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

这项研究的主要目标是进一步开发、校准和测试一种基于地震折射的方法来研究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，而是地表整个岩石的有效强度，在那里它与气候、山坡和生物变量相互作用。这种有效强度受到裂隙发育的调节，裂隙削弱了岩石，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。为了描绘浅层地下裂缝密度的变化，一种前景看好但基本上未被探索的方法是将基岩露头的浅层地震折射调查与完整的实验室分析相结合。样本。初步结果表明，随着深度的变化，有两种常见的破裂模式：一种是均匀破裂的岩石(显然是受到大规模构造力的作用)；另一种是上层岩石具有明显的断裂梯度(显然是由于气候和生物破裂过程)，后者覆盖在强度更大、裂缝更少的下层之上。虽然这种方法很有前途，但仍需要改进、测试和更彻底地探索。因此，这项研究的目标是。通过将方法的开发和校准集中在人工和自然的基岩暴露上，从而允许对裂缝特性进行详细的观察、测量和采样，可以通过与现场观测的直接比较来检验和验证从地震中得出的结果。有了改进的校准，关于近地表破裂的两个基本问题将被研究：近地表裂缝模式如何在整个地形上随深度和空间变化；以及对近地表裂缝形成的主要控制因素是什么？在科罗拉多州的一个山区现场，将调查(I)冻融过程导致岩石破裂的相对重要性，以及(Ii)由于山坡陡峭和弯曲而导致破裂的重力的相对重要性。为什么在一些山坡上会发生山体滑坡，而在另一些陡峭的山坡上却不会发生山体滑坡？为什么景观的某些部分比其他类似出现的区域侵蚀得更快？控制山坡侵蚀或坍塌的一个关键因素是下伏岩石的强度。虽然不同类型的岩石通常具有不同的内在强度(例如，花岗岩和泥岩)，但岩石中的裂隙密度也对其强度施加了根本控制：更高的裂隙密度和裂隙之间更大的连通性会削弱岩石，增加其对滑坡或侵蚀的易感性。最近的研究表明，在控制山坡稳定性方面，裂缝密度至少与固有岩石强度一样重要。尽管岩石破裂对山坡的稳定性很重要，但量化裂缝密度的方法仍然难以捉摸：通常基岩隐藏在一层土壤下，即使暴露出来，也只能看到基岩最上面的裂缝。一种很有前途的新方法是利用浅层地震勘探探测山坡顶部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。这项研究将通过在自然和人工基岩暴露中对这项新技术进行测试和校准来探索这项新技术，在那里裂缝密度以前已经被量化。随后，这种浅层地震方法(使用可背包的便携式阵列)将被用来测试(I)冻融过程的强度和频率以及(Ii)山坡的曲率和陡度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，以有效地评估山坡易受山体滑坡侵蚀或破坏的能力，以及植物、天气和地形对山坡稳定性和基岩破裂的影响。

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.