Excuse Me Sir, Will That Be One Millisecond … Or Two??
Mackenzie, Peter1 and Norm Cooper2
1 MacKenzie Land & Exploration, Ltd., Worthington, OH
2 Mustagh Resources, Ltd., Calgary, AB
There are times when more data is good, and there are times when more is worse. The Nyquist Theorem indicates that to sample a sinusoidally oscillating waveform, we must use at least two samples per wavelength; one to describe a positive lobe, and one to describe the negative lobe. If the input analogue data is limited to a bandwidth less than the Nyquist frequency 1/[2xSi], then the lowest frequency alternative of the infinite number of possibilities will be the correct re-construction.
Seismic recording instrumentation applies anti-alias filters to virtually eliminate frequencies above the Nyquist so that no aliased data will distort the frequencies of interest to us. Modern 24-bit recording systems utilize Delta-Sigma modulators that operate at high clock speeds to produce a stream of bits, whose average over time, is a very close digital estimate to the analogue input signal. At a 2 ms sample interval, today’s units are 512-times over-sampling AD converters. At 1ms sample intervals, they become only 256-times over-sampling. The average output for each sample does not have time to converge as precisely, and 3 dB of dynamic range is lost.
We recommend that the recording sample interval be selected such that the highcut filter of the recording system exceeds the highest useful frequency anticipated. For post acquisition down sampling, use a Sin-based interpolator (such as the sync function).
Decreasing sample intervals unnecessarily during recording may result in loss of precision (dynamic range), and will result in increased down time in the field, and increased program costs.
AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004