Although the application of seismic methods, active and passive, to monitor in-situ reservoir stimulation processes is not new, seismic arrays and array processing technology coupled with a new noise cancellation method has not been attempted. Successful application of seismic arrays to passively monitor in-situ reservoir stimulation processes depends on being able to sufficiently cancel the expected large amplitude background seismic noise typical of an oil or geothermal production environment so that small amplitude seismic signals occurring at depth can be detected and located. This report describes the results of a short field experiment conducted to test both the application of seismic arrays for in-situ reservoir stimulation monitoring and the active noise cancellation technique in a real reservoir production environment. Although successful application of these techniques to in-situ reservoir stimulation monitoring would have the greatest payoff in the oil industry, the proof-of-concept field experiment site was chosen to be the Geysers geothermal field in northern California. This site was chosen because of known high seismicity rates, a relatively shallow production depth, cooperation and some cost sharing the UNOCAL Oil Corporation, and the close proximity of the site to LLNL. The body of this report describes the Geysers field experimental configuration and then discusses the results of the seismic array processing and the results of the seismic noise cancellation followed by a brief conclusion. 2 refs., 11 figs.