| Abstract: | High resolution seismic reflection exploration for minerals places severe demands on field practice so as to maximize the signal-to-noise bandwidth. In particular, all horizontally propagating coherent noise, especially ground roll, must be attenuated. The blocking effect of a trench between source and receiver has been investigated by means of two-dimensional physical seismic model experiments. Rectangular, circular and wedge-shaped saw-cuts of various dimensions were studied. The results show that thin rectangular cuts of depth equal to one-quarter of the Rayleigh wave noise wavelength produce a 12 dB or better improvement in the signal-to-noise ratio. Rayleigh wave attenuation is greater than 30 dB at a cut depth of one wavelength. In the field applications envisaged, this corresponds to trenches up to a few metres deep. The trenches should be filled with foam or loose sand to dampen out mode conversion and diffraction noise. There are obvious practical difficulties of implementing such a technique in routine CMP operations. The technical effectiveness of the saw-cut is illustrated by imaging a deeply-buried small hole (diffractor) in an aluminium plate. Without the saw-cut between source and receiver, the seismic record is dominated by Rayleigh wave noise, masking P-wave arrivals from the target diffractor. However, with a saw-cut of depth three-quarters of a Rayleigh wave wavelength, the improvement is dramatic, making it easy to detect and identify the hole. When scaled to the field situation, this is equivalent to imaging a 6 m tunnel at a depth of 400 m, using a surface trench of depth 2 m to block ground roll. |
