Composition of high frequency ambient noise from cross-correlation: A case study using a small aperture array

Long-time cross correlation of ambient noise has been proved as a powerful tool to extract Green's function between two receivers. The study of composition of ambient noise is important for a better understanding of this method. Previous studies confirm that ambient noise in the long period (3s and longer) mostly consists of surface wave, and 0.25-2.5 s noise consists more of body waves. In this paper, we perform cross correlation processing at much higher frequency (30-70 Hz) using ambient noise recorded by a small aperture array. No surface waves emerge from noise correlation function (NCF), but weak P waves emerge. The absence of surface wave in NCF is not due to high attenuation since surface waves are strong from active source, therefore probably the high ambient noise mostly consists of body wave and lacks surface wave. Origin of such high frequency body waves in ambient noise remains to be studied.