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The Geoid and Geophysical Prospecting
Published in Petr Vaníček, Nikolaos T. Christou, GEOID and Its GEOPHYSICAL INTERPRETATIONS, 2020
Measurements of the gravity field were initially made by a pendulum or by spring gravimeters. Gravity surveys were more frequently undertaken when useful gravimeters became available in the 1930s. These instruments consisted essentially of a mass, acted upon by gravity, mounted on a spring providing the restoring force. The precision of these instruments was (even in those days) in the sub-mgal range. Gravity measurements offshore were initially made on the sea floor where a gravimeter was lowered to the sea bottom and measurements were made similar to those made on land. However, gravity measurements onboard a ship are much more problematic due to the accelerations of the ship caused by waves. Damping of the gravimeter is necessary which results in lowered precision. Additional error sources arise when measurements of the gravity field are made on a moving vessel. When measurements of the gravity field are made on moving platforms, e.g., ships and aircrafts, extraneous accelerations are encountered that have to be compensated for. These accelerations include the Eötvös correction, which adjusts for variations in the centrifugal acceleration around the Earth’s rotational axis, and horizontal and vertical accelerations due to motions of the platform. Gravity corrections applied in conventional marine gravity are further discussed by Dehlinger.3 A precision of marine gravity measurements better than 1.0 mgal can be obtained if a survey is especially designed for gravity. However, the precision of marine gravity surveys usually falls between a few and 5 mgal, because gravity in most geophysical surveys offshore is a by-product of seismic reflection profiling. Comprehensive gravity maps covering areas extending a few hundred kilometers in each lateral direction are practically always compiled on the basis of several different gravity surveys imposing problems of matching data sets of different quality and processing.
Symbols, Terminology, and Nomenclature
Published in W. M. Haynes, David R. Lide, Thomas J. Bruno, CRC Handbook of Chemistry and Physics, 2016
W. M. Haynes, David R. Lide, Thomas J. Bruno
by replacement of one or more carbon atoms of the fulvalene skeleton by a heteroatom). [5] Fulvenes - The hydrocarbon fulvene and its derivatives formed by substitution (and by extension, analogues formed by replacement of one or more carbon atoms of the fulvene skeleton by a heteroatom). [5] Fundamental vibrational frequencies* - In molecular spectroscopy, the characteristic vibrational frequencies obtained when the vibrational energy is expressed in normal coordinates. They determine the primary features of the infrared and Raman spectra of the molecule. - Name sometimes used for microgram. -rays* - Electromagnetic radiation (photons) with energy greater than about 0.1 MeV (wavelength less than about 1 pm). g-Factor of the electron* - The proportionality factor in the equation relating the magnetic moment µ of an electron to its total angular momentum quantum number J, i.e., µ = -gµBJ, where µB is the Bohr magneton. Also called Landé factor. Gal - A non-SI unit of acceleration, equal to 0.01 m/s. Also called galileo. Gallon (US) - A unit of volume equal to 3.785412 L. Gallon (UK, Imperial) - A unit of volume equal to 4.546090 L. Gauss (G) - A non-SI unit of magnetic flux density (B) equal to 10-4 T. Gaussian system of units - A hybrid system used in electromagnetic theory, which combines features of both the esu and emu systems. Gel - A colloidal system with a finite, but usually rather small, yield stress (the sheer stress at which yielding starts abruptly). [3] Genetic code* - The set of relations between each of the 64 codons of DNA and a specific amino acid (or other genetic instruction). Gibbs energy (G)* - An important function in chemical thermodynamics, defined by G = H-TS, where H is the enthalpy, S the entropy, and T the thermodynamic temperature. Sometimes called Gibbs free energy and, in older literature, simply "free energy". [2] Gibbs phase rule - The relation F = C - P + 2, where C is the number of components in a mixture, P is the number of phases, and F is the degrees of freedom, i.e., the number of intensive variables that can be changed independently without affecting the number of phases. Glass transition temperature* - The temperature at which an amorphous polymer is transformed, in a reversible way, from a viscous or rubbery condition to a hard and relatively brittle one. [10] Glow discharge mass spectroscopy (GDMS) - See Techniques for Materials Characterization, page 12-1. Gluon - A hypothetical particle postulated to take part in the binding of quarks, in analogy to the role of the photon in electromagnetic interactions. Glycerides - Esters of glycerol (propane-1,2,3-triol) with fatty acids, widely distributed in nature. They are by long-established custom subdivided into triglycerides, 1,2- or 1,3-diglycerides, and 1- or 2-monoglycerides, according to the number and positions of acyl groups. [5] Glycols - Dihydric alcohols in which two hydroxy groups are on different carbon atoms, usually but not necessarily adjacent. Also called diols. [5] Grain (gr) - A non-SI unit of mass, equal to 64.79891 mg.
