Reports: ND854935-ND8: Geophysical Characterization of Biogenic Structures in Siliciclastic and Carbonate Media
Ilya V. Buynevich, PhD, Temple University
The funded research was the first high-resolution geophysical investigation of three decapod species inhabiting adjacent coastal biotopes on San Salvador Island. Extensive (>2 km) 800 MHz georadar surveys captured a diverse suite of >120 burrows of ghost (Ocypode quadrata), blackback (Gecarcinus lateralis) and blue (Cardisoma guanhumi) land crabs. Site-specific post-processing protocols were used to establish characteristic attributes that distinguish unfilled burrows from other discordant anomalies based on signal polarity structure. Variably oriented 2D and quasi-3D (~0.75-1.25 wavelength line separation) images allow the diagnostic signature of G. lateralis burrows to be distinguished from those of O. quadrata based on: 1) higher density of point-source diffractions (0.9-1.9/m vs. 0.01-0.06/m) and 2) shallower inclination angle (21-46° vs. >60°). C. guanhumi burrows are differentiated by: 1) larger diameter; 2) presence of an end chamber (where the radar signal is not attenuated by saline groundwater); 3) low electromagnetic signal velocity (~9 cm/ns vs. ~11 cm/ns) in organic-rich muddy substrate and 4) rapid signal decay. Target density, migrated inclination angle and the effective range offer the best combination that differentiates between the three end-member structures and matrix properties. This study establishes GPR as a viable non-invasive subsurface imaging technique, with potential implications for: 1) identification of shifting ichnocoenoses, as exemplified by intense hurricane impacts (2015/2016); 2) recognition and mapping of similar biogenic structures in buried or lithified carbonates and, 3) quantification of a near-surface ichnofabric index and the dual porosity/permeability structure of prospective hydrocarbon reservoirs. Furthermore, the basic attributes of subsurface visualization can be readily extended to other mesoscale biogenic structures in evaporite and siliciclastic media.
During Year 2 of the project, fieldwork on San Salvador Island, the Bahamas focused on continuing high-resolution geophysical (GPR) characterization of bioturbation (3D grids), with additional drone imaging and ichnological measurements of Cardisoma guanhumi burrows in a mangrove biotope. These important yet understudied brachyurans are widely distributed throughout tropical and subtropical estuarine regions and are prolific landscape engineers. Furthermore, the burrows may have potential as an indicator of past biotope boundaries (wetland/upland) and sea-level position, which became an important new direction in the current research effort.
For the first time, a three-dimensional image of C. guanhumi burrow network has been post-processed, so the structures can be visualized based on electromagnetic signal return patterns (see the TOC figure). This allows direct comparison to bioturbation (ichnofabric) indices, with applications to macroporosity structure of potential petroleum reservoirs. To better understand burrow morphology, sediment movement, and the relationship to sea-level fluctuations, casting techniques were conducted on nine blue land crab structures. Field measurements (depth to water table, total length, average diameter of cast segments, and angle orientation) were collected for statistical characterization of burrow morphometry. Additionally, extensive photographs were taken to aid in photogrammetric analysis through 2D software processing (Image-J) and statistical analysis conducted using NCSS. In total, 29 burrow-morphological parameters were identified and calculated to determine index attributes that are representative of the biogenic structures produced by C. guanhumi. In addition to the morphometric analysis, in situ measurements from 150 burrows (entrance diameter, and orientation of inclined shaft) were collected to investigate densities and the extent of bioturbation related to the blue land crab.
To better understand the relationship between the water table and the burrow end chamber constructed by Cardisoma guanhumi, three monitoring wells were excavated. These were installed at sites where burrow casts have been extracted. Measuring stakes were placed in the pits to observe the influence of tidal fluctuations at the freshwater-seawater interface. Salinity measurements were taken from 20 burrows and 11 water bodies on the island and used to constrain the upland extent of the blue land crab. These data will be used to examine tidal range thresholds for this species.
The research to date has been incorporated into one completed master’s thesis (Karen Kopcznski, 2017), one published journal article and one currently in press (not listed in publication count), with 7 published conference abstracts (since Year 1) presented at regional and national GSA conferences.