Suppression of multiple reflected refraction in shallow low-velocity zones by vertical combination of seismic sources

A special type of multiple reflected refraction often appears in seismic records in areas with low-velocity structures. It seriously affects the closed reflection wave, which causes the distortion of the reflection wave shape and affects the correct interpretation of the seismic data. Based on the forward simulation of the multiple reflected refraction equation, the generation mechanism of multiple reflected refraction in the shallow low-velocity zones was set forth. A method of suppressing multiple reflected refraction by vertically combined dual sources was proposed. The formation parameters(formation thickness and velocity)are obtained according to the methods of small refraction and micro logging in the early stage of exploration. Firstly, according to the relative position relationship between the multiple reflected refraction and the effective wave, the prerequisites for the suppression of the multiple reflected refraction was given. Secondly, the optional range of vertical combination parameters was calculated according to the source combination formula, which was used to adjust the relative position of the two sources set vertically in the low-velocity zone. Based on the above studies, forward modeling and loess plateau exploration area tests were conducted to compare the conventional single shot records with the source vertical combination records. Models verification and practical data applications show that the vertical source combination method can suppress multiple reflected refraction in shallow low-velocity zones and effectively improve the signal-to-noise ratio of seismic data.