A new basin depth map of the fault-bound Wellington CBD based on residual gravity anomalies
Published in New Zealand Journal of Geology and Geophysics, 2023
Gravity methods are sensitive to variations in subsurface density and when combined with basement depth constraint from boreholes or seismic methods are a useful tool for citywide determination of basin geometry, as measurements can be taken in an urban setting with minimal space or time requirements. The only gravity surveys previously undertaken for Wellington City were made in the 1960s (Cowan and Hatherton 1968; Hatherton and Sibson 1969). These surveys estimated elevations using barometers and data from Wellington City Council motorway and drainage plans, while terrain corrections were done manually with visual estimations of elevation in the field and with topographic maps and graticules using the method of Hammer (1939). Resultant total Bouguer anomalies were estimated at ±0.2 mGal in flat areas and as high as ±2 mGal in hilly areas, largely due to errors in elevation. Since the 1990s Global Positioning System (GPS) technology and Digital Elevation Models (DEMs) have allowed the calculation of Bouguer anomalies with uncertainties <0.1 mGal (Yule et al. 1998; Styles et al. 2006), but to date, this has not been done for Wellington City. Neither has a whole basin depth map for Wellington City been created from gravity data. The Hatherton and Sibson (1969) survey was used in the first basin depth map for Wellington City (Grant-Taylor et al. 1974), but only to supplement limited borehole data.
Sparsifying spherical radial basis functions based regional gravity models
Published in Journal of Spatial Science, 2022
Haipeng Yu, Guobin Chang, Shubi Zhang, Nijia Qian
From Table 2, Figure 2 to Figure 4, we have the following four observations. First, using different regularization schemes in the same test area, the optimal depth of RBFs is different. More specifically, the optimal depth of the Tikhonov-based regularization scheme is relatively deep, and Lasso is shallow. Second, prediction errors at the control points for the final solutions in noiseless areas are 0.85 mGal and 1.18 mGal, respectively. It proves the accuracy is insufficient, because we assume the 2 mGal accuracy level of input airborne gravity. In that case, the error due to the representation of the gravity field reaches more than a half of the measurement errors. So, it is important to point out that the experimental setup in this study still has a lot of room for improvement. Third, in terms of accuracy, the Tikhonov-based regularization scheme is the best. In regions with rough gravity field features, Tikhonov regularization outperforms the Lasso by 20% which is not slightly. But the latter is simpler due to its sparsity. Fourth, in summary, the proposed sparse method, in some cases, can achieve comparable accuracy to the state-of-the-art Tikhonov regularization method, but the solution is sparser. We would like to stress that this work does not aim to completely replace the traditional Tikhonov method with the proposed sparse method. Rather, in this work, a promising alternative is proposed, which is flexible to balance the complexity of the model in terms of non-zero parameters and the modelling accuracy.
The basement morphology under Tongariro National Park, southern Taupo Volcanic Zone
Published in New Zealand Journal of Geology and Geophysics, 2018
Edwin I. Robertson, Frederick J. Davey
For depth models across the andesitic volcanic cones (Figure 5) the andesite density was first estimated and the elevation of the base of the andesitic cover derived (see above). The underlying basement surface was then derived using a horizontal slab model with inclined edges for the margins and an average wet density (2.35 Mg/m3) for the depth range of the Tertiary sediments underlying the andesites. A change of 100 m in the assumed elevation of the base of the andesites produces a change in Bouguer anomaly of only 0.4 mGal. The existence of Tertiary sediments underlying the andesites is supported by a 3.0 km/s P-wave speed layer recorded by Sissons and Dibble (1981) just east of Mt Ruapehu, and by their occurrence to the south and west of the andesites. These volcanoes probably have solid cores but no stations (maximum station elevation was 1700 m above sea level) were located close enough to the peaks (Mt Ruapehu 2800 m, Mt Ngauruhoe 2290 m) to detect this. As massive andesite has a wet density of about 2.67 Mg/m3 (e.g. Whiteford and Lumb, 1975; Sissons 1981) the bulk density of 2.25 Mg/m3 corresponds to a porosity of 25%. Gravity studies of volcanic islands in the Southwest Pacific Ocean by Robertson (1967) gave similar results – olivine basalt core of 2.87 Mg/m3, with bulk island density of 2.35 Mg/m3 corresponding to a bulk porosity of 28%.