A combined acoustic and electromagnetic wave-based techniques for bathymetry and subbottom profiling in shallow waters

Response of bottom sediment stability after carp removal in a small lake

“…By examining the contrast of mechanical impedance (product of acoustic speeds in medium and bulk density of medium) and acoustic reflectivity, one can delineate the interface of different mediums, such as water-sediment, sand-clay layers or soft and hard patterns over lake floors (Nitsche et al, 2004;Lin et al, 2010). For example, sub-bottom profilers (SBP) have been employed to delineate lake bathymetry and sediment sublayers (Lin et al, 2009). Sidescan sonar (SSS) has been used to illustrate surficial sediment textures (Huvenne et al, 2002), sedimentary bedforms (Shawab et al, 1996) and objects such as shipwrecks over lake or sea floors (Ballard et al, 2002).…”

“…For example, ground-penetrating radar (GPR) images bathymetry and sublayer by distinguishing contrasts of dielectric properties in different mediums (Sambuelli and Bava, 2012). GPR performs well in coarse-grained sediments and vegetated environments, and can be a useful alternative when acoustic signals fail to map bathymetry and sediment sublayers (Annan and Davis, 1992;Lin et al, 2009). Nevertheless, the use of GPR is limited in high electrical-conductivity environments such as fine-grained sediments and water with high dissolved salt (NaCl) due to significant energy attenuation (Annan, 2005).…”

“…Nevertheless, the use of GPR is limited in high electrical-conductivity environments such as fine-grained sediments and water with high dissolved salt (NaCl) due to significant energy attenuation (Annan, 2005). The combination of acoustic and EM wave-based geophysical techniques can provide complimentary information and a more complete picture of sediment subsurface (Lin et al, 2009).…”

“…The reflected signals are emitted at the locations where mechanical impedance (product of density and acoustic speed in medium) significantly changes, i.e., at the interface of water and vegetation, water and sediments or different sediment types. The SBP system is therefore used to obtain the water depth, sediment sublayers (Lin et al, 2009) and the presence of submerged vegetation (Sabol et al, 2002). Unlike the SBP system to provide the bathymetry and sublayer information, the SSS system emits highfrequency and wide-angled acoustic signals to map substantial areas over lake floors.…”

“…In terms of the central frequency of about 50 MHz and the EM wave speed in water is V water EM ¼ 3:3r10 7 m:s À1 and in sediments is V sed EM ¼ 10 8 m:s À1 , the GPR theoretical resolution in water is: Res water ¼ ðV water EM =4f Þ ¼ ð3:3r10 7 =4:50r 10 6 Þ ¼ 0:17ðmÞ, and in sediments is Res sed ¼ 0:50ðmÞ. Sixty-four stacks per trace were adopted to reduce data smearing during continuous sublayer profiling (Lin et al, 2009). The geophysical measurements including SBP, SSS and GPR were conducted along several transects normal to the shoreline and extending from nearshore to offshore, in Lake Wingra.…”