An investigation of seawater and sediment depth using a prototype airborne electromagnetic instrumentation system – a case study in Broken Bay, Australia

Abstract

A B S T R A C TPrevious studies using commercial airborne electromagnetic equipment that is not optimized for marine surveying have demonstrated the use of airborne electromagnetic methods for measuring water depth and estimating sediment thickness. A new prototype helicopter time-domain airborne electromagnetic system, SeaTEM (0), is now under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, New South Wales, Australia, in shallow water up to ∼30 m in depth. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Water and Pittwater, which join in Broken Bay give rise to paleovalleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ∼200 m. The survey area also included a tombolo with a beach either side, which provided the opportunity to measure water depth through a surf zone. Sediment thickness and water depth is predicted from stitched layered-earth inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ∼ ±1 m and ∼ ±0.5 m with known water depths in areas less than 20 and 6 m deep respectively. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleovalleys to depth limits of ∼60 to 80 m below sea level in 20 to 30 m water depth, as well as resolving bedrock ridges and exposed reefs in shallow waters. I N T R O D U C T I O NThe use of airborne electromagnetic methods to detect seawater depth in turbid waters is under investigation because a significant proportion of Australia's coastal waters within depths of about 50 m is affected by water turbidity. If one assumes that the turbidity levels found in seawater do not affect the bulk seawater conductivity, then the interpreted layer thickness of the upper conductive layer (seawater) of a layeredearth model derived from the inversion of airborne electromagnetic data should not be influenced by water turbidity. *

Demonstration of Two Portable Scanning LiDAR Systems Flown at Low-Altitude for Investigating Coastal Sea Surface Topography

“…The AEM bathymetry method usually assumes a one-dimensional (1D) layered-earth model consisting of at least two layers (seawater and unconsolidated sediment) overlying a less conductive basement, for interpretation of AEM data [20]. Apart from investigating wave propagation in coastal regions, knowledge of the sea surface topography (i.e., instantaneous wave surface) obtained from 2D laser scanning during AEM survey can support AEM bathymetry studies by measuring the deviation of the sea surface from 1D, thereby invoking the use of 2D and 3D EM interpretation methods if necessary.…”

Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

“…Comparatively few studies have used EMI methods in coastal environments (Paine et al, 2004;Seijmonsbergen et al, 2004;Vrbancich, 2009;Christensen and Halkjaer, 2010;Nenna et al, 2013;Delefortrie et al, 2014b), with most of these focusing on mapping saltwater intrusion. Most of these studies use Geonics EM31, 34, 38, and similar frequencydomain sensors; Geonics EM47, 63, and similar time-domain electromagnetic (TDEM) sensors in addition to various airborne electromagnetic (AEM) systems.…”

“…AEM surveys are important for coastal studies but are beyond the scope of this paper. Previous coastal EMI studies have explored subsurface σ because it is related to framework geology (Seijmonsbergen et al, 2004;Vrbancich, 2009), classification of coastal wetlands (Paine et al, 2004), and investigation of coastal groundwater dynamics and pollution (Goldman et al, 1991;Fitterman and Deszcz-Pan, 1998;Christensen and Halkjaer, 2010;Nenna et al, 2013). Seijmonsbergen et al (2004) use the EM34 (albeit not a portable multifrequency EMI profiler) at 20 m station spacing and 20 m coil separation to acquire a 14.5 km transect along a segment of the Dutch coast, Netherlands.…”

Airborne Electromagnetic Bathymetry

“…Combined with interpreted water depths (i.e. AEM bathymetry), the AEM method can therefore be used to (i) estimate the bedrock topography by combining the water depth and sediment thickness (Vrbancich, 2009;Vrbancich & Fullagar, 2007a) and (ii) map the seafloor resistivity (Won & Smits, 1986a). Combined with empirical relationships such as Archie's Equation (Archie, 1942) and assumed cementation factors for unconsolidated marine sediments (Glover, 2009), the derived seafloor resistivities can be used to estimate important seafloor properties such as density, porosity, sound speed and from these properties, acoustic reflectivity (Won & Smits, 1986a).…”

“…Commercial systems were initially used from 1998 to 2005 for field trials to appraise the accuracy of the AEM bathymetry method based on (i) time-domain fixed-wing AEM using the GEOTEM and QUESTEM systems (Vrbancich et al, 2005a,b;Wolfgram & Vrbancich, 2007), (ii) time-domain helicopter AEM using the HoistEM system (Vrbancich & Fullagar, 2004, 2007b and (iii) frequencydomain helicopter AEM using the DIGHEM V system (Vrbancich et al, 2000a,b) and DIGHEM_Resistivity system (Vrbancich, 2004). Between 2006 and 2010, field trials were conducted with a prototype SeaTEM system (Vrbancich, 2009), a floating system equivalent to the commercial RepTEM system and the SeaTEM system . The SeaTEM system is essentially a slightly smaller version of RepTEM, developed for DSTO by Geosolutions Pty Ltd. as a research instrument.…”