Station Maintenance

Shown in Figure 5.3 are power spectral density (PSD) distributions of background noise for a sample of 8 NHFN land and bridge site stations. In general, background noise levels of the borehole HFN stations is more variable and generally higher than that of the Parkfield HRSN borehole stations (Figure 6.2). This is due in large part to the significantly greater level of cultural noise in the Bay Area, and to the fact that noise reduction efforts on the much more recently installed NHFN stations are still underway. For example the two noisiest stations (i.e. BBEB and W02B) are located on the Bay Bridge which is currently undergoing earthquake retrofit and east span reconstruction. These stations have also only recently come back on-line with upgraded infrastructure and instrumentation, so the full complement of noise reduction modifications have not yet been implemented.

Figure: Plot showing the HFN.BK.DP1 background noise, PSD, for 8 of the NHFN stations. Plotted are the background low-noise PSD estimates. Ten minutes of .BK.DP1 data starting at 2005.268.0900 (2 AM PDT) were used in the analysis. Note that there is considerable variation in the general level and structure of the individual station background noise PSD estimates. Some of the stations show peaks at 60 Hz and its harmonics while others have a high average background level. The two bridge sites, BBEB and W02B are the noisiest while land site BRIB in Briones Regional Park (well away from the heavy cultural noise of the more populated region of the Bay Area) is the quietest. Two stations, CMSB and HERB show a peak in the 20-30 Hz range. The peak at CMSB is probably due to excitation of modes in the open bore hole and the peak at HERB is due to excitation of the local structure by the adjacent railway line and highways 4 and 80. The three stations in the middle of the group (RFSB, SMCB and CRQB) are responding to the local cultural noise. There are numerous ongoing experiments at the Richmond Field Station which are affecting the noise level at RFSB, CRQB is sited near a sewage treatment plant and the Carquinez bridge, and SMCB is currently only installed at post hole depth (3.5 m) on the St. Mary's campus.

On average the MPBO NHFN sites are more consistent and quieter (Figure 8.4). This is due in large part to the greater depth of the MPBO sensors, the locations of MPBO stations in regions of generally less industrial and other cultural noise sources, and possibly to the absence of powered sensors (i.e. accelerometers) in their borehole sensor packages.

One of the most pervasive problems at NHFN stations equipped with the Q4120 data loggers is power line noise (60 Hz and its harmonics at 120, 180, and 240 Hz). This noise reduces the sensitivity of the MHH detectors. Whenever a NHFN station is visited, the engineer at the site and a seismologist at the BSL work together to expedite the testing process, especially when attempting to identify and correct ground-loop faults which generally induce significant 60, 120, 180, and 240 Hz seismic signal contamination due to stray power line signal pickup, generally inductively coupled and aggravated by the presence of ground loops.

Below is a synopsis of maintenance efforts performed over the past year for several NHFN stations that gives some idea of the ongoing maintenance and performance enhancing measures that we are continuing to implement.