An adaptive method for determining an acquisition parameter t0 in a modified CPMG sequence

[Display omitted] •An adaptive method to determine t0 (duration of the first window) is proposed.•The internal magnetic field gradient is main controlling factor of discriminant σ.•The maximum value of σ is the criterion for optimizing t0.•The adaptive method greatly reduces the difficulties of experimental operation. The modified CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence is a common sequence used for measuring the internal magnetic field gradient distribution of formation rocks, for which t0 (the duration of the first window) is a key acquisition parameter. In order to obtain the optimal t0, an adaptive method is proposed in this paper. By studying the factors influencing discriminant factor σ and its variation trend using T2-G forward numerical simulation, it is found that the optimal t0 corresponds to the maximum value of σ. Then combining the constraint condition of SNR (Signal Noise Ratio) of spin echo, an optimal t0 in modified CPMG pulse sequence is determined. This method can reduce the difficulties of operating T2-G experiments. Finally, the adaptive method is verified by the results of the T2-G experiments for four water-saturated sandstone samples.