It is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area. Considering earthquake-generating stress to be uniform in a studied area, we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of P waves. First, the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution, with intervals of 10 ø. This calculation is made for all the events used. Then, four parameters def'ming the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum. A numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 x 25 readings. It is found that a stable solution of the stress tensor is determined by the inversion and about 60% of fault planes are distinguished from auxiliary planes. There are no events whose auxiliary planes are incorrectly determined as fault planes. This method is used to estimate the earthquake-generating stress in the aftershock area of the 1984 western Nagano prefecture earthquake, central Japan. The results obtained show that the maximum principal stress is horizontal in the direction of N80øW-S80*E. There are strike-slip events having fault planes almost perpendicular to the fault plane of the main shock. ments of orientations of in situ stress made in the area near the San Andreas fault show that the direction of the maximum principal stress is almost perpendicular to the strike of the San Andreas fault [Mount and Suppe, 1987; Zoback et al., 1987; Shamir et al., 1988; Zoback and Healy, 1992]. There is a large difference in the orientation of the principal stress estimated from focal mechanisms of individual events and that by the in situ stress measurements.Gephart and Forsyth [1984] considered that earthquakes occur along weak planes whose orientations are distributed randomly, and they developed a novel method to determine the stress Copyright 1995 by th• Am,6can •y•i•alUnion.method to focal mechanism solutions of microseismic events which were determined by using polarity data observed at 64 and 32 stations at two mines.However, in general, there is a large estimation error in individual focal mechanism solutions, especially in solutions determined by the use of a small number of P wave polarity observations. The large amount of estimation error makes it difficult to distinguish the fault plane from the auxiliary plane. Actually, most of the authors who use Gephart and Forsyth's method do not determine orientations of the fault planes. It is 8327 8328 HORIUCHI ET AL.: STRESS TENSOR INVERSION USING POLARITY DATA

Downhole seismic monitoring has become an alternative to more conventional surface seismic in cases where the latter has low signal-to-noise ratio or poor repeatability, or when frequent monitoring of a local target is required. Downhole 3D VSP monitoring has been applied to both onshore (O'Brien et al., 2004; Kiyashchenko et al., 2011) and offshore environments (Wu et al., 2011). In onshore environments, surface seismic repeatability often suffers from surface noise and near-surface variations caused by water-table changes or surface facilities construction. For VSP surveys, the wavefield separation into downgoing and upgoing fields presents an opportunity to discriminate between near-surface and reservoir changes, thus making time-lapse applications more effective.

A steam flood re-development of a mature field in the South of Oman is well underway with the first pattern of steam injection active since late 2008. The areal surveillance programme designed by a joint effort between Petroleum Development Oman (PDO) and Shell International Exploration and Production (SIEP) includes the simultaneous recording of a variety of seismic methods of different resolution. The main purpose of the time-lapse work is to monitor the steam conformance and sweep efficiency as the steam is injected in this heavy oil reservoir over 1,000 m below surface. Key aspects of the data acquisition campaigns, state-of-the-art processing and time-lapse interpretation will be discussed. The ultimate objective of the programme is to evaluate the effectiveness of the techniques used in order to select the most appropriate and economically viable reservoir surveillance tools for full field deployment.

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