The transient electromagnetic (TEM) method becomes more urgent than ever for marine exploration due to abundant resource reserves and the increasing undersea engineering construction activities, especially in the offshore exploration of mineral deposits such as Sanshandao gold mine. However, the research and application of TEM method in marine environment are still challenged by many problems. Such contradiction motivates our study on the coincident-loop TEM in seafloor exploration. The TEM response of coincident loops is firstly derived in the integral form, based on the potential functions in Helmholtz equations for a magnetic source locating in the whole-space layered model. The frequency-domain vertical magnetic field is described as the Hankel integral with double first-order Bessel functions of first kind. Secondly, the time-domain induced voltage is obtained by transforming the frequency-domain response through the cosine transform and then taking the derivative of time. To simultaneously solve the Hankel transform and the cosine transform, a novel algorithm is introduced by adapting the fixed-point quadrature and extrapolation via the Shanks transformation. Finally, a typical conductivity model for marine polymetallic deposit is designed to investigate the characteristic of TEM response under various conditions. Numerical results demonstrate that existence of conductive seawater causes the TEM response to increase significantly and decay slower. The air-sea reflected electromagnetic waves lead to a significantly large fake negative response (NR) in shallower seawater with depth less than 300 m. Increase in the height of loops will weaken and delay the anomaly response and shorten the observation time-window. The height of configuration should be no more than 100 m for shallower targets and 50 m for deeper targets, respectively. The observation time-window should cover 10–1 000 ms. Increase in the radius of loops only enhances the TEM response proportionally but hardly improves the relative anomaly. The vertical resolution on the low-resistivity target approximates 20 m for the configuration considered in the study. Decreases in D.C. resistivity and chargeability cause the positive response (PR) to increase significantly and decay more rapidly. Meanwhile, the NR is advanced and enlarged significantly and decays slower compared with the PR. The influence of time constant is not monotony and there exists an optimal value for producing the maximum NR. As the frequency parameter increases, the PR is caused to decay more rapidly without magnitude change and the NR is advanced and decays more rapidly with significant increase in magnitude. The influence of frequency parameter is more pronounced than that of time constant.

由于丰富的资源储量和海底工程建设活动的增加，瞬态电磁（TEM）方法比以往任何时候都更加迫切需要进行海洋勘探，尤其是在三山岛金矿等矿床的近海勘探中。但是，TEM方法在海洋环境中的研究和应用仍然受到许多问题的挑战。这种矛盾促使我们对海底勘探中的重合回路TEM进行研究。首先，基于位于整个空间分层模型中的磁源的Helmholtz方程中的势函数，以整数形式导出重合回路的TEM响应。频域垂直磁场被描述为具有第一类双重一阶贝塞尔函数的汉克尔积分。其次，通过余弦变换对频域响应进行变换，然后取时间的导数，即可得到时域感应电压。为了同时解决Hankel变换和余弦变换，通过Shanks变换采用定点正交和外推法，引入了一种新颖的算法。最后，设计了一种典型的海洋多金属矿床电导率模型，以研究各种条件下的TEM响应特性。数值结果表明，导电海水的存在导致TEM响应显着增加而衰减较慢。空气-海洋反射的电磁波在深度小于300 m的较浅海水中导致明显较大的假阴性反应（NR）。循环高度的增加会减弱并延迟异常响应，并缩短观测时间窗口。对于较浅的目标，配置高度应分别不超过100 m，对于较深的目标，配置高度应分别不超过50 m。观察时间窗口应涵盖10–1000 ms。回路半径的增加仅按比例提高了TEM响应，但几乎没有改善相对异常。对于研究中考虑的配置，低电阻率目标的垂直分辨率约为20 m。直流电阻率和可充电性的降低会导致正响应（PR）显着增加并更快地衰减。同时，与PR相比，NR显着提前和扩大，衰减更慢。时间常数的影响不是单调的，并且存在用于产生最大NR的最佳值。随着频率参数的增加，PR会更快地衰减而不改变幅度，并且NR会提前并随着幅度的显着增加而更快地衰减。频率参数的影响比时间常数的影响更为明显。