Tidal calibration of Plate Boundary Observatory borehole strainmeters: Roles of vertical and shear coupling
Tidal calibration of Plate Boundary Observatory borehole strainmeters: Roles of vertical and shear coupling
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DOI：
10.1029/2009jb006407
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发表时间：
2010-06
影响因子：
--
通讯作者：
E. Roeloffs
中科院分区：
文献类型：
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作者：
E. Roeloffs
[1]多分量钻孔应变仪直接测量其圆柱形外壳在几个方位角处的直径变化。为了将这些测量值转换为地层应变，需要校准矩阵，该校准矩阵必须通过分析安装的应变仪对已知应变的响应来估计。通常，地球潮汐应变的理论计算作为已知应变。本文对12个板块边界观测站（PBO）钻孔应变仪进行了这样的分析，假设应变仪的四个应变计中的每一个都对地层应变张量的所有三个水平分量以及垂直应变作出响应（“耦合”）。还估计了方向校正。每个PBO应变计中的第四个引伸计提供冗余信息，用于减少耦合系数可能被误导地拟合到不适当的理论潮汐的机会。满意的拟合观测和理论计算的潮汐之间得到了三个PBO应变计在加州，其中计算的潮汐是由其他仪器，以及六个应变计在俄勒冈州和华盛顿，没有其他仪器记录地球潮汐应变证实。几种应变仪具有意想不到的大垂直应变耦合系数，这增加了应变仪对大气压力的响应。垂直耦合减小，甚至改变的迹象，明显的响应面应变引起的地球潮汐或地球深部过程，因为附近的自由表面，垂直应变是相反的符号面应变。然而，垂直耦合不损害剪切应变响应。PBO钻孔应变仪可以提供与构造或岩浆过程相关的瞬态应变的校准的剪切应变时间序列。
[1] A multicomponent borehole strainmeter directly measures changes in the diameter of its cylindrical housing at several azimuths. To transform these measurements to formation strains requires a calibration matrix, which must be estimated by analyzing the installed strainmeter's response to known strains. Typically, theoretical calculations of Earth tidal strains serve as the known strains. This paper carries out such an analysis for 12 Plate Boundary Observatory (PBO) borehole strainmeters, postulating that each of the strainmeters' four gauges responds (“couples”) to all three horizontal components of the formation strain tensor, as well as to vertical strain. Orientation corrections are also estimated. The fourth extensometer in each PBO strainmeter provides redundant information used to reduce the chance that coupling coefficients could be misleadingly fit to inappropriate theoretical tides. Satisfactory fits between observed and theoretically calculated tides were obtained for three PBO strainmeters in California, where the calculated tides are corroborated by other instrumentation, as well as for six strainmeters in Oregon and Washington, where no other instruments have ever recorded Earth tidal strain. Several strainmeters have unexpectedly large coupling coefficients for vertical strain, which increases the strainmeter's response to atmospheric pressure. Vertical coupling diminishes, or even changes the sign of, the apparent response to areal strain caused by Earth tides or deep Earth processes because near the free surface, vertical strains are opposite in sign to areal strain. Vertical coupling does not impair the shear strain response, however. PBO borehole strainmeters can provide calibrated shear strain time series of transient strain associated with tectonic or magmatic processes.