scholarly journals An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins

2021 ◽  
Vol 11 (20) ◽  
pp. 9462
Author(s):  
José Maringue ◽  
Esteban Sáez ◽  
Gonzalo Yañez
Keyword(s):  
Gravity Anomaly ◽  
Soft Soil ◽  
Soil Column ◽  
Geophysical Methods ◽  
Soil Layer ◽  
Sedimentary Basins ◽  
Soft Soils ◽  
Predominant Period ◽  
Residual Gravity ◽  
Residual Gravity Anomaly

The study of site amplification effects is crucial to assess earthquake hazards that can produce great damage in urban structures. In this context, the gravity and the ambient noise horizontal-to-vertical spectral ratio (H/V) are two of the most used geophysical methods to study the properties of the subsoil, which are essential to estimate seismic amplification. Even though these methods have been used complementarily, a correlation between them has not been thoroughly studied. Understanding this correlation and how it depends on geology could be important to use one method as an estimator of the other and to make a distinction between the seismic and gravimetric basement. In this research, a comparison between the residual gravity anomaly and the H/V predominant period is performed using a long dataset from different projects on sedimentary basins in a group of the most important cities in Chile. To simplify the geological information, a seismic classification is used for soils, which considers the Vs30 and the predominant period of vibration (T0). The results of this comparison show a direct correlation between both parameters, the higher the negative residual gravity anomaly the higher the H/V predominant period. This correlation improves when only soft soils are considered, increasing the R2 value in more than a 50% in all the individual cities with respect to the overall correlation. When all the cities are considered, the R2 value for soft soils increases up to 0.87. These results suggest that the ideal geological background for this correlation is when a soft soil layer overlies a homogeneous bedrock. Heterogeneities in the bedrock and in the soil column add dispersion to the correlation. Additionally, the comparison between the depth to basement inferred by both methods show differences of less than 15% in soft sites; in denser sites, the difference increases up to 30% and the definition of a clear H/V peak is more difficult. In general, the gravimetric basement is deeper than the seismic one. However, gravimetric depths to basement can be under/over-estimated in zones with a heterogeneous soil column.

scholarly journals RESIDUAL GRAVITY ANOMALY OF BRAZILIAN MARAJÓ BASIN USING CRUSTAL MODELING: IDENTIFYING STRUCTURAL AND TECTONIC FEATURES

2019 ◽  
Vol 37 (2) ◽  
Author(s):  
Gilberto Carneiro dos Santos Junior ◽  
Cristiano Mendel Martins ◽  
Nelson Ribeiro-Filho
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Sedimentary Basins ◽  
Data Interpretation ◽  
Earth’S Crust ◽  
Residual Gravity ◽  
North Brazil ◽  
Residual Gravity Anomaly ◽  
Earth's Crust ◽  
Tectonic Features

ABSTRACT. Dealing with gravity data at complex geological environments is a hard task because regional and residual anomalies are unknown. Due to the fact former techniques do not apply geologic information for separating gravity data, interpretation could lead to common mistakes. In order to allow a better interpretation at sedimentary basins, we applied a different approach for separating regional and residual anomalies for gravity data: the crustal modeling procedure. This approach consists on discretizing the Earth’s crust in prismatic cells and calculating the predicted signal due to Earth’s crust. We set horizontal dimensions of each prism, while the top and bottom are defined by Earth’s topography and depth of crust-mantle boundary, usually called Moho. Additionally, when the predicted signal is calculated, the residual anomaly is obtained from simple subtraction. We applied our methodology at Marajó basin (North, Brazil), where previous geological studies identified a system of faults and grabens, also known as Marajó graben system. Moreover, our results are well compared with previous interpretation through the seismic method, exemplifying the approach’s quality and efficiency. We believe, therefore, that the crustal modeling approach should be considered for studying any Brazilian sedimentary basin and other interesting areas.Keywords: crustal modeling; residual gravity anomaly; Marajó basin; Marajó graben system. RESUMO. Interpretar dados gravimétricos em ambientes geológicos de grande complexidade é uma tarefa difícil de ser realizada, visto que anomalias regionais e residuais são desconhecidas. Devido ao fato de que conhecidas técnicas de separação regional-residual não consideram informações geológicas, a interpretação final pode fornecer resultados equivocados. A fim de permitir uma melhor interpretação nas bacias sedimentares, aplicamos uma diferente abordagem para separação regional-residual: a modelagem crustal. Esta abordagem consiste em discretizar a crosta terrestre em células prismáticas e calcular o sinal regional predito. Definimos as dimensões horizontais de cada prisma, enquanto o topo e a base são definidos pela topografia e profundidade da interface crosta-manto, respectivamente. Após o cálculo do sinal predito, a anomalia residual é calculada via subtração. Aplicamos nossa metodologia na bacia do Marajó (região Norte, Brasil), onde estudos geológicos identificaram um sistema de falhas e grábens, definido por sistema de gráben do Marajó. Nossos resultados apresentam boa correspondência quando comparados com interpretações realizadas via método sísmico, o que exemplifica a qualidade e eficiência da nossa proposta. Acreditamos, portanto, que esta abordagem de modelagem crustal deve ser considerada para o estudo de qualquer bacia sedimentar brasileira e de outras regiões de interesse.Palavras-chave: modelagem crustal; anomalia gravimétrica residual; bacia do Marajó; sistema de gráben do Marajó.  

