Borehole array in Japan
Guralp Systems has recently provided instrumentation and technical assistance during an upgrade to a medium-sized array in Japan.
Before the upgrade, the array comprised 8 sites over an area 10 km in diameter. Boreholes had been drilled at six of these sites, and sensors concreted into the bottom of each borehole. By the time of installation, these boreholes had filled with water. Two further instruments had been installed in tunnels in the hillside.
As part of the upgrade:
- 7 new boreholes were drilled and equipped with CMG-3V instruments;
- CMG-3V instruments were installed in the 6 boreholes which already contained sensors;
- a final CMG-3V instrument, modified with a vault conversion base, was installed in one of the tunnels.
The station, now with 14 Guralp instruments installed, was fully tested on site.
The sensor, hole lock and winch were shipped separately and assembled in the field.
An aluminium tripod and winch, supplied by Guralp Systems, was used to install the borehole instruments.
The CMG-3V is supplied as standard with a three-jaw hole lock, which is suitable for installing in boreholes between 89 and 178 mm in diameter.
All Guralp instruments are comprehensively soak tested before shipping, so a water-filled borehole is not an obstacle.
Where instruments were installed in boreholes which already contained sensors, the hole lock units were modified with a groove to allow the data cable from the older instrument to pass.
Above the instruments, data and load-bearing cables were securely clamped together to avoid the data cable taking any weight.
All thirteen boreholes were equipped with strain relief units, modified where necessary with rings to allow the data cable from pre-existing instruments through.
The sensor installed in the tunnel is identical to those in the boreholes, but with a conversion base unit instead of the hole lock.
This instrument was thermally insulated and protected from extraneous vibration with a sand-filled enclosure.
Instruments were tested on-site with Guralp DM24 digitizers and Scream! software. Matlab extensions to Scream! were used to produce PSD plots from real-time data, allowing engineers to monitor the noise performance of the sensor during installation.