Analysis of noise in the Rookhope area

The main purpose of this research has been to investigate the attenuation and frequency characteristics of noise generated by a single source. About seven weeks were spent from October to late November, 1963 measuring the vertical ground velocity of the noise over a network of recording stations on the southern slopes of the valley at Rookhope, County Durham. Special recording methods were used to eliminate the effects of any variation in noise level with time. This was necessary as the ground velocity at all stations over the network had to be normalised to values that would be obtained for a constant source, before they could be used in the construction of attenuation curves. Two methods of frequency analysis were performed on a number of the station records: one a power spectral analysis using a computer to facilitate calculations, and the other employing a range of band-pass filters on records obtained on magnetic tape. Both techniques showed that the noise consisted essentially of two frequency components, 6 c.p.s. and 11-12 c.p.s., the latter being confined to an area in the immediate vicinity of the source. Attenuation analyses revealed the presence of two areas to the west and south of the source which showed noticeably different rates of attenuation. Two possible hypotheses were put forward to explain this phenomenon: one embodying a change in subsurface conditions between the two areas, and the other constant subsurface conditions, but a change in the predominant wave types propagating over the two regions. Since no knowledge could be obtained on the precise distribution and nature of the wave types, as the vertical ground velocity only was measured, both theories were considered to be open to some doubt, and thus preference was given to neither. It was found that the values of the rate of absorption obtained for the two areas were generally consistent with those usually found from laboratory experiments on sedimentary rocks.