Double-difference attenuation tomography method and pilot applications

地震学家利用地震波构建地球内部结构的图像，称为地震断层扫描，类似于医生使用x射线通过CAT扫描（计算机轴向断层扫描）构建人脑图像的方式。在地震学中，断层扫描最常使用测量到的地震波的传播时间来推断地球内部的地震波速度结构。另一种可能性是利用不同频率地震波的振幅来推断地球的衰减结构，也就是说，确定地震波比正常情况下损失能量更多或更少的区域。在这个项目中，研究人员将开发和测试一种新的方法来成像衰减结构，具有精细的空间分辨率，使用测量到的衰减差异从对附近地震的波。该方法的有效性将在日本本州北部和加州间歇泉地热田的地震数据上进行评估。在前一种情况下，研究人员将努力对地球深处的三维衰减变化进行成像，这将为了解“火环”火山喷发的岩浆起源区域以及深层地震的性质提供洞见。在后一种情况下，目标是检测地热系统内衰减结构随时间的变化，这可以用来监测由于地热开采或增产而导致的压裂变化。这个项目将开发一种新的方法，称为？双差衰减层析成像？，这将在合成数据上进行测试，并应用于两个感兴趣的区域。这个概念是对地震衰减成像所做的，就像双差定位对震源定位和双差层析成像对震源定位和地震速度成像所做的那样。关键的发展是事件对微分t*分析，它与以前的微分t*分析方法形成对比，后者通常使用相对或站对方法。将使用附近事件对的频谱比方法来获得事件对微分t*值，类似于如何使用波形互相关来获得双差定位和双差断层扫描的微分时间。这种方法将在合成数据和实际数据上进行彻底的测试。选择了两个地区应用新的双差衰减层析成像方法：加利福尼亚的Geyser地热田和日本的北本州。这些领域的选择既有其内在的兴趣，也有以往研究结果的可用性，我们的结果可以进行比较。这种方法有可能在地震发生的地区获得非常高分辨率的地震衰减结构图像。地震衰减是地震速度的有力补充，有助于区分地下岩性、饱和度、压裂和温度，这些因素会对速度和衰减产生不同的影响。在我们设想的一些应用中，衰减时间变化的高分辨率检测是一个可以实现的目标。许多可能的应用都是针对具有高度社会重要性的问题，如地热能、采矿、废水处理、水力压裂、二氧化碳封存和火山。该项目将支持一名女性早期职业科学家和一名研究生。

Seismologists use seismic waves to construct images of the Earth's internal structure, termed seismic tomography, analogous to the way doctors use X-rays to construct images of the human brain with CAT scans (Computerized Axial Tomography). In seismology, tomography is most often done using measured travel times of seismic waves to infer the seismic wave velocity structure within the Earth. Another possibility is to use the amplitudes of seismic waves at different frequencies to infer the Earth's attenuation structure, that is, to identify regions where seismic waves lose energy more or less than normal. In this project, the researchers will develop and test a new method to image attenuation structure, with fine spatial resolution, using measured attenuation differences between waves from pairs of nearby earthquakes. The effectiveness of the method will be evaluated on earthquake data from northern Honshu, Japan, and The Geysers geothermal field, California. In the former case, the researchers will endeavor to image three-dimensional attenuation variations deep within the Earth, which will provide insight into the zone where magmas originate that erupt at "Ring of Fire" volcanoes and also into the nature of deep earthquakes. In the latter case, the goal is to detect changes in attenuation structure within the geothermal system over time, which can be used to monitor fracturing changes due to geothermal exploitation or stimulation.This project will develop a new method termed ?double-difference attenuation tomography?, that will be tested on synthetic data, and applied to two regions of interest. The concept is to do for seismic attenuation imaging what double-difference location has done for hypocenter locations and double-difference tomography has done jointly for hypocenter locations and seismic velocity imaging. The key development is event-pair differential t* analysis, which contrasts with previous differential t* analysis methods, which have generally utilized either a phase-pair or a station-pair approach. A spectral ratio approach for nearby event pairs will be used to obtain the event-pair differential t* values, analogous to how waveform cross-correlation is used to obtain differential times for double-difference location and double-difference tomography. This method will be thoroughly tested on both synthetic and real data. Two areas have been selected for applying the new double-difference attenuation tomography method: The Geyser geothermal field, California, and Northern Honshu, Japan. These areas were chosen both for their intrinsic interest and for the availability of results from previous studies to which our results can be compared. This method has the potential for obtaining very high resolution images of seismic attenuation structure in the regions where earthquakes occur. Seismic attenuation is a strong complement to seismic velocity in efforts to distinguish lithology, saturation, fracturing, and temperature in the subsurface, and these factors can affect velocities and attenuation differently. In some of the applications that we envision, high-resolution detection of temporal changes in attenuation is a realizable goal. Many of the possible applications are to problems of high societal importance, such as geothermal energy, mining, wastewater disposal, fracking, CO2 sequestration, and volcanoes. The project will support a female early career scientist and a graduate student.