在将钻井垂直地震剖面(VSP)数据的时深拟合公式应用于深部地层的时深转换时,拟合深度与计算的层速度常常不够准确。本文首先利用多项式和幂函数给出了东海陆架盆地中部某凹陷41口钻井VSP数据的时深拟合公式,并用双程旅行时(TWT)最深达8 s的三维地震速度体数据与多道地震剖面对拟合公式在深部地层的适用性进行分析。在TWT为8 s时,速度体数据表明41口钻井位置的平均深度为18 140 m,平均层速度为6 208 m/s,二次多项式的平均拟合深度较之偏高9.2%,计算的层速度偏高36.2%,幂函数则分别偏低28.9%与35.6%,拟合效果都不理想。对此,本文采用通过识别VSP数据的增速拐点并对增速拐点前的VSP数据进行二次多项式拟合,对增速拐点后的VSP数据进行幂函数拟合的分段拟合模型,将TWT为8 s时的平均拟合深度和层速度的误差降到3.3%与4.7%。地震剖面显示研究区莫霍面深度约为TWT=11 s,分段拟合模型在TWT=11 s的平均拟合深度为27 516 m,层速度为7 334 m/s,更接近前人研究成果,表明该模型能显著提高深部地层时深转换的精度。
Abstract
The fitting depth and calculated interval velocity will be inaccurate when the time-depth fitting of vertical seismic profile (VSP) data is applied to deep strata. In this study, polynomials and power functions were used to fit the time-depth relation based on VSP data from 41 drilling holes in the East China Sea Shelf Basin. The applicability of the fitting in deep strata was analyzed by analyzing the seismic velocity volume data and seismic profiles. The three-dimensional seismic velocity volume from migration has a maximum depth of 8 s in two-way travel time (TWT). The average fitting depth from the quadratic polynomial is 9.2% higher and the average interval velocity is 36.2% higher, respectively, whereas the power fitting depth and interval velocity are 28.9% lower and 35.6% lower, respectively in the deep strata (TWT=8 s), where the seismic velocity volume data shows that the average depth and interval velocity at 41 drilling holes are 18 140 m and 6 208 m/s, respectively. To solve this problem, a segmented fitting model was proposed, in which the interval velocity increase turning point of the VSP data was firstly identified, then the quadratic polynomial function was used to fit the VSP data above the turning point and the power function was used to fit the VSP data beneath the turning point. At TWT=8 s, the errors of the average fitting depth and interval velocity are reduced to 3.3% and 4.7%, respectively. The segmented fitting model shows that the average fitting depth and interval velocity are 27 516 m and 7 334 m/s, respectively, and these values basically are consistent with the observed Moho depth(TWT=11 s) in the seismic section of the study area. The segmented fitting method can significantly improve the accuracy of time-depth conversion of the strata, especially at large depths.
关键词
垂直地震剖面 /
地震速度 /
时深转换 /
拟合公式 /
深部地层 /
东海陆架盆地
Key words
vertical seismic profiling /
seismic velocity /
time-depth conversion /
fitting equation /
deep strata /
East China Sea Shelf Basin
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基金
国家科技重大专项项目(2016ZX05027-001);国家自然科学基金项目(41906053,41761134051,91858213,41776057);中国博士后科学基金(2018M642409)
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