2:45 PM - 3:00 PM
[HTT20-05] A preliminary report on a seismic reflection experiment using underwater speaker along the Arakawa river
Keywords:extremely shallow water, environment-friendly seismic source, Tokyo Bay, fault, gas
Recently, the activity of seismic reflection surveys has been restricted from the impact on environmental loads to marine ecosystem. As a result, for example, seismic reflection surveys were rarely conducted in the northern part of the Tokyo Bay, since the late 2000s. Moreover, no seismic survey was done along the rivers discharging into the Tokyo Bay, such as the Arakawa river. Accordingly, the restriction in seismic reflection surveys leads to lack of geological information, such as stratigraphy and fault, in the coastal zone of the Tokyo Bay. In other words, it makes geological evaluation in the boundary zone between land and sea so difficult.
In such a circumstance, we have been developing a new seismic survey system since 2017, which is available in extremely shallow waters that water depth is less than 10m. A non-impulsive seismic source is used for mitigating the environment impact on marine ecosystem. In addition, we need to make the survey system small because only a small boat can be used in the coastal areas, and then we adopt an underwater speaker: 20cm in diameter, 4.5kg in weight and 40W in standard input power.
In November 2021, a seismic reflection experiment has been conducted along the Arakawa river: from the estuary to the position that runs up 5km to the north. Data acquisition parameters are as follows:
Survey boat: Dai-san-amibenmaru (3.9tons)
Boat speed: 2-3knot
Source: an underwater speaker
Shot interval: 12s in standard
Shooting time: 10s
Recording: a hydrophone cable
Number of channels: 10
Channel spacing: 1m
Sampling interval: 0.25ms
As a result of the experiment, it is revealed that the seafloor gas, which was discovered in the northern part of the Tokyo Bay by the previous seismic experiments, extends to the estuary of the Arakawa river. Furthermore, several faults that appear to cut the reflections from the river bottom and/or near the bottom were observed. Interestingly, low-velocity anomalies suggesting gas were also observed around the faults.
In such a circumstance, we have been developing a new seismic survey system since 2017, which is available in extremely shallow waters that water depth is less than 10m. A non-impulsive seismic source is used for mitigating the environment impact on marine ecosystem. In addition, we need to make the survey system small because only a small boat can be used in the coastal areas, and then we adopt an underwater speaker: 20cm in diameter, 4.5kg in weight and 40W in standard input power.
In November 2021, a seismic reflection experiment has been conducted along the Arakawa river: from the estuary to the position that runs up 5km to the north. Data acquisition parameters are as follows:
Survey boat: Dai-san-amibenmaru (3.9tons)
Boat speed: 2-3knot
Source: an underwater speaker
Shot interval: 12s in standard
Shooting time: 10s
Recording: a hydrophone cable
Number of channels: 10
Channel spacing: 1m
Sampling interval: 0.25ms
As a result of the experiment, it is revealed that the seafloor gas, which was discovered in the northern part of the Tokyo Bay by the previous seismic experiments, extends to the estuary of the Arakawa river. Furthermore, several faults that appear to cut the reflections from the river bottom and/or near the bottom were observed. Interestingly, low-velocity anomalies suggesting gas were also observed around the faults.