scholarly journals A new basin depth map of the fault-bound Wellington CBD based on residual gravity anomalies

2021 ◽  
pp. 1-15
Author(s):  
Alistair Stronach ◽  
Tim Stern
Keyword(s):  
Gravity Anomalies ◽  
Depth Map ◽  
Residual Gravity

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>

Iberian Crust Analyzed by, Residual Gravity Anomalies - Moho Prediction and Isostasy

1997 ◽  
Author(s):  
G. Stapel ◽  
J. Verhoef ◽  
H. Kooi ◽  
S. Cloetingh
Keyword(s):  
Gravity Anomalies ◽  
Residual Gravity

A Hybrid PCG- Bat Algorithm for 2D Gravity Inversion: Applications for Ore Deposits Exploration and Interpretation of Sedimentary Basins

2020 ◽  
Author(s):  
Mohamed Abdrabou ◽  
Maha Abdelazeem ◽  
Mohamed Gobashy
Keyword(s):  
Conjugate Gradient ◽  
Hybrid Algorithm ◽  
2D Gravity ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Bat Algorithm ◽  
Ore Deposits ◽  
Preconditioned Conjugate Gradient ◽  
Residual Gravity

&lt;p&gt;Geophysical data such as gravity data can be inverted to get a subsurface image, which depicts the subsurface distribution of physical property. Consequently, inversion of geophysical data has an effective role for interpreting measured geophysical anomalies in hydrocarbons and mineral applications. Interest about ore deposits exploration and sedimentary basins interpretation is associated with their economic importance. The presence of sedimentary basins gives lower amplitude of gravity anomalies with negative signals, due to the negative density contrast as these sedimentary basins have lower density than that of the neighboring basement rocks. In prospecting ore deposits, studying the spatial distributions of densities in the subsurface is essential of significance.Two dimensional forward modelling strategy can be done via locating the rectangular cells with fixed size directly underneath the location of the observed data points using regular grid discretization. Density vector of the subsurface rectangular cells are obtained via solving the 2D gravity inverse problem by optimizing an objective function (i.e., the differences between observed and inverted residual gravity data sets). In this work, a hybrid algorithm merging a bat (BAT) algorithm with the preconditioned conjugate gradient (PCG) method is suggested as a mean for inverting surface gravity anomalies to obtain the density distribution in the subsurface. Like the hybrid, minimization algorithm has the capability to make use of the advantages of both two techniques. In this hybrid algorithm, the BAT algorithm was utilized to construct an initial solution for the PCG technique. The BAT optimizer acts as a rapid build-up of the model, whereas the second modifies the finer model approximated solution. This modern algorithm was firstly applied on a free-noise synthetic data and to a noisy data with three different levels of random noise, and good results obtained through the inversion. The validity and applicability of our algorithm are applied to real residual gravity anomalies across the San Jacinto graben in southern California, USA, and Sierra Mayor - Sierra Pinta graben, USA and prospecting of the Poshi Cu-Ni deposits, Xinjiang, northwest China. The obtained results are in excellent accordance with those produced by researchers in the published literature.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Keywords: &lt;/strong&gt;Gravity data, 2D Inversion, BAT algorithm, Preconditioned Conjugate Gradient, Sedimentary Basins.&lt;/p&gt;

CHART TO CHECK ELEVATION FACTOR EFFECTS ON GRAVITY ANOMALIES

Geophysics ◽  
1957 ◽  
Vol 22 (3) ◽  
pp. 643-645 ◽  
Author(s):  
L. F. Ivanhoe
Keyword(s):  
Basic Assumption ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Topographic Feature ◽  
Residual Gravity ◽  
Factor Effect ◽  
Surface Geology ◽  
Topographical Relief

The effect of varying surface densities on gravity anomalies is a more common problem in areas of topographical relief than is generally recognized. There is an unjustified tendency to assume that gravity maps are unique and final, even though one basic assumption (density of surface rocks) is inherent in all gravity maps. The use of incorrect elevation factors will produce gravity anomalies over any topographic feature. Both positive and negative gravity anomalies can be produced by either a topographic hill or valley depending on the degree of error in the elevation factor. These “elevation factor anomalies” are especially troublesome on residual gravity maps. The interpretation of gravity data should always include an analysis of the elevation factor effect as well as a study of the surface geology.

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