Characteristics of magnetic field for ground-based transient electromagnetic methods considering full transmitting-current waveforms
In the past four decades,most three-dimensional(3D)modeling and inversion studies on transient electromagnetic(TEM)methods have primarily focused on the time derivative of the magnetic field(∂b/∂t),with relatively fewer studies on the magnetic field itself(B-field).Nowadays,with the advancement of the superconducting quantum interference device(SQUID)technique,collecting B-field data in TEM surveys has become more popular.Based on the vector finite-element method,tw0 3D forward-modeling approaches that are suitable for calculating the B-field response were developed.The two methods use spectral and time-stepping techniques,respectively,to calculate time-domain B-field response.Considering a model with a good conductor beneath a conductive overburden and a model with a dipping,plate-like conductor in a homogeneous half-space,the characteristics of the B-field and ∂b/∂t responses with full-waveform effects were investigated using the developed forward-modeling methods.The detectability to good conductors of the two data types was also analyzed.The modeling results reveal that the late-time B-field response initially increases and then decreases as the conductivity of the anomalous body increases when a full waveform is considered.However,even with a step-off waveform,the ∂b/∂t response demonstrates similar characteristics as seen in the full-waveform B-field response.In both conductive and resistive host scenarios,the B-field response is more sensitive to deep conductors compared with the ∂b/∂t response.Furthermore,the late-time B-field response was found to be less susceptible to interferences from the background noise of the geomagnetic field compared with the ∂b/∂t response.Hence,the B-field response is more suitable for detecting deep conductive targets.It is important to note that full-waveform effects can cause difficulties in the detection of deep conductive targets.Therefore,it is recommended to use low base-frequency transmitting-current waveforms as much as possible when exploring blocky or vein-type sulfide ore deposits using TEM methods.
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