GNGTS 2013 - Atti del 32° Convegno Nazionale

of the survey to check the design parameters and to properly set the minimum and maximum offset. Fig. 1a) shows the acquisition layout used in the field. The receiver spacing was 1.25 m to reduce the surface waves and air wave aliasing effects. As many as 36 source records corresponding to 36 single blows spaced of 1.25 m were recorded, leaving the receiver spread in the indicated position. The whole source-receiver layout was then advanced of 5 m downhill along the profile and the acquisition procedure was repeated again. The flexibility of the hardware equipment and in particular of the source, allowed us to record the data with a source-array geometry. From an estimate of the velocity and of the frequency content of the surface waves in the test records, which were found to be approximately 350 m/s and 35 Hz as the pick frequency respectively, we decided to realize the source-array layout shown in Fig. 1a), which is characterized by the first notch at k≈0.1 [cycles/m]. Recording each single blow separately we left the task of array-forming for spatial filtering to the processing lab. We ended up having 868 single blow gathers for a total profile length of about 200 m. A typical single blow gather is shown in Fig. 1b) after a trace by trace normalization. Some reflections are clearly visible between 80 ms and 100 ms at far offset, but the high amplitude coherent noise due to the air blast wave and, particularly, to the surface waves (ground roll) that contaminate the data overwhelm any signal outside the optimum window. Fig. 1c) illustrates the corresponding f-k spectrum. The high amplitudes of the ground roll, its partially aliased nature and the partly overlapping frequencies of the ground roll with the useful signal make it difficult for subsequent processing tools to achieve a satisfactory noise attenuation and we may often have to resort to drastic measures such as the muting of entire portions of the observed data. Chebyshev polynomials. The Chebyshev polynomials are defined using the following recursion formula (Holzman, 1963): (1) Some examples are: , , ... , etc. Fig. 1 – a) Acquisition layout. The stars represent the source locations and the triangles represent the receiver locations. b) An example of a single blow gather. A trace by trace normalization is applied to the seismogram. c) f-k spectrum of the seismogram showed in frame b). 67 GNGTS 2013 S essione 3.1