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A denoising method based on wavelet analysis is presented for the removal of noise (background noise and random spike) from time domain electromagnetic (TEM) data. This method includes two signal processing technologies: wavelet threshold method and stationary wavelet transform. First, wavelet threshold method is used for the removal of background noise from TEM data. Then, the data are divided into a series of details and approximations by using stationary wavelet transform. The random spike in details is identified by zero reference data and adaptive energy detector. Next, the corresponding details are processed to suppress the random spike. The denoised TEM data are reconstructed via inverse stationary wavelet transform using the processed details at each level and the approximations at the highest level. The proposed method has been verified using a synthetic TEM data, the signal‐to‐noise ratio of synthetic TEM data is increased from 10.97 dB to 24.37 dB at last. This method is also applied to the noise suppression of the field data which were collected at Hengsha island, China. The section image results shown that the noise is suppressed effectively and the resolution of the deep anomaly is obviously improved.

Time-domain transient electromagnetic method (TEM) measurements sometimes exhibit a sign reversal in the secondary field during the off-time, which is usually attributed to the induced-polarization (IP) effect. In contrast with the conventional IP method, which uses a current source, TEM with an ungrounded transmitting loop operates using a pure voltage source, which is induced by the primary field switching on and off. We performed TEM measurements in a resistive survey area showing an IP effect, and we used a low-temperature superconducting quantum interference device (LT-SQUID) with sensitivity of [Formula: see text] as a magnetic field sensor. A sign reversal in all of our measurements was observed; furthermore, the negative amplitude reached [Formula: see text]. In-depth analysis with an extended version of a wire-filament circuit reveals that the large negative signal may be due to discharging of in-ground capacitance, an IP effect. The conduction response of the ground can be restored by subtracting the fitted discharging response (negative valued) from the observed data. To verify this operation, we compared TEM measurements with and without wire-loop targets, which can induce a conduction field with a known decay time constant during the off-time. The extracted conduction responses of the wire-loop targets match the expected ones well. This research reveals that the primary field switch-off must always be included when interpreting TEM data with sign reversal and an LT-SQUID may be a good alternative sensor for studying the IP effect in TEM.

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