Quantifying Near-Surface Patterns of Bedrock Fractures and Assessing Controls on Fracture Formation.

本研究的主要目标是进一步开发、校准和测试基于地震折射的方法，以调查地下岩石强度特性和裂缝分布。 岩石的强度和凝聚力在塑造景观、抵抗侵蚀和调节滑坡灾害方面起着关键作用。然而，关键因素不是完整岩石的强度，而是整个岩体在地表的有效强度，在地表，岩体与气候、山坡和生物变量相互作用。这种有效强度受到裂缝发育的影响，裂缝会削弱岩体，使其更容易受到侵蚀、物理和化学风化、生物活动或坍塌的影响。为了描绘裂缝密度在浅层地下的变化，一个有前途的，但在很大程度上未探索的方法结合浅层地震折射调查基岩露头的实验室分析？完好无损？样品初步结果表明，两种常见的断裂模式与深度：岩石是均匀断裂（显然是由大规模的构造力）;和岩石与一个明显的断裂梯度在上层（显然是由于气候和生物压裂过程），覆盖在一个更强大的，较少断裂的下层。虽然这种方法很有前途，但需要更彻底地改进、测试和探索。这就是本研究的目的。通过将方法开发和校准的重点放在允许对断裂特性进行详细观察、测量和取样的人工和天然基岩暴露上，可以通过与现场观察结果的直接比较来测试和验证地震导出的结果。随着改进的校准在手，两个基本的问题，近地表压裂将被调查：近地表断裂模式如何随着深度和空间的景观变化;以及什么是近地表断裂形成的主要控制？在科罗拉多的一个山区现场，将研究（i）导致岩石破裂的冻融过程与（ii）由于山坡陡度和曲率导致破裂的重力的相对重要性。为什么有些山坡会发生山泥倾泻，而其他同样陡峭的山坡却不会？为什么某些地区的侵蚀速度比其他类似地区快得多？控制山坡侵蚀或崩塌的其中一个关键因素，是其下岩石的强度。虽然不同类型的岩石通常具有不同的内在强度（例如花岗岩与泥岩），但岩石中裂缝的密度也对其强度产生了根本性的控制：裂缝密度越高，裂缝之间的连通性越大，就会削弱岩石，并增加其对滑坡或侵蚀的抵抗力。最近的研究表明，在控制山坡稳定性方面，裂缝密度至少与岩石固有强度一样重要。尽管岩石断裂对山坡稳定性很重要，但量化断裂密度的方法仍然难以捉摸：通常基岩隐藏在一层土壤下，即使暴露在外，也只有基岩最高表面的断裂可见。一种很有前途的新方法是利用浅层地震勘探来探测山坡顶部10-20米，并将地震速度随深度的变化转换为裂缝密度随深度的变化。这项研究将通过在天然和人工基岩暴露中测试和校准这种新生技术来探索这种技术，其中裂缝密度先前已经量化。随后，这种浅层地震方法（使用背包式便携式阵列）将用于测试（i）冻融过程的强度和频率以及（ii）山坡曲率和陡度的变化如何影响岩石破裂的密度和深度。总体目标是提高我们的能力，以有效地评估两个山坡的侵蚀或滑坡失败的脆弱性和植物，天气和地形对山坡稳定性和基岩断裂的影响。

The primary goal of this study is to further develop, calibrate, and test a seismic refraction-based methodology to investigate subsurface rock-strength properties and fracture distributions. The strength and coherence of rocks play key roles in shaping landscapes, resisting erosion, and modulating landslide hazards. The key factor, however, is not the strength of intact rock, but rather the effective strength of the entire rock-mass at the surface where it interacts with climatic, hillslope, and biotic variables. This effective strength is modulated by the development of fractures that weaken the rock mass and make it more susceptible to erosion, physical and chemical weathering, biologic activity, or collapse. To delineate variations in fracture density in the shallow subsurface, a promising, but largely unexplored methodology combines shallow seismic refraction surveys of bedrock outcrops with laboratory analyses of ?intact? samples. Initial results indicate two common fracture patterns versus depth: rock that is uniformly fractured (apparently by large-scale tectonic forces); and rock with a distinct fracture gradient in an upper layer (apparently due to climatic and biotic fracturing processes) that overlies a much stronger, less fractured lower layer. Although very promising, this methodology needs to be refined, tested, and explored more thoroughly. Hence the goal of this study. By focusing the method development and calibration on artificial and natural bedrock exposures that permit detailed observation, measurement, and sampling of fracture properties, the seismically-derived results can be tested and validated by direct comparison to field observations. With an improved calibration in hand, two fundamental questions about near-surface fracturing will be investigated: how do near-surface fracture patterns vary both with depth and spatially across the landscape; and what are the dominant controls on near-surface fracture formation? In a mountainous field site in Colorado, the relative importance of (i) freeze-thaw processes in causing rock fracturing versus (ii) gravitational forces that cause fracturing due to hillslope steepness and curvature will be investigated. Why do landslides occur on some hillslopes, but not on others of equal steepness? Why do parts of the landscape erode much more quickly than other, similar appearing areas? One key control on erosion or collapse of hillslopes is the strength of the underlying rock. Whereas various rock types typically have different intrinsic strengths (a granite versus a mudstone, for example), the density of fractures in a rock also exerts a fundamental control on its strength: higher fracture densities and greater connectivity among the fractures weaken a rock and increase its susceptiblility to landsliding or erosion. Recent research suggests that fracture densities are at least as important as intrinsic rock strength in controlling hillslope stability. Despite the importance of rock fracturing for hillslope stability, methodologies for quantifying fracture densities have remained elusive: commonly the bedrock is hidden under a layer of soil, and even when exposed, only fractures on the topmost surface of the bedrock are visible. A promising, new approach uses shallow seismic surveys to probe the top 10-20 m of a hillslope and convert variations in seismic velocity with depth into changes in fracture density with depth. This research will explore this nascent technology by testing and calibrating it in natural and artificial bedrock exposures where the fracture density has been previously quantified. Subsequently, this shallow seismic methodology (which uses backpack-able portable arrays) will be used to test how variations in (i) the intensity and frequency of freeze-thaw processes and (ii) hillslope curvature and steepness influence the density and depth of rock fracturing. The overall goal is to improve our ability to efficiently assess both hillslope vulnerability to erosion or failure by landsliding and the impact of plants, weather, and topography on hillslope stability and bedrock fracturing.