the majority of the readers of Eos do, day in and day out, and most of us consider ourselves reasonably accomplished at it. Thus, why should we be interested in a book on Doing Science? I will try to convince you in the following few paragraphs that you will find this book enjoyable and filled with
A 70-km-long seismic reflection profile in western Michigan provides new insight into the nature,... more A 70-km-long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5-s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ~2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault-bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic-filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate-to high-angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region. ß ß ß ß ß ß ß ß ß ß o o %ø ß oø .... øøøø• ? ß o ß ß 'øø% Pap. 81-10, edited by F.H.
The exact solution for the oblate spheroidal layer model of the earth is applied to recent gravit... more The exact solution for the oblate spheroidal layer model of the earth is applied to recent gravity data measured at the Hilton mine, Mount Isa, Queensland. We find that our extracted values of the gravitational constant from the Hilton mine data are consistent with the laboratory value within the accuracy of the mass density profile determination made at the Hilton mine and the surrounding area.
Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within th... more Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within the Precambrian sedimentary rocks to a depth of 4 km in the McClure-Sparks No. 1-8 drill hole. The inplace densities of these units were calculated from the corrected observations. The densities of the Phanerozoic rocks vary considerably owing to the presence of evaporites contrasting with the clastic and carbonate rocks, and the mean density of the Phanerozoic rock column is 2.61 g/cm 3. The borehole gravimeter density of the Precambrian clastic rocks is 2.77 g/cm 3, 0.05 g/cm s greater than the density obtained from sample cuttings. The source of this discrepancy is unknown, but one possible explanation is the gravimetric effect of mafic igneous rocks, which may be more common within the Precambrian sedimentary rocks than is indicated by intersection of these rocks in the drill hole.
Two igneous bodies were encountered during the drilling of the Michigan Basin deep drill hole. Th... more Two igneous bodies were encountered during the drilling of the Michigan Basin deep drill hole. The upper body (unit 1) extended from 4970 to 4998 m, while the lower body (unit 2) was observed from 5252 m to the bottom of the hole at 5324 m. Samples used in this investigation were taken at 1.5-m intervals in the core section from 5306 to 5322 m. In addition, one sample from the lower contact of unit I was examined. The assemblage observed in unit 2 consists of clinopyroxene-chlorite-albite-epidote-actinolitesphene, while the one sample from unit 1 contains albite-chlorite-carbonate-epidote. Both of these assemblages can be characterized as spilitic and are representative of conditions which prevail during greenschist facies metamorphism. Comparison of the compositions of the various phases coexisting in the two assemblages with published experimental studies suggests conditions of equilibration of approximately 450øC and a fluid pressure of 1-2 kbar.
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