Geophysical Applications of Adaptive Noise Cancellation

Abstract

ABSTRACT Adaptive Noise Cancellation (ANC) was applied to three different noise-contamination problems encountered in seismic data acquisition. The solution to each problem required modifications to the generic ANC algorithm, as well as a unique means of obtaining a noise estimate. In all three cases ANC dramatically attenuated the noise without disturbing the underlying signal. INTRODUCTION The traditional means of dealing with noise that contaminates seismic data is to try and solve theproblem in the field. For example, a land crew experiencing 60-Hz power-line interference might insert a 60-Hz notch filter into their recording system. Sometimes, source and receiver array patterns are designed so as to attenuate bothersome noise. For some environmental noise, all too often a crew has no choice but to cease recording and wait for a quiter time. None of these solutions are satisfactory. Using special filters and arrays can degrade the desired signal; waiting for a quiter time is costly. Modern seismic recording systems, especially the new 24-bit systems, have a dynamic range wide enough to faithfully record the seismic signal, even in the presence of relatively strong noise. Under these renditions it becomes feasible to deal with noise during processing rather than during data acquisition. Generally, if the time and spatial properties of a particular noise allow it to be delineated from a seismic signal in some domain, then one can find a digital filter that attenuates the noise without harming the signal. For some types of noise though, a standard-type filter is not the best choice. For example, a digital 60-Hz notch filter corrupts the seismic signal, just as does its analog field counterpart. Adaptive Noise Cancellation is an alternative way of removing noise contamination from a signal ANC is, in essence, a scheme for optimally estimating the noise component of a measurement and then subtracting the estimated noise from the measurement. In this paper first explain how a generic ANC algorithm works. Then, I discuss attempts to use this technique to attenuate three kinds of obstinate noise sometimes found in seismic data. These were:single-frequency noise, such as power-line interference,coherent noise picked up by geophones in a dual-sensor oceanbottom cable, andenvironmental noise, such as that from nearby shipping lanes, found in streamer shot records. ADAPTIVE NOISE CANCELLATION The block diagram in Fig. 1 shows the main features of a simple, one-channel, digital adaptive noise canceller. Notice that the noise canceller has two inputs, one output, and a feedback loop. The primary input is a desired signal that has been corrupted by additive noise. The secondary input is an in exact "measurement" of the noise that contaminates the primary input. in practice (as will be seen below), the secondary input can be obtained in several ways: it may bean actual measurement, it maybe an educated guess at the noise, or it may even be derived from the primary input in some fashion. An ANC is based on the premise that the secondary input, when convolved with some digital filter.