Collaborative Research: Borehole Logging to Classify Volcanic Signatures in Antarctic Ice

Part I: Nontechnical

One of the most interesting historical records that science can provide is contained in the ice of Antarctica. Layer by layer over hundreds of thousands of years, snow has precipitated on the ice sheet, become compacted, and turned into additional ice. Any dust or other impurities in the air or snow have been precipitated as well and thus each snowfall leaves a snapshot record of the atmosphere that existed at or near the time of deposition. A detailed chronology of volcanic eruptions can be obtained from the ice layers where ash and other volcanic products were deposited. Normally, the analysis of volcanic layers requires the physical extraction of a core from the ice sheet; however, chronologies from cores have discontinuities and are difficult, time-consuming, and expensive to obtain.

Borehole logging is a measurement method where one lowers instrumentation into a drilled hole in the ice, whether or not core has been retrieved. To date, this technology has only been used to measure optical systems to identify volcanic ash and other impurity layers. In this program, a profiling technology will be developed that measures the conductivity of the ice. A radio-frequency emitter lowered into the borehole will create a return signal that changes depending on the local conductivity, which depends on the concentration of dissolved ions. For example, dissolved sulfates are a critical marker of volcanic activity that may not be coincident with deposited ash. Other dissolved ions, such as chloride, can be indicative of other processes. It is expected that this borehole profiling instrument will be able to help rapidly identify volcanic eruptions that had potentially global impact, distinguish between different dissolved ions via their frequency dependencies, and assist in establishing chronologies between different ice cores and boreholes.

Part II: Technical Description

Borehole logging of the polar ice sheets is one of the most important methods that earth scientists have to identify and date volcanic eruptions. However, current technology only indicates the presence and depth of ash from an eruption. In order to extract more detailed information, one must obtain an ice core, and laboriously measure each section in the laboratory using electrical conductivity or dielectric measurements to determine the presence or absence of dissolved sulfate and its location relative to the corresponding ash, if any. This program will investigate and demonstrate a borehole logging-compatible radio-frequency dielectric sensor to detect and measure spikes in dissolved major ions chemistry in ice, particularly in intervals corresponding to volcanically produced sulfates. The sulfate layers are one of the primary signatures of volcanic products. However, other ions, such as chlorides, calcium, and others are also commonly seen in ice, and the dielectric logging technology of this program would also measure these. It is expected that certain sets of ions will be distinguishable by their frequency dependencies. This technique could guide other investigators, who are using conventional core scanning and sampling methods, to regions of special interest in corresponding core.

We plan to construct a ring-based electrode system and test this system on a variety of artificial ice boreholes and ice cores. This unit will not include a pressure vessel or other borehole logger packing. We will test different means of applying electrical signals including short pulses and periodic waves. We will further utilize differential measurements with low noise circuits and filters to achieve maximum sensitivity. We will correlate the signals extracted with known molarities of sulfates and other ions and measured ECM records. We will perform scaled-down experiments using real ice cores stored in Bay?s lab at UC Berkeley. This will permit testing of different designs in ice with natural impurities and polycrystalline structure. This small collection includes cores from a variety of locations in Antarctica and Greenland, and a variety of ages as old as a million years.