Seismic source specially designed for the Meso-Paleozoic strata and its applicaton to South Yellow Sea

Successful seismic survey in the South Yellow Sea has been a great challenge to both researchers and oil industry for years. The seismic reflection signal from deep is always very weak and the seismic wave field very complex. Therefore, the seismic imaging quality of the deep strata has been considerably poor in the past decades. The main reasons come from: 1. there are a strong reflector (T₂ seismic reflection interface) and multi-sets of carbonate strata with high seismic velocity widely distributed in the region. 2. the impedance contrast of strata is small in the region as both the Mesozoic and Paleozoic strata have suffered long-term compaction. 3. the structure of the region is too complicated as the result of strong tectonic movement. In this paper, we proposed a specific and targeted seismic survey design, especially a special air-gun source, for the purpose to improve the images from the deep part. A systematic review is made on airgun theories. The wavelet features of sigle airguns, strongly-interacting clustered airguns, weakly- interacting tuning airguns and sub-array airguns are reviewed one by one. Above it, key factors for multi-level source are analyzed, such as the sub-array depth combination and firing time-delay. Then a design approach on plane-distributed airgun source and cubic-distributed airgun source(multi-level source) are developed. A 6390in³ airgun source composed of 4 sub-arrays is specially designed for the South Yellow Sea 2016 Project. Two sets of seismic source is optimized, one is 10m in depth for all 4 sub-arrays and the other is 7m-10m-10m-7m in depth in shape of an inverted trapezoid. Field testing demonstrates that the plane-distributed airgu system with stronger low frequency components absorbs less energy and thus is more effective. It is a breakthrough indeed. Some deep strata become visible, and the seismic signal of the deep strata strengthened. The general quality of seismic images below the strong reflector (T₂ seismic reflection interface) is greatly improved. The breakthrough and improvement have laid a solid foundations for future exploration of petroleum in deep strata of the region.