A Low-Power Global Positioning System-Acoustic Payload to study the subduction zones offshore the Pacific Northwest and Alaska

由于地震本身和由此引发的海啸，日本、智利和早些时候的苏门答腊岛附近发生的巨大俯冲带地震造成了广泛的破坏。类似类型的俯冲带断层位于太平洋西北部的近海，即卡斯卡迪亚俯冲带(CSZ)。它上一次破裂是在1700年，早在人口稠密的社区广泛建立之前很久。从那时起，CSZ一直在以每年几厘米的速度在地壳中建立弹性应变。在未来，这种储存的弹性能量很可能会以强烈的地震和海啸的形式释放出来。大部分弹性应变正在积累在海岸附近的海底。通过测量近海这种应变的缓慢积累，可以更好地了解潜在的地震和海啸风险。通过将全球定位系统和海面平台上的声波测距系统以及海底的声波应答器结合起来，可以测量弹性应变积累。小型(冲浪板大小)的波浪和太阳能平台现在可以取代大型船只进行大量数据收集。然而，这需要对车载电子元件进行重新设计，使其更小、更低功率，这是本项目的目标。全球定位系统-声学(GPS-A)方法已被应用于波浪滑翔机，这是一种遥控、波浪和太阳能海面飞行器。波浪滑翔机的资金将持续到2017年，以测量卡斯卡迪亚俯冲区三个海底地点的板块变形。在9月2014年，我们在俄勒冈州中部90海里3000米深的海域收集了30小时的GPS和声学数据。初步结果证实，浪潮滑翔机是卡斯卡迪亚高成本船只的替代品，可能在其他全球地点也是如此。然而，我们发现，目前的GPS-惯性导航系统(GPS-INS)和波浪滑翔机上的计算机的功率需求大约为22瓦(W)。这将数据收集限制在24-30小时，在波浪滑翔机上的660瓦时电池组耗尽之前。此时，GPS-A的运行必须暂停2-3天，同时太阳能电池板为机载电池充电。我们和日本研究人员的研究表明，厘米级定位需要4-5天的GPS-A测量。由于数据收集必须在大约24小时后暂停才能为电池充电3天，因此需要16-20天才能在单个站点收集累积的5天GPS-A数据。再加上3-5天的站点间过境，在卡斯卡迪亚3个月的夏季天气窗内，只能访问和测量3到5个GPS-A站点。这为天气延误、机械故障等留出了很少的应急时间。随着更多卡斯卡迪亚近海海底站点的提出和增加，效率低下的情况被放大。该项目将用低功耗版本取代现有的GPS-INS和车载CPU，这些版本只消耗6-8W的功率，然后GPS-A数据可以连续收集长达5天。随着波浪滑翔机在海底地点之间移动，船上的电池会在几天内充电。这是在卡斯卡迪亚、阿留申群岛以及可能的其他地区使用的一种有效的操作方法。

Great subduction zone earthquakes offshore Japan, Chile, and earlier Sumatra caused extensive damage due the earthquake itself and the resulting tsunami. A similar type of subduction zone fault lies offshore the Pacific Northwest, the Cascadia Subduction Zone (CSZ). It last ruptured in 1700 long before the extensive establishment of populated communities. Since then the CSZ has been building up elastic strain in the crust at the rate of a few centimeters per year. In the future, this stored elastic energy will likely be released as a powerful earthquake and tsunami. Much of the elastic strain is accumulating the seafloor offshore the coast. By measuring the slow buildup of this strain offshore, a better understanding of the potential earthquake and tsunami risk can be established. By combining the Global Positioning System and an acoustic ranging system on a sea surface platform, and acoustic transponders on the sea floor, the elastic strain buildup can be measured. Small (surfboard-size) wave- and solar-powered platforms can now replace large ships for much data collection. However, this requires re-engineering the onboard electronic components to be smaller and lower power, which is the goal of this project. The Global Positioning System-Acoustic (GPS-A) approach has been adapted to a Wave Glider, a remotely controlled, wave- and solar-powered sea-surface vehicle. The Wave Glider is funded through 2017 to measure plate deformation at three seafloor sites in the Cascadia Subduction Zone. In Sept. 2014, we collected 30 hours of GPS and acoustic data at a 3000-m-deep site 90 Nm offshore central Oregon. Preliminary results validate the Wave Glider as a replacement for high-cost ships in Cascadia, and likely at other global sites. We found, however, the power requirements of the present GPS-Inertial Navigation System (GPS-INS) and the computer onboard the Wave Glider draw about 22 Watts(W). This limits data collection to 24-30 hours, before the 660 Watt-Hour battery bank aboard the Wave Glider is depleted. At this point, GPS-A operations must be suspended for 2-3 days, while the solar panels recharge the onboard batteries. Our research and that of Japanese researchers demonstrated that 4-5 days of GPS-A measurements are needed for centimeter-level positioning. Because data collection must be suspended after only ~24 hours to charge batteries for 3 days, it takes 16-20 days to collect the cumulative 5 days of GPS-A data at a single site. Adding 3-5 days for transiting between sites, only three to five GPS-A sites can be visited and measured over a 3 month summer weather window in Cascadia. This allows little contingency time for weather delays, mechanical problems, etc. As more seafloor sites offshore Cascadia are proposed and added, the inefficiency is magnified. This project will replace the existing GPS-INS and onboard CPU with low power versions that consume only 6-8 W. GPS-A data could then be collected continuously for up to 5 days. The onboard batteries are recharged over the few days as the Wave Glider transits between seafloor sites. This is an efficient operational approach for use in Cascadia, the Aleutians, and potentially other regions.