L017 & L027 Cookernup seismic reflection and refraction survey, WA, 1955_1956

Seismic reflection traverses were surveyed across the Perth Basin at Cookernup, W.A. These traverses were planned to find the thickness and dip of the Basin sediments adjacent to the Darling Scarp and to discover any faulting or folding within them; also to determine the applicability of the seismic method as a tool for both regional and detailed investigation in this area. Seismic refraction traverses were surveyed to help in the solution of problems encountered in the interpretation of the reflection cross-sections. The survey indicated a considerable thickness of sediments about 20,000 ft, at the eastern margin of the Basin near the Darling Scarp, and suggested tectonic structure that is not indicated in surface geology, The reflection traverses indicated that sediments (presumably Lower Palaeozoic or Precambrian) lying deep in the Perth Basin may continue underneath the Darling Scarp and abut the granitic gneisses etc. of the Western Australian Shield on an overthrust fault plane. The overthrust fault, if it exists, does not reach the surface, but is covered to a depth of possibly some few hundred feet by younger sediments and also by alluvium eroded from the Darling Scarp. Some reflection and refraction shooting was done in an attempt to test this and other hypotheses, but the results crc inconclusive. Gravity results strongly suggest a normal fault, and if normal faulting is the case, the reflections from beneath the outcropping basement are possibly derived from shear zones, Some probable 'reflected refractions' were also observed. There is scope for further seismic testing but it is considered that conclusive evidence could only be provided by drilling.

本次研究在西澳大利亚州库克纳普地区的珀斯盆地（Perth Basin）开展了地震反射测线（seismic reflection traverses）勘测工作。本次测线的设计目标包括：查明达令断崖（Darling Scarp）周边盆地沉积物的厚度与倾角，探明沉积物内部是否存在断裂或褶皱；同时验证地震方法在该区域开展区域与精细化勘探的适用性。此外，本次勘测还布设了地震折射测线（seismic refraction traverses），以辅助解决反射剖面解释过程中遇到的问题。
勘测结果显示，在达令断崖附近的盆地东缘，沉积物厚度可达约20000英尺，且存在地表地质调查未揭示的构造结构。地震反射测线结果表明，珀斯盆地深部的沉积物（推测为下古生界或前寒武纪地层）可能延伸至达令断崖下方，并以逆冲断层面（overthrust fault plane）与西澳大利亚地盾（Western Australian Shield）的花岗片麻岩（granitic gneisses）等岩体接触。该逆冲断层若真实存在，则并未出露地表，其上覆有厚度可能达数百英尺的较年轻沉积物以及来自达令断崖的冲积层。
为验证该假说及其他相关推测，本次研究开展了部分反射与折射地震数据采集工作，但结果尚无定论。重力勘探结果强烈指向存在正断层（normal fault）；若确为正断层构造，则出露基底下方的反射信号可能源自剪切带（shear zones）。本次观测还发现了若干疑似“反射折射波”的信号。本区域仍具备进一步开展地震勘探的潜力，但学界普遍认为，仅通过钻井勘探才能获得确凿证据。