Important design considerations for inboard airborne magnetic gradiometers
Important design considerations for inboard airborne magnetic gradiometers
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DOI：
10.1071/eg984266b
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发表时间：
1984-11
影响因子：
0.9
通讯作者：
C. Hardwick
中科院分区：
文献类型：
--
作者：
C. Hardwick
人们普遍认识到磁梯度测量作为总场测绘的辅助手段的优点，一些飞机已配备了梯度仪。这些梯度仪是从高灵敏度总场磁力仪系统衍生出来的，这些系统本身存在某些误差，而这些误差在常规勘测中通常是可以容忍的。然而，在梯度仪中，非常大的总场值是不同的，这些误差在许多情况下，可以大大超过系统所需的基本精度。在内侧梯度测量系统中存在两个主要误差源。第一个，也是最重要的，是由于飞行器不可避免的磁干扰，或者由于目前可用的补偿系统不能充分处理磁干扰。无源补偿方法对于梯度测量来说不够全面，目前使用的为军事应用而设计的有源补偿系统不能保证在零频率（dc）或关注长波长异常的天文学家感兴趣的非常低的频率下进行补偿。第二个误差源是通常用于将拉莫尔频率转换为环境总场的频率计数技术。计数过程在某种程度上类似于以相对低的速率进行的数字采样，因此对于来自更高频率干扰源（包括飞机机动频率处的分量）的混叠几乎没有提供保护。本文用实例说明了这两类错误。一个列表的设计标准，并介绍了实现这些标准的几种技术。最后，补偿和调查线的结果显示，在加拿大的康维尔580的国家研究理事会的三轴梯度仪系统。这架飞机使用非定向铯磁力计，每个翼尖和尾翼尖端各一个。在飞机所有航向的整个正常机动包线上的补偿给出了从直流到1Hz的3毫克/米的典型标准偏差误差。因此，该系统能够测量下至非地质背景水平的梯度。
The advantages of magnetic gradiometry as an adjunct to total field mapping are generally recognized and a few aircraft have been equipped with gradiometers. These gradiometers are derived from high-sensitivity total-field magnetometer systems that are in themselves subject to certain errors that can usually be tolerated in conventional surveys. However, in a gradiometer, where very large total-field values are differenced, these errors can, in many cases, greatly exceed the basic accuracy required of the system. There are two principal sources of error in inboard gradiometry systems. The first, and most significant, results from the inevitable magnetic interference of the aircraft or from the inability of currently available compensation systems to deal with the magnetic interference adequately. Passive methods of compensation are not sufficiently comprehensive for gradiometry and the active compensation systems currently in use, which were designed for military applications, cannot guarantee compensation at zero frequency (dc) or at the very low frequencies of interest to the geophysicist concerned with long-wavelength anomalies. The second source of error is the frequency-counting technique usually employed to convert a Larmor frequency to ambient total field. The counting process is somewhat analogous to digital sampling at a relatively low rate and as such affords little protection against aliasing from higher frequency interference sources, including components at aircraft maneuvering frequencies. This paper, using examples, illustrates the two types of error. A list of design criteria is presented and several techniques are described for realizing these criteria. Finally, compensation and survey line results are shown for a three-axis gradiometer system in the National Research Council of Canada's Convair 580. This aircraft uses nonoriented cesium magnetometers, one in each wingtip and one at the tip of the tail fin. Compensations over the entire normal maneuver envelope of the aircraft on all headings give typical standard deviation errors of 3 mg/m from dc to 1 Hz. Thus, the system is capable of measuring gradients down to nongeologic background levels.