Seismic and Gravity Investigations of the Central Volcanic Region, North Island, New Zealand

<p>Gravity and seismic refraction studies were undertaken in order to investigate the geological structure of the Central Volcanic Region. A detailed analysis of density determinations from bore-hole rock samples, three seismic refraction surveys and a spectral analysis of the magnetic anomaly field are described. Interpretation of the observed gravity anomaly fie ld for the Central Volcanic Region is initially undertaken by analytically separating the observed anomaly field into its regional and residual components; the almost entirely negative residual anomaly field is then interpreted in terms of varying thicknesses of near-surface, low-density volcanic rocks. At Mangakino and just west of Taupo, however, it is found that the calculated gravity anomaly effect of the seismically determined thickness of low-velocity, and hence low-density, volcanic rocks is less negative than the observed residuals; at both locations "secondary residuals" of about -200 μN/kg remain unexplained. Models are presented that account for these secondary residuals as being due to discrete volumes of low-density molten rhyolite emplaced within the seismic basement. The second method of gravity interpretation used in this study involves modelling all components of the observed gravity anomaly field . This necessitated giving consideration to both the gravity effect of the subducted Pacific plate and to seismic data bearing upon the variation of crustal thickness and mantle density throughout the central North Island. A gravity model for the central North Island is developed for which the important features are: i) The crust of the Central Volcanic Region is deduced to be only about half the normal continental thickness, and underlying the crust is an "anomalous", low-density upper mantle. This finding from the gravity model is supported by the results of a previous study of upper mantle seismic velocities and from the interpretation of a longrange seismic refraction survey carried out within the Region. These seismic data indicate the depth to, and the velocity of the upper mantle beneath the Region to be 15 km and 7.4 km/s respectively. ii) The positive gravity anomalies that predominate over the western and northwestern North Island can largely be explained by gravity edge-effects associated with variations in the crustal thickness and mantle density within the back-arc areas of the North Island. The gravity model is interpreted as lending support for a previously made proposal that the Region is the site of asymmetric back-arc spreading, and that the crustal rocks now being created are transitional in character between typical oceanic and typical continental.</p>

Seismic and Gravity Investigations of the Central Volcanic Region, North Island, New Zealand

<p>Gravity and seismic refraction studies were undertaken in order to investigate the geological structure of the Central Volcanic Region. A detailed analysis of density determinations from bore-hole rock samples, three seismic refraction surveys and a spectral analysis of the magnetic anomaly field are described. Interpretation of the observed gravity anomaly fie ld for the Central Volcanic Region is initially undertaken by analytically separating the observed anomaly field into its regional and residual components; the almost entirely negative residual anomaly field is then interpreted in terms of varying thicknesses of near-surface, low-density volcanic rocks. At Mangakino and just west of Taupo, however, it is found that the calculated gravity anomaly effect of the seismically determined thickness of low-velocity, and hence low-density, volcanic rocks is less negative than the observed residuals; at both locations "secondary residuals" of about -200 μN/kg remain unexplained. Models are presented that account for these secondary residuals as being due to discrete volumes of low-density molten rhyolite emplaced within the seismic basement. The second method of gravity interpretation used in this study involves modelling all components of the observed gravity anomaly field . This necessitated giving consideration to both the gravity effect of the subducted Pacific plate and to seismic data bearing upon the variation of crustal thickness and mantle density throughout the central North Island. A gravity model for the central North Island is developed for which the important features are: i) The crust of the Central Volcanic Region is deduced to be only about half the normal continental thickness, and underlying the crust is an "anomalous", low-density upper mantle. This finding from the gravity model is supported by the results of a previous study of upper mantle seismic velocities and from the interpretation of a longrange seismic refraction survey carried out within the Region. These seismic data indicate the depth to, and the velocity of the upper mantle beneath the Region to be 15 km and 7.4 km/s respectively. ii) The positive gravity anomalies that predominate over the western and northwestern North Island can largely be explained by gravity edge-effects associated with variations in the crustal thickness and mantle density within the back-arc areas of the North Island. The gravity model is interpreted as lending support for a previously made proposal that the Region is the site of asymmetric back-arc spreading, and that the crustal rocks now being created are transitional in character between typical oceanic and typical continental.</p>

