Gravity anomalies of three-dimensional bodies with a variable density contrast

1989 ◽  
Vol 130 (4) ◽  
pp. 711-719 ◽  
Author(s):  
I. V. Radhakrishna Murthy ◽  
P. Rama Rao ◽  
P. Ramakrishna
Keyword(s):  
Three Dimensional ◽  
Gravity Anomalies ◽  
Variable Density ◽  
Density Contrast

Three‐dimensional Fourier gravity inversion with arbitrary density contrast

Geophysics ◽  
1992 ◽  
Vol 57 (1) ◽  
pp. 131-135 ◽  
Author(s):  
F. Guspí
Keyword(s):  
Frequency Domain ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Variable Density ◽  
Density Contrast ◽  
Gravity Inversion ◽  
Linear Quadratic ◽  
Space Domain ◽  
Depth Functions

The use of variable‐density contrasts in gravity inversion has gained increasing importance in recent years due to the necessity of constructing more realistic models of geophysical structures such as sedimentary basins. Linear, quadratic, and exponential variations, either in the space or in the frequency domain, are the basis of several methods. See, among others, the papers by Granser (1987), Chai and Hinze (1988), Reamer and Ferguson (1989), and Rao et al. (1990). Guspí (1990) used polynomial density‐depth functions for inverting gravity anomalies into 2-D polygons in the space domain.

Three-dimensional gravity interpretations of the Round Lake batholith, northeastern Ontario

1970 ◽  
Vol 7 (1) ◽  
pp. 156-163 ◽  
Author(s):  
R. A. Gibb ◽  
J. van Boeckel
Keyword(s):  
Volcanic Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Density Contrast ◽  
Upper Crust ◽  
Normal Faulting ◽  
Maximum Thickness ◽  
Dimensional Gravity ◽  
Facies Change ◽  
Basic Volcanic

Gravity surveys of the Timmins–Senneterre mining belt of northeastern Ontario and western Quebec were made by the Dominion Observatory during the period 1946–1964. The Round Lake batholith is one of several composite granitic plutons of the Algoman series which are outlined by intense negative gravity anomalies. The anomaly over the Round Lake batholith can be explained by the large density contrast (0.22 g/cm3) between the granite and surrounding Keewatin volcanic rocks.Two possible models of the batholith are presented which depend on different assumptions as to the composition of the upper crust. The first model involves normal faulting of the batholith to explain the variations in anomaly level within the batholith. In this model the granite is assumed to be homogeneous in density and extends to a maximum depth of 10 km. Alternatively density variations corresponding to a facies change within the pluton may be the major cause of the local internal anomaly variations. In this interpretation the true thickness of the granite cannot be evaluated as the whole region is assumed to be underlain by granite, but the maximum thickness of the surrounding basic volcanic rocks is 5 km.

Gravity anomalies of 2-D bodies with variable density contrast

Geophysics ◽  
2001 ◽  
Vol 66 (3) ◽  
pp. 809-813 ◽  
Author(s):  
Jianzhong Zhang ◽  
Benshan Zhong ◽  
Xixiang Zhou ◽  
Yun Dai
Keyword(s):  
Cross Section ◽  
Northeast China ◽  
Polynomial Function ◽  
Gravity Anomalies ◽  
Lateral Position ◽  
Variable Density ◽  
New Method ◽  
Density Contrast ◽  
Liaohe Basin ◽  
The Cross

A new method is presented to compute gravity anomalies that result from 2-D bodies with variable density contrast. The cross‐section of a body is approximated by a polygon. Density is assumed to vary as any order of polynomial function with depth and lateral position. Results calculated by the proposed method for models with variable density contrast compare well with other methods. Liaohe basin, northeast China, is modeled from field gravity anomalies using the formulas given, showing the method is valid and effective.

A versatile solution for the gravity anomaly of 3D prism-meshed bodies with depth-dependent density contrast

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. G77-G86 ◽  
Author(s):  
Li Jiang ◽  
Jianzhong Zhang ◽  
Zhibing Feng
Keyword(s):  
Los Angeles ◽  
Half Space ◽  
Numerical Stability ◽  
Analytic Solution ◽  
Gravity Anomalies ◽  
Variable Density ◽  
Density Contrast ◽  
Contrast Variation ◽  
Stability Range ◽  
Limit Values

We have developed a generalized solution for computing the gravity anomalies of 3D irregular-mass bodies with complicated density-contrast variation. The 3D irregular-shaped bodies can be approximated flexibly by a collection of finite-juxtaposed right-rectangular prisms. The complicated density-contrast variation of each prism can be well-represented by a depth-dependent polynomial function. A novel analytic solution of gravity anomalies due to a right-rectangular prism with an arbitrary order of polynomial density-contrast function of depth is then derived. The solution is singularity free in the upper half-space over the prism, and its singularity in the lower half-space containing the prism is resolved by assigning their limit values to the singular terms. The numerical stability of the solution is also evaluated through numerical tests. Hence, the solution can be used to compute the gravity anomalies of 3D irregular bodies with variable density contrasts without singularities when computation points are within the numerical stability range. Based on synthetic models with variable density contrast, our solution is validated by using other solutions in the literature. We also simulated the gravity anomalies of the Los Angeles basin and compared them with the observed anomalies and with those computed using the analytic solutions of other workers. These tests confirm the accuracy and efficiency of our analytic solution.

scholarly journals Basin Depth Mapping Beneath Wellington City Based on Residual Gravity Anomalies

2021 ◽  
Author(s):  
◽  
Alistair Stronach
Keyword(s):  
Gravity Anomaly ◽  
Sedimentary Basin ◽  
Gravity Data ◽  
Three Dimensional ◽  
Maximum Amplitude ◽  
Central Business District ◽  
Gravity Anomalies ◽  
Depth Map ◽  
Capital City ◽  
Detailed Knowledge

<p><b>New Zealand’s capital city of Wellington lies in an area of high seismic risk, which is further increased by the sedimentary basin beneath the Central Business District (CBD). Ground motion data and damage patterns from the 2013 Cook Strait and 2016 Kaikōura earthquakes indicate that two- and three-dimensional amplification effects due to the Wellington sedimentary basin may be significant. These effects are not currently accounted for in the New Zealand Building Code. In order for this to be done, three-dimensional simulations of earthquake shaking need to be undertaken, which requires detailed knowledge of basin geometry. This is currently lacking, primarily because of a dearth of deep boreholes in the CBD area, particularly in Thorndon and Pipitea where sediment depths are estimated to be greatest.</b></p> <p>A new basin depth map for the Wellington CBD has been created by conducting a gravity survey using a modern Scintrex CG-6 gravity meter. Across the study area, 519 new high precision gravity measurements were made and a residual anomaly map created, showing a maximum amplitude anomaly of -6.2 mGal with uncertainties better than ±0.1 mGal. Thirteen two-dimensional geological profiles were modelled to fit the anomalies, then combined with existing borehole constraints to construct the basin depth map. </p> <p>Results indicate on average greater depths than in existing models, particularly in Pipitea where depths are interpreted to be as great as 450 m, a difference of 250 m. Within 1 km of shore depths are interpreted to increase further, to 600 m. The recently discovered basin bounding Aotea Fault is resolved in the gravity data, where the basement is offset by up to 13 m, gravity anomaly gradients up to 8 mGal/km are observed, and possible multiple fault strands identified. A secondary strand of the Wellington Fault is also identified in the north of Pipitea, where gravity anomaly gradients up to 18 mGal/km are observed.</p>

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