Seismometer off Kushiro-Tokach

Seismometer off Kushiro-Tokachi

There are three seabed seismometers installed in the Kushiro-Tokachi System.

* An expanded version of an exterior photograph of the seabed seismometer may be viewed by clicking the image at the right.

1. Outline
2. Specifications
3. Correction Parameters, etc.

1. Outline

Each seabed seismometer is loaded with an acceleration detector of three orthogonal constituents X,Y and Z, and one hydrophone sensor. Each acceleration detector converts servo circuit current output into direct current voltage and outputs it. Moreover, each acceleration detector is provided with a thermometer, and temperature data is outputted by voltage output. Acceleration detector output is branched out in sensor units. Then there is output upon amplification of two gains. At this point, sensitivity is set.

2. Specifications
2.1 Sensor Unit	Detector	1) Detector model	Servo accelerometer (JA-5III-A, 3 units)
		2) Number of constituent parts of detector	3 components
		3) Detector sensitivity	1.33 mA/G
		4) Detector resolution	1×10^-6p-p or less
		5) Detector bias error	± 4 mG or less
		6) Detector bias temperature coefficient	10 μG / °C
		7) Detector current sensitivity, temperature coefficient	200 ppm / °C
		8) Detector bias stability	100 μG per annum or less
	Amplifier	1) Input signal number	Seismometer signal	3 components
			Thermometer signal	3 components
		2) Output signal number	Seismometer signal	Low gain : 3 ch High gain : 3 ch
			Thermometer signal	3 ch
		3) Amplification range	Low gain	DC to 200 Hz
			High gain	0.05 to 200 Hz
	General Characteristics	1) Amplification sensitivity	Seismometer signal	Low Gain : ± 1.5 V/G High Gain : ± 150 V/G
			Thermometer signal	approx. 110 mV / °C
		2) Thermometer stability	Output stability of temperature during certain time: 0.02 °C per annum or less
2.2 Transmission Part Unit	A/D Converter	1) A/D conversion bit number	24 bits
		2) Sampling frequency	1 kHz ± 100 ppm
		3) Input level range	± 4.5 V
		4) Filter band	DC to 200 Hz
2.3 Data Processing Part	Digital Filter	24-bit data sent from seabed seismometer is converted into 32 bits by land apparatus, digital filtering (Fc = 40 Hz ) processing is performed, 100 Hz sampling is performed, and recordings are made and sent to JAMSTEC in Yokosuka.
2.4 General Characteristics of OBS Data	Low gain	Transit area : 0 to 40 Hz, block area : 50 Hz or more
	High gain	Transit area : 0.5 Hz to 40 Hz, block area : 50 Hz or more, 6 dB/oct (low area).

3. Adjustment Parameters, etc.
3.1 Location of Installation	As shown in the diagram of the cable route map , the seabed seismometers are installed approx. 100 to 150 km off Kushiro in offshore waters. The location of installation is as shown in Table 1.
3.2 Direction of Installation	Regarding the seabed seismometers, for construction reasons, the direction of the cables is not in a north-south direction, because several devices are installed in a series. Therefore, in order to have earthquake analysis data (N-S, E-W, U-D), it is necessary to adjust the direction. The direction of installation of the devices is as shown in Table 2. 0 degrees (360 degrees) is in a northerly direction, with angle indication shown clockwise (see Fig. 1 for reference).
3.3 Angle of Installation and Polarity	The OBS is cylindrical, and its polarity is as shown in Fig. 2. As mentioned previously, the OBS is of an enclosure structure, so when laying the cable, the cable is grounded at the radial angle of the cable. Meanwhile, the surface of the seabed is not entirely flat. Thus, as the device is not equipped with a gimbal, a dynamic range has been provided to enable sensing in line with gravity. Consequently, in order to have earthquake analysis data (N-S, E-W, U-D), it is necessary to have adjustment for the offset gravity portion. The current offset is as shown in Table 3.

Table 1 Location of Installation of Seismometers(WGS-84)

	Lat.(°)	Long.(°)	Depth (m)
OBS1	41.6807N	144.3945E	2,329
OBS2	41.9408N	145.0562E	3,428
OBS3	42.2528N	144.8107E	2,124

Table 2 Direction of Installation of Seismometers

	OBS1	OBS2	OBS3
Direction (degrees)	243.82	170.69	122.53

Table 3 Seismometer Offset

	X-axis(V)	Y-axis(V)	Z-axis(V)	remarks
OBS1	0.060	1.104	-1.043	Cable deployment (1999/07)
OBS1	-0.006	-1.513	-0.002	as of 2007/01/31
OBS2	0.017	-0.033	-1.504	Cable deployment (1999/07)
OBS2	0.054	-1.511	0.036	as of 2007/01/31
OBS3	-0.034	0.168	-1.503	Cable deployment (1999/07)
OBS3	0.057	-1.353	0.674	as of 2007/01/31

Fig. 1 Direction of Seismometer Respective of Position List

Fig. 2 Diagram of Seismometer Sensor Polarity