Development of a Monitoring System on the Methane Hydrate Exploration
Authors: Hayashi Tsutomu , Asakawa Eiichi , Okamoto Taku , Takahashi Hiroo , Saeki Tatsuo 
Published: March 2015

A 2D multi-component seismic survey was carried out in the Nankai Trough using the RSCS (Real-time Seismic Cable System) in FY2006. The RSCS is the newly developed ocean bottom cable system which is usable in more than 2000m water depth. The results of the PP and PS components gave us much information of the methan hydrate bearing zone. Based on RSCS technology, we are developing a new monitoring system (DSS: Deep-sea Seismic System) using multi-component seismic sensors to delineate the methane hydrate dissociation zone for the offshore methane hydrate production test scheduled in FY2012.

Conventional RSCS is composed of three component gimbaled geophones which require a large volume inside the receiver. We will adopt accelerometers to achieve a small receiver that is 2/3 the size of conventional RSCS. The accelerometer data can be corrected into horizontal or vertical directions based on the gravity acceleration. The receiver case has a protective metallic exterior and the cable is protected with steel-screened armoring, allowing for burial usage using ROV for sub-seabed deployment. It will realize a unique survey style that leaves the system on the seabed between pre-test baseline survey and post-test repeated survey, which might be up to 6 months. The fixed location of the receiver is very important for time-lapse monitoring survey. We conducted a feasibility study to detect the methane hydrate dissociation with this monitoring system.

The first experiment of the new monitoring system in a marine area is planned for autumn FY2011, just during this conference. We will present our monitoring system, the result of the numerical simulation, and the outline of the first experiment.

Introduction

A 2D RSCS (Real-time Seismic Cable System) Survey was carried out by JGI and NSC (Nippon Salvage Corp.) under the financial support of JOGMEC in Dec. 2006, in the area Shima-Oki, near the edge of the Nankai Trough (Asakawa et al., 2008, Asakawa et al., 2010, Takahashi, 2005 and Takahashi et al., 2007). The survey area covered part of the JOGMEC 3D survey and is known to have a strong BSR event. The RSCS is the newly developed ocean bottom cable system which is usable in more than 2000m water depth. The quality of the field data is excellent with high vector fidelity. The vertical component data was imaged using P-wave OBS pre-stack time migration, including Vp migration velocity analysis. The resulting sections showed excellent agreement with the 3D survey migrated data volume. The in-line horizontal component data was imaged using C-wave (PS converted wave) OBS pre-stack time migration, including Vc migration velocity analysis and updates to the gamma ratios. The resulting C-wave sections showed amplitude anomalies above the BSR level. This fact indicates S-wave velocity anomaly and gives useful information to delineate the elastic properties of Methane Hydrate Deposit in the Nankai Trough.

JOGMEC is planning an 'offshore methane hydrate production test' in the Nankai Trough in FY2012. During the production test by depressurization method, methane hydrate shall dissociate. We plan to estimate the dissociation area of methane hydrates using the technology of multi-component seismic with RSCS. Because methane hydrate dissociates to fluid (methane and water), we consider that P-wave attenuates quickly. Therefore PS converted wave is essential to monitor the dissociation process.

We designed the new RSCS (DSS: Deep-sea Seismic System) by adopting accelerometers to achieve a small receiver that is about 2/3 the size of those for conventional RSCS. The accelerometer data can be corrected into horizontal or vertical directions based on the gravity acceleration without gimbal mechanism. The receiver case has a protective metallic exterior and the cable is protected with steel-screened armoring, allowing for burial usage using ROV for sub-seabed deployment.

The fixed location of the receiver by burial is very important for long term monitoring surveillance.