The Permafrost Density Logger
Authors: McKay A.S. , O&apos;Connell L.P. 
Published: March 2010

Permafrost soils of high and variable ice content, capable of providing thaw subsidence, have long been recognized as a serious problem affecting almost every phase of engineering activity in worldwide permafrost areas. Knowledge of the unpredictable distribution and ice content of permafrost soils that can vary widely both, vertically and horizontally is essential in construction practices for roads, pipelines, well drilling sites, structures, utilities, etc., all of which may sustain damage from thawing of ice-rich permafrost soils.

Of the methods used to date in the determination of the distribution of ice in frozen ground, the soil-boring technique, with laboratory analyses of the core, is the method primarily used for engineering purposes. However, where large numbers of investigation holes are required for the assessment of soil conditions throughout the length of pipeline, road or right-of-way projects, this method has proved to be time consuming and costly.

This paper describes a completely portable logging instrument designed to accurately measure the relatively low densities of permafrost in situ throughout the entire vertical distribution of soil-ice conditions in a borehole. The density logger, based on the gamma-gamma principle, provides a continuous measurement of permafrost density which, being directly related to the permafrost ice content, can also provide a prediction of thaw shrinkage per unit length along a borehole and, when integrated, provide an estimate of total earth surface thaw settlement to be expected in the event of total thawing of the ice in the earth down to any designated depth on the log.

Introduction

The increased activity and development in the permafrost regions of northern Canada and Alaska in recent years has demanded a high input of engineering technology for use in virtually all construction practices where low-density permafrost soils of variable ice content are capable of providing large thaw subsidence. The paper describes the technique and results obtained using a logging instrument specifically designed to provide density measurements of the permafrost for use in determining potential thaw subsidence rather than measuring core densities.

Technique

The Permafrost Density Logger, designed and constructed by Texaco Inc., Bellaire Laboratories, is a compact, highly portable logging tool that comprises a manually operated reel-type winch of 300-ft cable capacity, a case containing the surface electronics and paper recorder and a tripod, all of which, when assembled, combine for a total weight of approximately 75 pounds. The downhole portion of the equipment is a metal sonde measuring 3–1/2 feet in length by 2–1/4, inches in diameter, complete with interchangeable decentralizer springs to accommodate borehole, diameters of from 3–1/2 to 10 inches. Power requirements for both surface and downhole electronics, including the recorder, are provided by low-voltage rechargeable nickel-cadmium flashlight cells housed in the recorder case, which supply adequate power for a normal operating day.

The logging tool or density measuring instrument uses a scattered gamma-ray method, and is specifically designed to accurately make density measurements of materials ranging from 2.0 gm/cc to less than .9 gm/cc.