A Calculation Method of Magnetic Anomaly Field Distribution of Ellipsoid with Arbitrary Posture

The ellipsoidal magnetization model has a wide range of application scenarios. For example, in aviation magnetic field prospecting, mineral prospecting, seabed prospecting, and UXO (unexploded ordnance) detection. However, because the existing ellipsoid magnetization formula is relatively complicated, the detection model is usually replaced by a dipole. Such a model increases the error probability and poses a significant challenge for subsequent imaging and pattern recognition. Based on the distribution of ellipsoid gravity potential and magnetic potential, the magnetic anomaly field distribution equation generated by the ellipsoid is deduced by changing the aspect ratio, making the ellipsoid equivalent to a sphere. The result of formula derivation shows that the two magnetic anomaly fields are consistent. This paper uses COMSOL finite element software to model UXO, ellipsoids, and spheres and analyzes magnetic anomalies. The conclusion shows that the ellipsoid model can completely replace the UXO model when the error range of 1nT is satisfied. Finally, we established two sets of ellipsoids and calculated the magnetic anomalous field distributions on different planes using deduction formulas and finite element software. We compared the experimental results and found that the relative error of the two sets of data was within [Formula: see text]‰. Error analysis found that the error distribution is standardized and conforms to the normal distribution. The above mathematical analysis and finite element simulation prove that the calculation method is simple and reliable and provides a magnetic field distribution equation for subsequent UXO inversion.

Variations of Seasonal Precipitation in the Yellow River Basin and Its Relationship to General Circulation and SST

Based on the precipitation data from 1961 to 2017 in the Yellow River Basin and NCEP/NCAR reanalysis data, the variation characteristics of seasonal precipitation in the Yellow River Basin and its relationship to the circulation patterns were analyzed. Furthermore, effect of sea surface temperature (SST) on the seasonal precipitation was discussed. The results were as follows: (1) The precipitation in spring, summer and autumn all presented non-significant decreasing trends, while that in winter presented a non-significant upward trend. (2) The inter-annual and inter-decadal fluctuations of the precipitation of each season were evident. The fluctuation in winter was the most obvious, followed by that in spring and autumn, with that in summer most stable. Moreover, the precipitation in summer, autumn and winter in 1990s was the least. (3) When the precipitation in spring, autumn and winter was significantly above (below) normal, the 500 hPa height anomaly field over Eurasia mid-high latitude of the corresponding period presented “positive (negative) in east and negative (positive) in west”. While, when the precipitation in summer was significantly above (below) normal, the height anomaly field presented “+(-), -(+), +(-)”, and the West Pacific subtropical high was stronger (weaker) and lying more northward (southward). Additionally, when the precipitation in each season was significantly above (below) normal, the 850 hPa wind anomalies showed abnormal southerly (northerly) winds in Eastern China, which was favorable (unfavorable) to transport water vapor from the South China Sea and the Western North Pacific to the Yellow River Basin. (4) The spring precipitation was above (below) normal during the period of El Niño (La Niña), while the summer, autumn and winter precipitation presented the opposite characteristics.

Computation of Density Perturbation and Energy Flux of Internal Waves from Experimental Data

We hereby present two different spectral methods for calculating the density anomaly and the vertical energy flux from synthetic Schlieren data, for a periodic field of linear internal waves (IW) in a density-stratified fluid with a uniform buoyancy frequency. The two approaches operate under different assumptions. The first method (hereafter Mxzt) relies on the assumption of a perfectly periodic IW field in the three dimensions (x, z, t), whereas the second method (hereafter MxtUp) assumes that the IW field is periodic in x and t and composed solely of wave components with downward phase velocity. The two methods have been applied to synthetic Schlieren data collected in the CNRM large stratified water flume. Both methods succeed in reconstructing the density anomaly field. We identify and quantify the source of errors of both methods. A new method mixing the two approaches and combining their respective advantages is then proposed for the upward energy flux. The work presented in this article opens new perspectives for density and energy flux estimates from laboratory experiments data.