A combined acoustic and electromagnetic wave-based techniques for bathymetry and subbottom profiling in shallow waters Y.-T. Lin a , C.C. Schuettpelz b , C.H. Wu a , D. Fratta c, ⁎ a Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA b Golder Associates, Denver Office, 44 Union Boulevard, Suite 300; Lakewood, CO 80232 c Geological Engineering Program, University of Wisconsin-Madison, Madison, WI 53706, USA abstract article info Article history: Received 7 April 2008 Accepted 11 November 2008 Keywords: Sub-bottom profiler Ground penetrating radar Shallow waters Bathymetry Sublayer imaging Acoustic-wave based sub-bottom profiler (SBP) and electromagnetic-wave based ground penetrating radar (GPR) are two complementary geophysical tools that were used to map bathymetry and sediment sublayers in shallow waters. Near shore regions in Great Lakes, inland lakes, and rivers in Wisconsin, USA were examined using both geophysical tools. In areas with high silt and clay contents, such as Lake Superior, the SBP was able to image the sediment sublayers, whereas in the areas with sand cover and vegetation, the GPR provided sediment stratigraphic information. The higher vertical and horizontal resolutions of the SBP surveys provided more accurate and detailed bathymetry information than GPR surveys. In the Yahara River, SBP surveys imaged blurring contrasts between sublayers due to the gradual deposition of sediments; however, GPR provided sharp delineations of sediment layers but was only able to image the top two sublayers because of the high silt and clay electromagnetic wave attenuation. To confirm these findings, Shelby tubes and hydraulic jetting were used to collect ground-truth information. Sublayer thicknesses are estimated by evaluating acoustic and electromagnetic wave velocities using a mixture-based equation. In Lake Michigan, both techniques show similar sediment stratigraphy, indicating that the average sediment particle size ranges between silt and fine sand. Three-dimensional maps of bathymetry and subbottom sediments are also constructed. Overall it is shown that the combination of GPR and SBP techniques compensates each survey's strengths in an effective methodology for imaging bathymetry and sub-bottom profiles in shallow water environments. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Accurate surveys of both bathymetry and subbottom layers in lakes, rivers, and oceans provide crucial information pertaining to a number of environmental and coastal issues (Lawler, 1993, Lane et al., 1994, Van Rijn, 1997). For example, successive surveys of bathymetry and sublayers before and after storm events help quantify the magnitude of bottom sediment erosion, deposition, and redistribution processes (Page et al., 1994; Walling et al., 1998; Ogston et al., 2000; Lee et al., 2004). Bathymetry and sublayer information helps in the evaluation of the role of cohesive sediments on lakebed downcutting and bluff recession along Great Lakes shorelines (Davidson-Arnott and Ollerhead 1995). In general, there are several in situ and geophysical techniques used to evaluate bathymetric or sublayer geometries. These techniques differ both in terms of their applicability and spatial and temporal scales. Two traditional methods of collecting quality data from a stationary boat include soundings obtained with a weight attached to a line and sediment coring that can be associated to GPS coordinates (Przedwpjski et al., 1995; Kalff, 2002). Sediment coring has the added advantage of allowing the collection of samples that can be used to validate geophysical and other non-intrusive surveying techniques. However, the number of sampling points is usually limited due to considerable logistical efforts. Airborne scanning laser altimetry (LIDAR — Irish and White, 1998) can be also used to map bathymetry in very clear water environments but cannot resolve sublayers. As an alternative to the traditional surveys described above, geo- physical techniques offer cost-effective and non-destructive methods that allows for continuous mapping of bathymetry and subsurface information. Geophysical techniques also provide larger penetration depths while maintaining a substantial survey area size (Scholz, 2001; Cagatay et al., 2003; Bradford et al., 2005). Acoustic-based geophysical methods are commonly used in aquatic environments (i.e., side scanning sonar and subbottom profiling techniques — Garcia et al., 2004; Nitsche et al., 2004; Schrottke et al., 2006) to image the bathymetry and subbottom sediment layers. Other techniques are based on the monitoring of electrical and magnetic properties of water and sediments at different frequencies. These techniques include electrical resistivity (ER), electromagnetic methods (EM), and ground penetrating radar (GPR). These techniques are used to map water depth and sublayers. In ER surveys, floating electrodes are Journal of Applied Geophysics 68 (2009) 203–218 ⁎ Corresponding author. E-mail addresses: yingtienlin@wisc.edu (Y.-T. Lin), cschuettpelz@golder.com (C.C. Schuettpelz), chinwu@engr.wisc.edu (C.H. Wu), fratta@wisc.edu (D. Fratta). 0926-9851/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2008.11.010 Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo