The impact of high-density spatial sampling versus antenna orientation on 3D GPR fracture imaging

Three-dimensional Ground Penetrating Radar (3D GPR) surveys are necessary to reconstruct complex fracture geometries in the subsurface. Two of the most important factors controlling image quality are antenna orientation relative to fractures and density of acquisition grids. This study, conducted in the Madonna della Mazza quarry (Italy), compares two acquisition methods with the goal of optimizing the imaging of fractures and related 3D fracture networks. We acquired two very dense, orthogonal 3D GPR surveys with a 250 MHz antenna and 5 cm trace spacing on the same day, covering the same area of 20 x 20 m. By decimation of the original raw datasets, reduced survey densities of 10 cm and 20 cm spacing are simulated. The results show differences in the imaging quality of the two methods to depths of 75 cm, while for a depth of 130 cm and deeper, image quality is similar. At the same trace density/m2, a single, unidirectional survey with a densely sampled grid is the preferred method rather than two surveys with orthogonal antenna orientations but larger profile spacing. The extra effort of conducting surveys with an acquisition grid of an eighth of wavelength of the antenna centre frequency guarantees that we can properly sample the high-frequency content of the GPR signal spectrum and results in optimum image quality regardless of fracture orientation. The simple survey design principle found in this study is universally applicable to any field condition, target geometry, and antenna frequency. Such high-density 3D GPR survey design enables the high-resolution characterization of 3D fracture networks on subsurface timeslices in near photographic quality

Ramp reef depositional facies model for the Mid-Pliocene Golden Gates Reef Member of the Tamiami Formation, South Florida

The depositional setting for northern most Atlantic coral reef development during the Mid-Pliocene Warm Period is a gentle sloping mixed carbonate–siliciclastic ramp. Five core transects document the distribution of the reef complex in an area approximately 65 by 12 km, and nine repetitive depositional facies are identified. Paleoecological and sedimentological evidence documents facies development along a water depth–energy gradient. The mid-Pliocene Tamiami Formation is characterized by coral boundstone developed over level skeletal rudstone depositional units dominated by mollusks. Hyotissa haitensis (Sowerby), one of the last Gryphaeid oysters, is the dominant fossil found in the most continuous skeletal facies and overlies deeper water green clay facies, the only facies with pelagic foraminifera. Ground penetrating radar documents reef depositional topography, onlapping stratigraphy and two episodes of reef growth. Two cycles of deposition are recognized, separated by subaerial exposure. The coral boundstone and the skeletal rudstone exhibit both high primary and secondary porosity and overlie the impermeable clay facies. The upper surface of the coral boundstone lies at ~ 4 m in elevation whereas contemporaneous estuarine deposits are found to the north at elevations of 20–25 m. High porosity bank reef complexes along a shallow dipping ramp provide an alternative to the standard model of reef and porosity development along the outer shelf margin. Understanding the differences in associated facies between these two depositional environments permits better interpretation of observed heterogeneities in subsurface geobodies associated with inner shelf and platform settings

Sinkhole Structure Imaging in Covered Karst Terrain

Ground penetrating radar (GPR) and resistivity techniques have been widely used to map the locations of sinkholes in covered karst terrain. To determine whether a sinkhole is a likely preferential conduit for groundwater flow, however, requires higher-resolution imaging than that used in conventional sinkhole mapping surveys. Field observations combined with simulated surveys for a 15-m diameter 3-m deep sinkhole in west-central Florida are used to assess the resolution of GPR and resistivity surveys targeting the semiconfining unit that floors the sinkhole depression. 2D resistivity surveys clearly show the central depression as well as resistivity contrasts between the cover sediments within and outside of the sinkhole, but are inadequate for resolving breaches in the semiconfining unit or underlying conduits. A 3D GPR survey resolves vertical structure on the order of tens of centimeters within the semiconfining unit, as well as indicators of conduits that extend several meters beneath the central depression. 3D GPR thus holds promise for imaging hydrologically significant features of sinkholes

AUV-Based Environmental Characterization of Deepwater Coral Mounds in the Straits of Florida

AbstractThe first AUV survey across five fields of deep-water coral mounds in the Straits of Florida reveals an unexpected high abundance and variability of mounds in water depths of 590 - 875 m. A drop camera and a series of dives with the Johnson- Sea-Link submersible confirmed living corals on each of the five investigated sites. The morphology of the mounds is highly diverse, ranging from isolated mounds to welldeveloped ridges with more than 100 m of relief. Along the toe-of-slope of western Great Bahama Bank antecedent topography seems to be the controlling factor for mound location while further west currents appear to control the formation of ridges. The comprehensive suite of sensors on board the AUV allows correlation of geophysical parameters and oceanographic observations. Acoustic Doppler current meter data document three different bottom current regimes consisting of unidirectional or bi-directional tidal flow. The bidirectional current pattern is not visible on backscatter data and only vaguely reflected in the mound morphology. In areas of uniform current direction mounds face the currents and align perpendicular to the current to form long ridges and intervening troughs. The synoptic seabed and oceanographic data recorded by the AUV characterize the dynamic and complex environments of entire coral mound fields at a resolution of 1-3 m.IntroductionDeep and cold-water coral ecosystems are less known but more widespread than their warm-water counterparts restricted to shallow tropical seas1,2. Cold-water corals and associated fauna flourish in oceanic waters of all latitudes at depths of several hundred to over one thousand meters with temperatures between 4° and 12°C and require no sunlight3. The limited availability and high cost of deep-water instrumentation has focused most research activity and related discoveries of deep-water coral habitats to the north and central Atlantic, the Gulf of Mexico and the north-east Pacific4,5. In the Straits of Florida, abundant mound-forming corals in water depths of 400-800 m have been documented in over 40 years of dredge sampling, submersible dives and seismic acquisition6-12 (Figure 1). This extensive collection of samples and observations however can not be put into a geomorphologic context as existing bathymetric charts do not resolve coral mounds. Such sparse information has proven inadequate to answer questions in regards to mound morphology, bottom current dynamics and nutrient sources supporting life at these depths. Furthermore the limited data set has so far prevented assessment of the biodiversity and the potential need for protection from over-fishing and underwater construction. High-resolution maps of morphology and oceanographic conditions resolving features at the 1-10 m scale are a basic requirement to make further progress in understanding deep-water coral mound distribution and genesis.Autonomous Underwater Vehicles (AUV) bring an integrated suite of mapping and oceanographic sensors close to the seabed for high-resolution data acquisition and give the opportunity to fill the scale gap of basic information in deepwater environments. Described here are the initial results of a 7 day cruise during which the AUV mapped five deep-water coral mound fields in the Straits of Florida covering a total area of 130 km2