High-resolution gravity study of the Gray Fossil Site

Geophysics ◽  
2008 ◽  
Vol 73 (2) ◽  
pp. B25-B32 ◽  
Author(s):  
J. L. Whitelaw ◽  
K. Mickus ◽  
M. J. Whitelaw ◽  
J. Nave
Keyword(s):  
High Resolution ◽  
Gravity Anomaly ◽  
Drill Hole ◽  
Structural Features ◽  
Gravity Measurement ◽  
Gray Fossil Site ◽  
Residual Gravity ◽  
Residual Gravity Anomaly ◽  
Fossil Site ◽  
Anomaly Map

The Gray Fossil Site, Washington County, Tennessee, has produced a remarkable Mio-Pliocene fauna and flora with no known correlative in the Appalachian region. After its discovery in 2000, a series of auger holes were drilled by the Tennessee Department of Transportation (TDOT) to determine the areal extent of the site. Drilling indicated that the fossils occurred in fill material within a paleokarst basin, but the distribution of boreholes does not permit details of sinkhole topography, and therefore its formation and fill history, to be adequately resolved. To better image the sinkhole basin, a high-resolution gravity survey, which included 1104 gravity measurement stations, was conducted. These data were used to create complete Bouguer and residual gravity anomaly maps and a 3D density model via inversionmethods. The residual gravity anomaly map compares favorably with 29 TDOT auger holes drilled to basement, but contains significantly more detail. The residual gravity anomaly map reveals the presence of seven separate sinkholes. However, 3D inverse modeling constrained by drill-hole depths and density data indicates that there are 11 separate sinkholes formed within the Knox Group carbonates. These sinkholes, which range between 20 and [Formula: see text] in depth, are aligned along northwest and northeast trending linear features that correlate to structural features formed during the Appalachian orogenies. It is possible that the overall sinkhole basin formed as the result of partial coalescence of multiple sinkhole structures controlled by a joint system and that the sinkholes then acted as a natural trap for the Gray Fossil Site fauna and flora.

Regional‐residual gravity anomaly separation using the minimum‐curvature technique

Geophysics ◽  
1991 ◽  
Vol 56 (2) ◽  
pp. 279-283 ◽  
Author(s):  
K. L. Mickus ◽  
C. L. V. Aiken ◽  
W. D. Kennedy
Keyword(s):  
Gravity Anomaly ◽  
Gravity Anomalies ◽  
Bouguer Gravity ◽  
Bouguer Gravity Anomaly ◽  
Residual Gravity ◽  
Residual Gravity Anomaly ◽  
Gravity Interpretation ◽  
Minimum Curvature

One of the most difficult problems in gravity interpretation is the separation of regional and residual gravity anomalies from the Bouguer gravity anomaly. This study discusses the application of the minimum‐curvature method to determine the regional and residual gravity anomalies.

scholarly journals Initial exploration of Hydrocarbon resources by Gravity Data: A Case Study in the South of Qom Province, Iran

2015 ◽  
Vol 2 (1) ◽  
pp. 52-57
Author(s):  
Payam Salimi
Keyword(s):  
Oil And Gas ◽  
Gravity Data ◽  
Geophysical Methods ◽  
Local Effect ◽  
Oil And Gas Exploration ◽  
Residual Gravity ◽  
3D Geometry ◽  
Residual Gravity Anomaly ◽  
Matlab Programming

Geophysical methods widely used in oil and gas exploration. Modeling of gravity data is used extensively to illustrate the geometry and interface between the sediments and bedrock. Which can help the salt dome, anticline folds, dome-shaped uplift of the continental platform and reef masses to be identified. There are various methods to illustrate the bedrock topography, and we will describe one of these methods in present paper. Using the upward continuation, we extract the residual gravity anomaly which in fact shows the local effect of bedrock gravity on the observed gravity. Then, according to the Oldenburg - Parker method, the residual gravity data are inversed and finally the 3D geometry the bedrock is illustrated. It should be noted that some software's like Surfer and Excel are used in this research but the program main code is written using Matlab programming.

