津軽海峡西口付近の音波探査とその成果 (1)

抄録

Various methods now available are employed by the Japanese National Railways in last 12 years to obtain informations on geology of the Tsugaru Strait between Hokkaido and Honshu (Main Island) for a project to drive an under-sea tunnel.
In 1959 a sonic survey was carried out with the continuous seismic profiler (So-called Sparker) of Marine Geophysical Services International Inc., U. S. A. Important records brought by this relatively new technique enabled us to revise geologic knowledge of the strait hitherto mapped.
In the first part of this paper, a procedure taken by authors to analyse the obtained sonic records will be presented.
§ 1. Procedure for analysing C. S. P.'s records.
1) All patterns due to diffractions or abnormal reflections are reduced from records, and only those reflections from physical boundaries showing geological structure are piked up.
2) By comparing these picked up reflections with clreaged samples or drilled cores so far already obtained from the sea bottom, it can be determined to what rock or formation they correspond. If we have no bottom rock samples to be compared with records, we must find similar patterns among many records on neighbouring survey lines, and temporary assumption on rock or formation will be made.
3) On every intersection point of two survey lines, after the depth of water and the charactor and apparent thickness of rock or formation are checked to see if they coincide with each other, the dip direction should be determined on every bedding plane, and the strike and true dip of that is calculated by equation (5) on page 15.
4) For these calculations, every survey line must be exactly positioned. If remarkable mispositioning is found by the above checks, the real track of the survey boat should be supposed by the relief of the sea bottom expressed in C. S. P.'s record, comaring with previously drawn detailed sea-bottom contour map.(scale 1: 20, 000, conters interval every 2 meters)
5) Faults, unconformities, synclinal and anticlinal structure can easily be found on well “Sparkered” records, and their extension will be traced considering the distribution of strike and dip.
6) All the date thus obtained are expressed into a geological map and, if necessary, the thickness of bed or formation is calculated by equation (6)&(7) on page 17 and geological section profiled.