In rock physics one of the important purposes of the determination of P- and S-wave velocities is to obtain the elastic constants and anisotropy parameters. This method is standardised in American Society for Testing and Materials (ASTM) by a standard D2845-95. The most challenging yet important part of these experiments is picking the first arrivals correctly and consistently. A sharp, clean first arrival of a wave is not always possible, especially when testing dry, porous sandstones at low pressures. The signal transmitted through such samples is weak due to a high damping effect, and picking the first break introduces uncertainties. Harvey sandstone is a sample type with high porosity and, therefore, some degree of anisotropy is expected. In order to study the anisotropy parameters of the Harvey 3 sandstone which belongs to Lesuer- Wonnerup Member of the Yalgorup Member, three core plugs in directions of horizontal, vertical and diagonal, were experimentally investigated. The laboratory measurements on three different samples which are cut at three different directions with regards to the base deposition system enabled us to calculate the anisotropy parameters using a standard acoustic equipment while applying stress. We concluded that Harvey 3 poses a more complex symmetry axis of anisotropy, and no sign of low-grade anisotropy was observed through the experimental data. To study the anisotropy degree of the Harvey 3 sandstone, a pressure cell, pore fluid injection pump, and ultrasonic system (consisting of P- and S-wave transducers, oscilloscope and pulser/receiver) were used to record the elastic waves passing through the samples. The specimens were fully saturated inside the pressure cell using vacuum injection for the best possible comparison of dry and saturated status. The results confirmed that the sample is far from weak anisotropy and possesses more axes of symmetries than simple layering anisotropy due to the pore orientation and distribution in the matrix of the background rock. The study also confirms that the anisotropy degree decreases under full compaction and full saturation, and the Harvey 3 anisotropy becomes elliptical when it’s fully saturated. Large, stress-dependent changes in the ultrasonic velocities for porous media were observed in this experimental study, which confirms the usefulness of such studies in examining the inner structure change process.

中文翻译：

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应力和流体对储集岩各向异性的影响：以西澳大利亚州哈维 3 CCS 站点的砂岩为例

在岩石物理学中，确定 P 波和 S 波速度的重要目的之一是获得弹性常数和各向异性参数。该方法在美国测试与材料协会 (ASTM) 中通过标准 D2845-95 进行了标准化。这些实验中最具挑战性但最重要的部分是正确且一致地挑选先到者。并非总是能获得清晰、清晰的波浪初至，尤其是在低压下测试干燥、多孔的砂岩时。由于高阻尼效应，通过此类样本传输的信号很弱，并且选择第一个中断会引入不确定性。Harvey 砂岩是一种具有高孔隙率的样品类型，因此预计会有一定程度的各向异性。为了研究Yalgorup段Lesuer-Wonnerup段的Harvey 3砂岩的各向异性参数，对水平、垂直和对角线三个方向的岩心塞进行了实验研究。三个不同样品的实验室测量结果在三个不同方向上根据基础沉积系统切割，使我们能够在施加应力的同时使用标准声学设备计算各向异性参数。我们得出结论，Harvey 3 提出了更复杂的各向异性对称轴，并且通过实验数据没有观察到低级各向异性的迹象。为了研究 Harvey 3 砂岩的各向异性程度、压力盒、孔隙流体注入泵和超声系统（由 P 波和 S 波换能器组成，示波器和脉冲发生器/接收器）用于记录通过样品的弹性波。使用真空注射使样品在压力盒内完全饱和，以便最好地比较干燥和饱和状态。结果证实，由于孔隙取向和在背景岩基质中的分布，样品远非弱各向异性，并且具有比简单分层各向异性更多的对称轴。研究还证实，在完全压实和完全饱和下各向异性程度降低，而Harvey 3各向异性在完全饱和时变为椭圆形。在这项实验研究中观察到多孔介质的超声波速度的大的、应力相关的变化，这证实了这些研究在检查内部结构变化过程中的有用性。