Shallow Borehole Convection Noise Study
Abstract
The purpose of this study was to determine the cause or causes of extraneous noise on the horizontal channels of the Borehole Seismometer System, Model 36000, and to develop methods of reducing or eliminating this noise. Similar noise had been observed on other borehole seismographs in the past and was later found to have been caused by the action of convection cells in the air within the sealed borehole. The plan of the tests was, first, to determine what input mechanism caused the characteristic noise (horizontal channel outputs with little or no vertical response) and, secondly, to investigate as many causes as possible of such an input, whether they be external to the instrument or due to forces acting directly on it in its borehole environment. Primary emphasis was placed on detection and identification of air convection activity by pressure and thermal sensors, and on repeatable generation of artificial convection activity to demonstrate effectiveness of various methods to reduce their effects. The overall conclusions of this study are (1) much of the noise at Garland thought to be convection-related is actually due to other sources; (2) natural convections caused by inverse thermal gradients in sealed boreholes are difficult to detect using present instrumentation due to their random nature; and (3) convection activity near the KS holelock is likely to induce tilt noise on the KS horizontal traces by changing the temperature of the support components. It is recommended that insulation be added below and inside the holelock in order to minimize thermal effects from convections or any other source.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 30, 1976
- Accession Number
- ADA033220
Entities
People
- John C. Cook
- John R. Sherwin
Organizations
- Teledyne Technologies