REFLECTION SURVEY IN ROUGH TOPOGRAPHY*

The problems connected with reflexion surveys in areas of rough topography consist of choosing proper equipment and of making careful near‐surface corrections. This paper illustrates some solutions to these problems that have been adopted in a survey which has recently started in a hily coastal area, in Southern Italy.

As the area is characterized by a youthful and very actively eroded topography of Post‐Miocene clays and litoral clastic sediments, both rotary and percussion drilling were necessary whilst all the seismic equipment had to be light to allow straight lines to be shot as far as possible. Details are given of the truckmounted, light equipment which has been designed for this special purpose.

In connection with the youthful topography of the area being surveyed, the criteria for the proper computation of the near‐surface corrections is discussed. The great differences in elevation, combined with variations in the surface and near‐surface materials, necessitated a careful analysis before deciding the thickness of the correction zone. The results are shown of the preliminary tests carried out in order to determine the average thickness of the low‐velocity zone. The solution adopted for drawing the reference surface determining the base of the correction zone for the whole of the area is subseqently explained.

Reference datum is drawn so as to follow gently, and with segments of constant slopes, the general features of the rough topographic relief at a depth ranging from 30 to 60 metres from the surface. Criteria for the routine checking of velocity variations in the correction zone are illustrated. Such checks are based on both uphole time analyses and refraction “first breaks” plots, with deeper shot holes at constant intervals.

Unsatisfactory results necessitate longer spreads for “low velocity tests” and uphole shooting from deeper holes and the reference datum is deepened for the particular area where this becomes necessary.

The final cross‐sections are shown and although, as yet, no deep holes, are available to check the seismic data, the general appearance of the cross‐sections seems to indicate that subsurface data are free from the influence of topography and of the velocities in the near‐surface formations.