Seismometer off Muroto Cape

Seismometer off Muroto Cape

Two seabed seismometers are installed in the Muroto System.

* Click the image at the right to enlarge and display the photograph of the exterior of the seabed seismometer.

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

1. Outline of Seismometer

Each seabed seismometer is loaded with an acceleration detector of three orthogonal constituents X, Y and Z. 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 outputting upon amplification of two gains. At this point, sensitivity is set.

2. Specifications
2.1 Sensor Unit	Detector	1) Model	Servo accelerometer (JA-5III-A, 3 units)
		2) Number of constituent parts of detector	3 components
		3) Sensitivity	1.33 mA/G
		4) Resolution	1×10^-5 Gp-p or less
		5) Bias error	± 4 mG or less
		6) Bias temperature coefficient	10 μG / °C
		7) current sensitivity, temperature coefficient	200 ppm / °C
		8) Bias stability	10 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 : 3ch High gain : 3ch
			Thermometer signal	3ch
		3) Amplification range	Low gain	DC to 200 Hz
			High gain	0.05 Hz to 200 Hz
	Characteristics of Earthquake Sensor Unit	1) Amplification sensitivity	Seismometer signal	Low gain : ± 5 V/3G High gain : ± 1,500 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 converter bit number	16 bit
		2) Sampling frequency	8 kHz ± 100 ppm
		3) Input level range	± 4.5 V
		4) Filter band	DC to 200 Hz
2.3 Data Processing Part	Digital Filter	16-bit data sent from seabed seismometer is converted into 20 bits by land apparatus, digital filtering (Fc = 40 Hz) processing is carried out, 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: 0.05 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 cable route map , the ocean bottom seismometers are installed approx. 80 km off the Muroto Cape, and approx. 110 km below the seabed. The location of installation is as shown in Table 1.
3.2 Direction of Installation	Regarding the ocean bottom 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 the installation of the seismometers 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 seismometer is cylindrical, and its polarity is as shown in Fig. 2. As mentioned previously, the seismometer 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 (N-S, E-W, U-D), it is necessary to have adjustments 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)	remarks
OBS1	32.4183N	134.4358E	2,087	Valid until 2005/08/16
OBS1	32.4200N	134.4375E	2,076	Valid after 2006/03/28
OBS2	32.6746N	134.3614E	1,267	-

Table 2 Direction of Installation of Seismometers

	Direction (degrees)	remarks
OBS1	131.79	Valid until 2005/08/16
OBS1	130.0	Valid after 2006/03/28
OBS2	137.64	-

Table 3 Seismometer Offset

	X-axis(V)	Y-axis(V)	Z-axis(V)	remarks
OBS1	-0.207	1.616	-0.353	Cable deployment (1997/03)
OBS1	-0.531	1.265	-0.871	as of 2007/01/31
OBS2	-0.112	-1.616	-0.073	Cable deployment (1997/03)
OBS2	-0.107	-1.617	-0.085	as of 2007/01/31

Fig. 1 Direction of Seismometer Respective of Position List

Fig. 2 Diagram of Seismometer Sensor Polarity