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This paper introduces new technique for qualitative analysis of multi-transient electromagnetic (MTEM) earth impulse response over complex geological structures. Instantaneous phase and frequency attributes were used in place of the conventional common offset section for improved qualitative interpretation of MTEM data by obtaining more detailed information from the earth impulse response. The instantaneous attributes were used to describe the lateral variation in subsurface resistivity and the visible geological structure with respect to given offsets. Instantaneous phase attribute was obtained by converting the impulse response into a complex form using the Hilbert transform. Conversely, the polynomial phase difference (PPD) estimator was favored over the center finite difference (CFD) approximation method in calculating the instantaneous frequency attribute because it is computationally efficient and has the ability to give a smooth variation of the instantaneous frequency over a common offset section. The observed results from the instantaneous attributes were in good agreement with both the subsurface model used and the apparent resistivity section obtained from the MTEM earth impulse response. Hence, this study confirms the capability of both instantaneous phase and frequency attributes as highly effective tools for MTEM qualitative analysis. [ABSTRACT FROM AUTHOR]

Methods based on pseudo-random coded emission technology can effectively suppress the noise in electromagnetic exploration and improve the depth and resolution of the detection. Additionally, the synthesis of electromagnetic response signals excited by pseudo-random code sources can provide not only theoretical data for data processing but also intermediate results for evaluations of field data. Currently, pseudo-random signal simulations ignore the presence of source effects generated by band-limited sources in the Earth's impulse response. Furthermore, the simulation methods are mainly completed based on one-dimensional and two-dimensional formats and thus cannot accurately reflect the pseudo-random response of three-dimensional geoelectric structures. In this paper, 3D pseudo-random electromagnetic signals with a band-limited source were synthesized according to the propagating routine of the electromagnetic field. First, Maxwell's Equations were solved using a three-dimensional time-domain finite volume method. To avoid the singularity of the source and incorporate source effect response, the Gaussian function was introduced to simulate the band-limited impulse sources. The Earth's impulse response was obtained, which contained source effects under three-dimensional geoelectric structures. Then, ideal pseudo-random electromagnetic signals could be composed by convoluting the Earth's impulse response with a pseudo-random source waveform. Since the transmitter and receiver instruments were band-limited, a low-pass filter with a Blackman window was introduced to simulate the instrument's system response. The simulated pseudo-random signals were obtained by adding noise and the ideal pseudo-random electromagnetic signals were filtered using a low-pass filter. By comparison with signals acquired in the field, the correctness of the pseudo-random signal synthesis with source effects was successfully verified. • MTEM response is composed of band-limited source, instrument response, and noise. • The practical earth impulse response includes the ideal response and source effect. • Gaussian sources can be used to directly simulate the band-limited impulse source of MTEM. • The source effect must be considered in the pseudo-random response simulation. [ABSTRACT FROM AUTHOR]