GNGTS 2013 - Atti del 32° Convegno Nazionale

• geophones with natural frequency of 14 Hz, un-damped; • data acquisition system ABEM - Seistronix RAS 24, two units with a maximum of 48 active channels; • energy source: 5 kg sledge hammer with vertical stacking for refraction; accelerated dropping weight, single shot, for reflection; • CMP cables and accessories. • refraction data processing has been performed by means of the SeisOptPro V5.0 package which simply requires the first-arrival times and the acquisition geometry, and works through a non-linear optimization technique, the so-called Adaptive Simulated Annealing (e.g. Lindsay and Chapman, 1993). Ultra-shallow reflection data have been processed by means of the SPW - Seismic Processing Workshop package. Ultra-shallow reflection section. The reflection section is shown in Fig. 1. It has been obtained with: • 36 channel off-end spread; • geophone interval = shot interval 1 m; maximum CMP fold 1800%; • in-line minimum offset 6 m, maximum offset 41 m; • shots 100; acquisition time 3,5 hours, spread assembly-disassembly included; • processing time, record uploading included, one full day. The processing flow has been rather simple and included the following main steps: trace edit, frequency analysis and filtering, spectral whitening, sorting into CMP gathers, velocity analysis, NMO correction and brute stack, spatial noise filtering, 2-traces horizontal stack. Though ambient noise level was very low, reflections were not very clearly detectable and several attempts to apply more sophisticated processing steps have not provided satisfactory results; eventually, the most convenient way for velocity analysis revealed the constant velocity sections method (CVS): the seismic section at hand has been obtained with a constant stack velocity of 850 m/s. At a glance, a near surface reflector affected by several discontinuities can be individuated; the dominant frequency of reflections is of the order of 100-170 Hz. It is located at a depth of 3-12 m from South to North, values estimated on the basis of the stack velocity which is the only available information for time-to-depth conversion. Several faults are also visible but, for the same lack of information said above, quantifying their possible throw is not easy. Actually, taking into account the geological information, namely the good knowledge of Fig. 1 – The utrashallow reflection section. CMP trace interval 0.5 m; red lines represent faults; red numbers indicate reflectors and structural details. 13 GNGTS 2013 S ESSIONE 3.1