Seismic Method and Hydrate

by Manabu Tanahashi

(This page is under-construction as other pages under www.aist.go.jp/GSJ/dMG.)

Multi-channel Seismic (MCS) method is one of the key technologies for detecting methane gas hydrate deposits in the deep ocean. Methane gas hydrate is supposed to exist in the fairly shallow subbottom (less than several 100 m) in the deep ocean because that the subbottom temperature increases and the hydrate is instable in the deep. The phase bondary between upper gas hydrate and lower gas and water makes a prominent seismic reflection because of the very big acoustic impedance difference between them.

What is the Multi-Channel Seismic (MCS)?

Multi-Channel Seismic reflection profiling is a popular exploration technology to find oil and gas resources in the offshore area. The principle of the method is same as sonar and radar. We use strong impulsive sound source such as air-gun array, and many channels of receiver (hydrophones in a very long hydrostreamer) to detect reflection signals from seabottom and subbottom geologic interfaces which have acoustic impedance contrast. Reflection signal time series data, which has a series of incident angles to the reflectors depending the variety of offsets between seismic source and the receiver channels, are collected for each source shots and recorded digitally without distortion. The shot and recording will be repeated periodically after moving the survey ship to the next shot point. The distance between shot points is kept as constant. Then we may edit the Common Mid Point (CMP) data from the record.

What is the Common Mid Point?

Choosing the suitable pairs of shot and data-channel as shown the figure, we may collect the reflection time series (seismic traces) data set which transmited through the "same" reflection point. The point is called as Common Mid Point, CMP (sometimes as Common Depth Point, CDP, too).

Normal MoveOut and CMP stack

Normal Move Out correction is the process to cancel the travel time delay caused by the geometry of reflection with the estimated velocity. On the offset and time chart, the reflection signal from the same reflector will be align horizontal. After the NMO correction the every traces of the CMP gather may simply be summed and result a noise reduced trace (CMP stack).

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This page is designed by M. Tanahashi
Last modified on 1996/04/19.