Enhanced gravity and magnetic grids for Northern Australia

Geoscience Australia (GA) generated a series of gravity and magnetic grids and enhancements covering Northern Australia. Several derivative gravity datasets have been generated 1) for the North-West Shield Western Australia region (approximately between latitudes 7‒26⁰ S and longitudes 110‒130⁰ E), 2) for the Northern Territory (approximately between latitudes 7‒26⁰ S and longitudes 125.5‒141⁰ E) and for Queensland (approximately between latitudes 7‒30⁰ S and longitudes 135‒160⁰ E). The magnetic dataset has been generated only for the North-West Shield Western Australia region (approximately between latitudes 7‒26⁰ S and longitudes 110‒130⁰ E). The magnetic and gravity data were downloaded from the Geophysical Archive Data Delivery System (GADDS), website (http://www.geoscience.gov.au/cgi-bin/mapserv?map=/nas/web/ops/prod/apps/mapserver/gadds/wms_map/gadds.map&mode=browse). Satellite Free-air (FA) gravity v27.1 (released March 11, 2019) and Satellite Topography v19.1 (released January 14, 2019) data were sourced from Sandwell et al. (2014) and downloaded from the Scripps Institution of Oceanography (SIO), National Oceanic and Atmospheric Administration (NOAA), U.S. Navy and National Geospatial-Intelligence Agency (NGA) (SIO Satellite Geodesy, website, http://topex.ucsd.edu/WWW_html/mar_grav.html). The Satellite Bouguer gravity grid with onshore correction density of 2.67 gcm-3 and offshore correction density of 2.20 gcm-3 was derived from the Free-air gravity v27.1 and Topography data V19.1. This Bouguer gravity grid was used for filling areas of data gaps in the offshore region. Data evaluation and processing of gravity and magnetic data available in the area of interest resulted in the production of stitched onshore-offshore Bouguer gravity grid derived from offshore satellite Bouguer gravity grid and GA’s onshore ground and airborne gravity survey data and a stitched Total Magnetic Intensity (TMI) grid derived from airborne and shipborne surveys (Tables 1 and 5). A Reduction to the Pole (RTP) grid was derived from the stitched TMI grid. The TMI, RTP, FA and terrain corrected Bouguer gravity anomalies are standard datasets for geological analysis. The free-air gravity anomaly provides the raw and basic gravity information. Images of free-air gravity are useful for first-pass interpretation and the data is used for gravity modelling. Magnetic anomalies provide information on numerous magnetic sources, including deep sources as arising from the structure and composition of magnetic basement and shallow sources such as intra-sedimentary magnetic units (e.g. volcanics, intrusions, and magnetic sedimentary layers). A standard TMI image will contain information from all these sources. Geosoft Oasis montaj software was used throughout the data processing and enhancement procedure and the montaj GridKnit module was used to generate the stitched gravity and magnetic grids. Enhancement techniques have been applied to the final processed Bouguer gravity and RTP magnetic grids to highlight subtle features from various sources and to separate anomalies from different source depths. These enhancement techniques are described in the next section. Enhancement processing techniques and results A summary of image processing techniques used to achieve various outcomes is described in Table 1. Data type Filter applied Enhancement/outcome Gravity/Magnetic First vertical derivative (1VD) Near surface features (e.g. intrabasinal) Gravity/Magnetic Upward continuation Noise reduction in data Gravity/Magnetic Low pass filter, or large distance upward continuation Enhancement of deep features (e.g. basement) Gravity/Magnetic High pass filter Enhancement of shallow features (e.g. surface anomalies) Gravity/Magnetic Tilt filter and 1VD Enhancement of structure (e.g. in basement) Gravity/Magnetic ZS-Edgezone and ZS-Edge filters Enhancement of edges Gravity/Magnetic horizontal modulus / horizontal gradient Enhancement of boundaries Magnetic RTP (reduction to the pole), Compound Anomaly, and Analytic Signal filter Accurate location of sources

澳大利亚地球科学局（GA）生成了一系列覆盖澳大利亚北部的重力与磁法网格及增强处理数据。已针对以下区域生成若干衍生重力数据集：1）澳大利亚西北部地盾区（大致位于南纬7‒26°、东经110‒130°之间）；2）北领地（大致位于南纬7‒26°、东经125.5‒141°之间）；3）昆士兰州（大致位于南纬7‒30°、东经135‒160°之间）。磁法数据集仅针对澳大利亚西北部地盾区（大致位于南纬7‒26°、东经110‒130°之间）生成。

重力与磁法数据下载自地球物理档案数据交付系统（GADDS）网站（http://www.geoscience.gov.au/cgi-bin/mapserv?map=/nas/web/ops/prod/apps/mapserver/gadds/wms_map/gadds.map&mode=browse）。卫星自由空气重力（FA）v27.1版本（2019年3月11日发布）及卫星地形v19.1版本（2019年1月14日发布）数据源自Sandwell等人（2014），并下载自斯克里普斯海洋研究所（SIO）、美国国家海洋和大气管理局（NOAA）、美国海军及美国国家地理空间情报局（NGA）（SIO卫星大地测量学网站：http://topex.ucsd.edu/WWW_html/mar_grav.html）。

陆地区域校正密度为2.67 gcm⁻³、海洋区域校正密度为2.20 gcm⁻³的卫星布格重力网格，由自由空气重力v27.1版本及地形数据v19.1版本推导而来。该布格重力网格用于填补海洋区域的数据空白。通过对目标区域现有重力与磁法数据的评估与处理，最终生成了两类拼接网格：一类是基于海洋卫星布格重力网格与GA陆地区域地面及航空重力调查数据的陆-海拼接布格重力网格；另一类是基于航空与船载调查数据的总磁强度（TMI）拼接网格（表1和表5）。由TMI拼接网格进一步推导得到化极（RTP）网格。

总磁强度（TMI）、化极（RTP）、自由空气（FA）及地形校正布格重力异常是地质分析的标准数据集。自由空气重力异常提供原始基础重力信息，其图像可用于初步解释，数据则用于重力建模。磁异常包含多种磁性源的信息，既包括磁性基底结构与成分产生的深部源，也包括沉积体内磁性单元（如火山岩、侵入体及磁性沉积层）等浅部源。标准TMI图像整合了所有这些源的信息。

数据处理与增强过程全程使用Geosoft Oasis montaj软件，陆-海拼接重力与磁法网格则通过montaj GridKnit模块生成。为突出不同源的细微特征并分离不同深度源的异常，已对最终处理的布格重力网格及化极（RTP）磁法网格应用增强技术，具体细节将在下一节阐述。

增强处理技术与结果
表1总结了用于实现各类目标的图像处理技术：

数据类型 | 应用滤波 | 增强效果/结果
重力/磁法 | 一阶垂直导数（1VD） | 近地表特征（如盆地内）
重力/磁法 | 向上延拓 | 数据降噪
重力/磁法 | 低通滤波或大距离向上延拓 | 深部特征（如基底）增强
重力/磁法 | 高通滤波 | 浅部特征（如地表异常）增强
重力/磁法 | 倾斜滤波与一阶垂直导数（1VD） | 构造（如基底构造）增强
重力/磁法 | ZS-Edgezone与ZS-Edge滤波 | 边缘增强
重力/磁法 | 水平模量/水平梯度 | 边界增强
磁法 | 化极（RTP）、复合异常及解析信号滤波 | 源定位精度提升