scholarly journals A gravity analysis of the Alpine Fault and the DFDP-2 drill site, Whataroa valley, South Westland, South Island, New Zealand

2021 ◽  
Author(s):  
◽  
Stephen Jenkins
Keyword(s):  
New Zealand ◽  
Gravity Anomaly ◽  
High Precision ◽  
River Valley ◽  
Gravity Models ◽  
Sediment Thickness ◽  
Residual Gravity ◽  
Alpine Fault ◽  
Drill Site ◽  
Residual Gravity Anomaly

<p>The second phase of drilling into the Alpine Fault (DFDP-2), was completed in the Whataroa River valley, a former glacial valley located in central Westland, South Island, New Zealand. The site is located next to a steep hillside on the hanging-wall, ~1 km southeast of the mapped surface trace of the Alpine Fault. Projection of the hillside suggests a sediment thickness of 100 ± 40 m at the drill site; however, the sediment thickness was approximately double pre-drill estimates. Additionally, the surface expression and shallow geometry of the Alpine Fault in the Whataroa River valley, is not well-defined due to post-glacial burial of the fault zone. This thesis describes a gravity study designed to better constrain sub-surface structure beneath the DFDP-2 drill site and across the Alpine Fault.  During this study, 466 new high-precision gravity observations were collected (standard error = 0.015 mGal) and amalgamated with 134 existing gravity stations, yielding comprehensive coverage of gravity data across the study area. A high density of observations was achieved within pre-determined zones, in addition to regional measurements so that residual gravity anomaly maps could be produced. The maps reveal: a negative residual gravity anomaly interpreted as a dextrally-offset glacial channel at least 350-450 m deep; steep localised gravity gradients near the Alpine Fault and DFDP-2 drill site that are interpreted as faulted and/or eroded boundaries; and a negative gravity anomaly adjacent to the DFDP-2 drill site that is interpreted as the deepest point of an over-deepened glacial lake.  Gravity models were used to estimate the bedrock-sediment interface geometry near the DFDP-2 drill site and Alpine Fault. Structural inversion of the density boundary next to the drill site suggests either a moderately-dipping reverse fault or sub-vertical erosional wall exists beneath the hillside. Additional constraints on physical properties from direct density measurements or seismic velocity determinations and direct constraints on sediment thickness and layer geometry from seismic surveys will in future allow this new high-precision gravity dataset to be modelled more effectively.</p>

scholarly journals A gravity analysis of the Alpine Fault and the DFDP-2 drill site, Whataroa valley, South Westland, South Island, New Zealand

2021 ◽  
Author(s):  
◽  
Stephen Jenkins
Keyword(s):  
New Zealand ◽  
Gravity Anomaly ◽  
High Precision ◽  
River Valley ◽  
Gravity Models ◽  
Sediment Thickness ◽  
Residual Gravity ◽  
Alpine Fault ◽  
Drill Site ◽  
Residual Gravity Anomaly

<p>The second phase of drilling into the Alpine Fault (DFDP-2), was completed in the Whataroa River valley, a former glacial valley located in central Westland, South Island, New Zealand. The site is located next to a steep hillside on the hanging-wall, ~1 km southeast of the mapped surface trace of the Alpine Fault. Projection of the hillside suggests a sediment thickness of 100 ± 40 m at the drill site; however, the sediment thickness was approximately double pre-drill estimates. Additionally, the surface expression and shallow geometry of the Alpine Fault in the Whataroa River valley, is not well-defined due to post-glacial burial of the fault zone. This thesis describes a gravity study designed to better constrain sub-surface structure beneath the DFDP-2 drill site and across the Alpine Fault.  During this study, 466 new high-precision gravity observations were collected (standard error = 0.015 mGal) and amalgamated with 134 existing gravity stations, yielding comprehensive coverage of gravity data across the study area. A high density of observations was achieved within pre-determined zones, in addition to regional measurements so that residual gravity anomaly maps could be produced. The maps reveal: a negative residual gravity anomaly interpreted as a dextrally-offset glacial channel at least 350-450 m deep; steep localised gravity gradients near the Alpine Fault and DFDP-2 drill site that are interpreted as faulted and/or eroded boundaries; and a negative gravity anomaly adjacent to the DFDP-2 drill site that is interpreted as the deepest point of an over-deepened glacial lake.  Gravity models were used to estimate the bedrock-sediment interface geometry near the DFDP-2 drill site and Alpine Fault. Structural inversion of the density boundary next to the drill site suggests either a moderately-dipping reverse fault or sub-vertical erosional wall exists beneath the hillside. Additional constraints on physical properties from direct density measurements or seismic velocity determinations and direct constraints on sediment thickness and layer geometry from seismic surveys will in future allow this new high-precision gravity dataset to be modelled more effectively.</p>

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