Construction and use of an inexpensive, lightweight free-fall penetrometer: applications to paleolimnological research Ian S. Spooner 1, *, Peter Williams 2 and Kathleen Martin 3 1 Department of Geology, Acadia University, Wolfville, NS, Canada B0P 1X0; 2 Department of Physics, Acadia University, Wolfville, NS, Canada B0P 1X0; 3 Environmental Science Program, Acadia University, Wolfville, NS, Canada B0P 1X0; *Author for correspondence (e-mail: ian.spooner@acadiau.ca) Received 3 April 2003; accepted in revised form 13 March 2004 Key words: Paleolimnology, Penetrometer, Physical limnology, Sediment characterisation Abstract This paper describes the construction of an inexpensive, lightweight, free-fall lake sediment penetrometer with application to paleolimnological research. This penetrometer is suitable for use in a variety of fresh water lake settings. Data are collected on a laptop PC, Palm OS handheld, or a Texas Instruments handheld data collection unit and are analysed using commercially available software. The quality of these data are adequate for determining changes in substrate type based on hardness. The unit can also be used to locate sites for optimum penetration of gravity cores and to calibrate sonar records. Addition of up to three optional transducers could expand the capabilities of the unit. Other potential uses include the determi- nation of specific geotechnical properties of the lake sediment including the undrained shear strength. Introduction Penetrometers have long been used to measure the physical properties of sediments in situ (Dayal et al. 1975; Chari et al. 1979; Akal and Stoll 2002), most commonly the undrained shear strength and relative density of sea floor sediment (Dayal 1981; Akal and Stoll 2002). Penetrometer data support a variety of applications including seabed classifica- tion, geotechnical site investigation, dredging sur- veys, and naval applications. Two main types of penetrometers are used in lakes or shallow marine settings. In situ designs are, in general, lowered to the lake bed and use the depth of penetration of modified weighted rods to characterise lake sedi- ment properties (Gaugush 1998). Free-fall designs impact the lake bottom and use acceleration and, in some cases, pressure sensors to monitor the sediment response to penetration (Dayal et al. 1975). Several types of free-fall penetrometers ex- ist. Scott (1967) correlated deceleration of a grav- ity corer with variations in marine sediment characteristics. Thompson and Colp (1970) used a 3¢¢ (76 mm) diameter, 5¢ (127 mm) long pene- trometer for ocean bottom exploration. Pene- trometers instrumented with accelerometers have been used to measure acceleration/deceleration and to calculate penetration velocity and depth of penetration by single and double integration of acceleration respectively, with time. These modern penetrometer systems tend to be large and heavy, or as size decreases, expensive. As a result pene- trometers have not been used extensively in lim- nological research and have seen little application in paleolimnology. The nature and the thickness of post-glacial lacustrine sediments in Atlantic Canada varies widely in lakes located short distances apart Journal of Paleolimnology 32: 305–310, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 305