Investigation of Fluid-Seismic Processes Associated with the Bayou Corne, LA Sinkhole

2012年6月，路易斯安那州康恩湾的居民开始报告发生地震，并在当地水域出现不寻常的气体冒泡。美国地质勘探局在该地区部署了宽带地震仪器，并证实在拿破仑维尔盐丘西侧附近发生了一群浅层地震。2012年8月2日，该地点形成了一个地陷，里面充满了水、原油和残骸的泥浆，导致当局宣布进入紧急状态，并疏散附近的居民。对地震记录的初步分析表明，个别地震的震源特征表明体积增加，这被认为是由于盐丘侧壁洞室坍塌导致的岩石裂缝导致高压流体和气体从深层储层迁移到地表。河口地震的许多地震记录都具有在火山和地热系统中观察到的特征，在这些系统中，深层流体可以产生各种地震和震颤现象。对Bayou Corne地震活动性的研究将揭示流体控制的震源机制，对地震信号随时间变化的调查将有助于了解系统如何演变，以及导致天坑形成的破坏过程。在研究过程中采用和改进的方法将更广泛地提高地震监测能力和与正在进行的采矿、石油和天然气生产、废水处理以及火山活动监测有关的危害评估。在整个项目过程中，将调查在天坑形成前几周内发生的数千次小地震（0到2级），以及在天坑形成后继续发生的地震。一种连续扫描地震信号的方法，能够自动检测、定位和估计震源参数，用于确定完整的地震矩张量，这些地震矩张量能够分辨剪切和拉伸分量，从而了解系统中发生的变形的性质以及充气流体在震源过程中的重要性。前期工作采用简化的一维地震速度模型来描述地震波从震源到受震源的传播，但盐丘环境的地质情况非常复杂，本研究将采用三维地震速度模型和适当的数值编码来生成三维传播传递函数，以提高震源机制的成像。除了微地震外，还有类似火山地震的调和地震观测，表明流体通过裂缝网络迁移的过程。也有长周期（20秒）震颤，初步模拟表明它们是体积源的时间脉冲。甚至还有更长的周期信号，可以解释为地面的广泛倾斜。利用流体-裂缝动力学模型和其他力学模型对各种信号进行建模，研究它们之间的关系，以及与盐丘内部和附近地质物质破坏过程和天坑形成的关系。

In June of 2012 the residents of Bayou Corne, Louisiana began reporting the occurrence of earthquakes, and unusual gas bubbling in the local waters. The USGS deployed broadband seismic instruments in the region and confirmed that a swarm of shallow earthquakes was occurring near the west flank of the Napoleonville Salt Dome. On August 2, 2012 a sinkhole formed at the location, filled with a slurry of water, crude oil and debris, leading to a declaration of emergency and evacuation of nearby residents. Preliminary analysis of the seismic records revealed that the individual earthquakes have a source signature that indicates a volume increase, which is believed to be due to high-pressure fluid and gas migrating from the deeper reservoirs to the surface through rock fractures resulting from the collapse of a side wall cavern in the salt dome. Many of the seismic records for earthquakes at Bayou Corne share features that are observed in volcanic and geothermal systems in which fluids at depth can generate a variety of earthquake and tremor phenomena. The study of the Bayou Corne seismicity will shed light on fluid-controlled seismic source mechanisms, and the investigation of how the seismic signals changed over time will lead to understanding of how the system evolved, and the process of failure that lead to the formation of the sinkhole. The methods employed and refined through the course of the study will more broadly improve seismic monitoring capability and hazard assessment associated with ongoing mining, oil and gas production, wastewater disposal, as well as the monitoring of volcanic activity.Through the course of the project the thousands of small (magnitude 0 to 2) earthquakes in the weeks prior to the formation of the sinkhole, and continuing afterward will be investigated. A method in which the seismic signals are continuously scanned enabling the autonomous detection, location and estimation of seismic source parameters used to determine full seismic moment tensors that are capable of resolving both shear and tensile components that will inform on the nature of deformation occurring in the system and the importance of gas-charged fluids in the seismic source process. Preliminary work utilized simplified one-dimensional seismic velocity models to describe the wave propagation from source to receiver, however the geology in salt dome environments is very complex, and the proposed research will utilize a three-dimensional seismic velocity model, and appropriate numerical codes, to generate three-dimensional propagation transfer functions to improve the imaging of the earthquake source mechanisms. In addition to the micro earthquakes there are observations of harmonic tremor that are similar to volcanic tremor and indicate a process of fluid migration through a network of fractures. There are also long-period (20s) tremor in which preliminary modeling has suggested they are the temporal pulsing of a volumetric source. There are even longer period signals that can be interpreted as broad tilting of the ground. The diverse signals will be modeled with fluid-crack dynamic models and other mechanical models to investigate the relationships between them, and to the failure process of the geologic materials within and adjacent to the salt dome, and the formation of the sinkhole.