ATEM shallow detection research based on pseudo-random transmitting current and digital fractional sampling

Conventional airborne time domain electromagnetic ( ATEM) detection emits bipolar trapezoidal waves, triangular waves and semi-sinusoidal waves with a base frequency of 25 Hz or 30 Hz. Shallow detection usually increases the base frequency and samples the very early time channel. In this paper, high frequency pseudo-random transmitting current and digital fractional sampling are used to realize ATEM shallow detection research. As high frequency pseudo-random transmitting current has uniform frequency spectrum other than odd harmonic frequency spectrum of traditional transmitting currents, it could excite shallow anomalies effectively. By flexibly changing its clock frequency and register order, we could obtain different detection depth and vertical resolution, which other currents could not do. Digital fractional sampling is actually a process of equivalent sampling, in which there is a fractional sample in one period and causes a sampling delay for constitutive periods. C onsequently we reconstruct one period by inserting the multi-period data together. In this way, we could improve the equivalent sampling rate for ATEM response excited by high frequency pseudo-random current . It need s to mention that the s a mpling delay should be carefully chosen based on the frequency stab i lity of transmitter and re ceiver. Then the ATEM response is calculated by the system identification to obtain the very early channel for shallow detection . In practice, we design a 8 - order m pseudo-random of 200 kHz as tra n smitting current , and employ a 1.25 MHz sampling rate . These leads to an equivalent sampling rate of 5MHz and the earliest channel of 0.2 m s .The vertical resolution of 10m within 100m is achieved by pseudo-random transmitting current and digital fractional sampling, which is proven by three-layer and half space forward modeling.