On the theory of sea-floor conductivity mapping using transient electromagnetic systems
On the theory of sea-floor conductivity mapping using transient electromagnetic systems
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DOI：
10.1190/1.1442296
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发表时间：
1987-02
期刊：
影响因子：
3.3
通讯作者：
S. Cheesman;R. N. Edwards;A. Chave
中科院分区：
文献类型：
--
作者：
S. Cheesman;R. N. Edwards;A. Chave
海底的电导率通常比其上的海水的电导率小得多.本文对几种常见的可控源电磁系统对相邻导电半空间的瞬态阶跃响应进行了理论研究，结果表明，水平同轴电偶极-偶极和水平同轴磁偶极-偶极两种系统，能够准确测量在海水存在下海底相对较低的电导率。对于这些系统，初始瞬变的时间位置指示海底的电导率，而在明显较晚的时间，瞬变的第二特征是海水电导率的测量。对于许多其他系统，包括通常用于陆地勘探的几个系统，瞬态响应的两个部分之间在时间上的诊断分离并不发生。海底电导率的变化会对基本上是海水响应的情况产生微小的扰动。使用位于海底附近的实用深拖同轴磁偶极-偶极系统可以观察到的一些瞬态响应是半空间、导电或电阻性基底上方的层以及具有中间电阻的半空间的响应。区域。系统响应的两个相邻的半空间，分别代表海水和海底，解析推导。只要电导率比大于约10或小于约1/10，该溶液在所有时间都有效。解析理论证实了这些层状地球模型的封闭形式的解决方案的数值评估的有效性。一个横向导体，如垂直的，无限的，导电岩脉露出海底延迟到达的初始地壳瞬态响应。延迟随堤的电导线性变化。这表明，时间延迟可以直接反演，以测量发射器和接收器偶极子之间及其附近的海底异常综合电导。
The electrical conductivity of the sea floor is usually much less than that of the seawater above it. A theoretical study of the transient step-on responses of some common controlled-source, electromagnetic systems to adjoining conductive half-spaces shows that two systems, the horizontal, in-line, electric dipole-dipole and horizontal, coaxial, magnetic dipole-dipole, are capable of accurately measuring the relatively low conductivity of the sea floor in the presence of seawater. For these systems, the position in time of the initial transient is indicative of the conductivity of the sea floor, while at distinctly later times, a second characteristic of the transient is a measure of the seawater conductivity. The diagnostic separation in time between the two parts of the transient response does not occur for many other systems, including several systems commonly used for exploration on land. A change in the conductivity of the sea floor produces a minor perturbation in what is essentially a seawater response. Some transient responses which could be observed with a practical, deep-towed coaxial magnetic dipole-dipole system located near the sea floor are those for the half-space, the layer over a conductive or resistive basement, and the half-space with an intermediate resistive zone. The system response to two adjoining half-spaces, representing seawater and sea floor, respectively, is derived analytically. The solution is valid for all time, provided the conductivity ratio is greater than about ten, or less than about one-tenth. The analytic theory confirms the validity of numerical evaluations of closed-form solutions to these layered-earth models. A lateral conductor such as a vertical, infinite, conductive dike outcropping at the sea floor delays the arrival of the initial crustal transient response. The delay varies linearly with the conductance of the dike. This suggests that time delay could be inverted directly to give a measure of the anomalous integrated conductance of the sea floor both between and in the vicinity of the transmitter and the receiver dipoles.