Distribution Of Gravity Anomalies Research Articles

Identifikasi Sesar dengan Menggunakan Metode Gaya Berat (Studi Kasus: Sulawesi Tengah)

The study of the geological structure of Sulawesi Island, especially the Central Sulawesi region, is very interesting because it has high complexity as a result of the dynamic interactions of the world's three main plates. Indications of high seismic activity and deformation processes in the Central Sulawesi region can be found from the morphological appearance and activity of several existing faults such as the Palu-Koro fault, the Lawanopo fault, and the Matano fault. The data used in this study are secondary data measuring the distribution of gravitational field anomalies (FAA) and topographical data using the Topex application. At CBA values ranging from 31 mGal to 56 mGal. The height anomaly on the CBA contour map is shown in red to light purple which has a value range of 40.7 mGal to 62.7mGal. Low anomaly is shown in light green to blue which has a value range of 27.3 mGal to 3.2 mGal. The regional gravity anomaly values range from 5.1 mGal to 62.8 mGal with a distribution of gravity anomalies from south to north. While in this study it can only be explained that the residual anomaly value is influenced by rock type and density. The high anomaly found in the southern part is thought to be caused by rocks having high density and being in a sedimentary rock environment which has low density.

A power law distribution of gravity anomaly values in cyclone-affected zones

This article considers the distributions in values of the vertical component of gravity anomalies in cycloneaffected regions of the northwestern Pacific and northwestern Atlantic oceans. It is shown that the density of the distribution of gravity anomalies and the respective cumulative distributions are characterized by a relatively strong negative power law dependence. The studied region of the northwestern Pacific is limited by the coordinates 0°-60° N, 108° E-150° W. It covers the whole zone where cyclones moving towards the American coasts originate, and also includes small land parts of eastern China and Russia. The values of the gravity anomaly (GA, δg) are deter� mined uniformly over the studied area. The interval between adjacent points where GA values were mea� sured is 0.08° both in latitude and longitude. The total number of δg values is 959 027, including 48.4% of negative ones. Note that many sequential sets of δg values are, in fact, typical examples of a random walk around zero. The especially high frequency of zero crossing is observed in the zones of tectonic plate boundaries; the highest gradients of GA are also reported here. Figure 1 presents the character of the distribution of gravity anomalies for the studied region. Hereinaf� ter, only absolute GA values are considered. The dis� tribution is given in regular and logarithmic forms. The entire set of δg values is distributed by classes in accordance with the Sturges rule (1): k ≈ 3.32 + 1, where k is the number of classes and N is the total number of δg values. In the considered case, extent of every class is 30 mGal. It was found in calculations that some marginal classes incorporating the greatest δg values distort the general regularity of GA distribution, and this is seen especially clearly in the logarithmic representation. This distortion was observed more or less to some N log degree for all versions of the calculated GA distribu� tion. The cause is that these classes contain very small numbers of δg values, and the relative fluctuations of their numbers also abruptly increase. Such a situation is typical for distributions of different natures but fol� low a relatively high power law (for example, distribu�

Tar sandstones in the Paraná Basin of Brazil: structural and magmatic controls of hydrocarbon charge

There are tar accumulations in Lower Triassic sandstones along the eastern margin of the Paraná Basin. The tar was derived from Permian source rocks that matured along the basic sills of the Serra Geral continental flood basalt cover. This paper exams the habitat of tar accumulation, integrating fieldwork with morphostructural data, digital elevation modelling, gravity and airborne magnetic data, remote sensing, and radiometric dates that constrain the timing of maturation and migration. The focus of this work is the Jacu structural complex, a large annular drainage anomaly, 20 km in diameter, that owes its origin to reactivation of pre-existing faults. Interpretation of the structural framework is based primarily on morphostructural analysis and the distribution of gravity anomalies. The Jacu structure consists of a central low-lying fault block that is surrounded by several structural highs, and is attributed to the interaction of NE- and NW-striking fault zones. The principal basement faults that encapsulate this structure were invaded by the dolerite dykes that fed the widespread flood basalt cover during the Early Cretaceous. An 40Ar/ 39Ar analysis directly dates this episode of magmatism to 134±2 Ma. Tar sandstones are restricted to the lower Pirambóia Formation and are present around the Jacu structure, especially near the structural highs or close to basic dikes. Morphostructural interpretation suggests the occurrence of tar sandstones at intermediate positions too, between the structural highs and lows. The most likely explanation for these tar accumulations relates oil maturation and migration to the intrusion of Lower Cretaceous Serra Geral sills into the Upper Permian black shales of the Irati Formation, and subsequent migration along faults and dyke walls into eolian sandstone reservoirs of the Triassic Pirambóia Formation. Not only did the basic dykes locally facilitate migration, but there were also places where they were barriers to lateral migration. Furthermore, the interdune and fluvial facies of the Pirambóia Formation formed local seals and barriers to vertical migration.

Convective support of long‐wavelength topography in the Apennines (Italy)

Two different free‐air gravity datasets from the Apennines have been analysed in the frequency domain to investigate the mechanisms of support of topography. At short wavelength (λ<150 km) the best fit between the observed and the calculated admittance gives a value of the elastic thickness of 3.7 km. The observed behaviour of the admittance at larger wavelengths (λ>150 km) shows that such topography is not supported by elastic stresses in the lithosphere but is dynamically supported by mantle convection. The distribution of gravity anomalies and topography suggests that the mantle is rising beneath the main topographic bulge of the Apennines and that this flow is maintained by buoyancy forces, rather than being driven by movement of the subducted slab.

A note on the errors involved in the calculation of elevations of the geoid

The paper discusses the errors likely to occur in calculations of the elevation of the geoid by Stokes’s formula when gravity data outside a given cap are neglected, and it is shown that they are comparable with the root-mean-square elevation. It is shown that these estimates and those of similar errors in deflexions of the vertical made earlier (Cook 1950) are not seriously affected by the assumptions made about the distribution of gravity anomalies between the harmonics up to degree 18.