Experimental investigation of interference from other seismic crews

Abstract

In a study of the contamination of reflection seismic data by interfering noise from other seismic crews, controlled experiments were performed in the Gulf of Mexico and the North Sea. In each experiment, a survey ship traversed a line several times collecting both data free of and data contaminated by interfering crew noise. In the Gulf of Mexico experiment, the “noise” ship followed a prescribed course about 11 km from the survey ship. In the North Sea experiment, the noise ship was positioned at stationary locations 10 and 40 km broadside to the survey line. Recorded interference noise in both experiments had peak amplitudes well above the 0.5 to 1.5 Pa (5 to 15 μbar) limit beyond which crews typically must agree on time‐sharing. Despite recorded crew noise that was three to eight times higher than levels typically considered acceptable, the conventionally processed common‐midpoint stack of the contaminated Gulf of Mexico data shows only slight evidence of the interference noise; in contrast, the North Sea stack is severely contaminated by crew noise as early as 1 s. However, when each unstacked trace is scaled by time‐varying weights that vary inversely with the local power in the trace, the crew noise is no longer visible in the contaminated stack of either data set. Trace‐weight normalization in this process is designed to ensure that stacked signal amplitudes are generally preserved. A simulated line wherein the actual Gulf of Mexico data are contaminated by crew noise five times stronger than that recorded in the field [yielding effective peak noise values of 7.5 to 20 Pa (75 to 200 μbar)] also shows no evidence of crew noise after inverse‐power weighted stacking. When data processing includes conventional stacking, we recommend that the specified tolerable amount of crew noise be based upon the root‐mean‐square amplitude of the crew noise computed over an entire record. With burst suppression techniques, such as inverse power‐weighted stacking, we recommend that the specified level be based upon the duration of the strong‐amplitude burst as well. With both criteria, field specifications can be chosen that remain conservative while tolerating considerably more crew‐interference noise than in the past. Issues of the influence of crew noise on the analysis of prestack data remain for future study.