Instruments and Methods In situ measurements of firn compaction profiles using borehole optical stratigraphy Robert L. HAWLEY, * Edwin D. WADDINGTON Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA E-mail: Robert.L.Hawley@Dartmouth.Edu ABSTRACT. We have developed a technique in which we use a borehole video camera and post- processing software to make a record of the optical brightness as a function of depth in polar firn. We call this method borehole optical stratigraphy. To measure firn compaction, we note the positions of optical features on the borehole wall detected by an initial ‘baseline’ log. We track the displacements of these features in subsequent logs. The result provides a measurement of the relative vertical motion and thus compaction of the firn over the survey period. We have successfully used this system at Summit, Greenland, to measure the depth distribution of firn column shortening experienced in a borehole over three 1 year periods. The uppermost 30 m of the firn at Summit is compacting as predicted by a simple steady-state model, implying that the firn density profile at Summit is at or close to steady state over the past 70 years. INTRODUCTION Detailed borehole measurements of vertical motion in ice and snow have been made in the past using several techniques (Paterson and others, 1977; Raymond and others, 1994; Hamilton and Whillans, 1996; Hawley and others, 2002; Elsberg and others, 2004). In polar firn, measurements of vertical motion can add valuable information to a glaciological field program. For example, such measure- ments can provide insight into the mechanics of firn compaction by constraining the actual rates of compaction (Arthern and others, 2010), and they can be used to assign a depth–age relationship to the shallow sections of an ice core (Hawley and others, 2002). Altimetry measurements from aircraft or spacecraft can determine the surface height of an ice sheet, and mass balance can be estimated from changes in this height (Helsen and others, 2008). However, this height change involves several factors, including isostatic rebound, accu- mulation variability, ice flow, and firn compaction. In situ measurements of densification can help in the estimation of mass balance by radar or laser altimetry, by determining the amount of motion of the surface that is due to firn compaction. This compaction could also have a seasonal cycle (Zwally and Li, 2002). Traditionally, detailed profiles of vertical motion in boreholes required the placement of artificial markers in the firn. It was sometimes time-consuming and difficult to place these markers, and occasionally they were displaced by the measuring tool (Hawley and others, 2004). With the development of borehole optical stratigraphy (BOS; Hawley and others, 2003), we have overcome this limitation, and produced a technique that is detailed, accurate, fast, and easy to implement in the field. BOS emerged from a novel technique (Hawley and others, 2002) for determining the locations of metal marking bands, in an effort to measure a profile of vertical strain. This technique compared favorably with the previous method, which used a metal-detecting tuned coil to locate the marking bands (Hawley and others, 2004). Subsequent studies revealed that BOS could distin- guish annual layers in the firn (Hawley and others, 2003), and that the optical signal from BOS had a complex relationship with density (Hawley and Morris, 2006). BOS tracks the natural variations in density and grain size in the firn; these properties together produce the optical variations recorded by the borehole camera, making it possible to measure firn column shortening by tracking optical features. BOREHOLE OPTICAL STRATIGRAPHY Overview BOS is a borehole logging method that uses a video camera and image processing to measure the relative brightness of the wall of a borehole. In this way, we obtain information in situ that is similar to that which visual stratigraphers obtain from a core. Our vertical-motion measurement technique exploits the fact that the BOS log of a borehole contains distinctive features that can be identified and tracked as they move down in the firn column in subsequent logs. Field methods Borehole preparation The borehole to be used for a BOS vertical-motion study can be a dry borehole to any depth. The hole should be drilled with an electromechanical drill, to keep the diameter variation to a minimum and maintain the physical structure of the firn (thermal drills would alter the grain structure). The hole must not be fully cased, because the camera must see the details of the firn in the borehole walls. A short casing is desirable, however, to minimize the damage to the top of the borehole from repeated transits of the camera. The casing should be clamped to a platform on the surface so that it does not affect compaction. A typical installation is sketched in Figure 1. Journal of Glaciology, Vol. 57, No. 202, 2011 *Present address: Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA. 289