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CSEM, acoustic and optical imaging of heavy mineral enrichments in coastal and shelf systems



Shallow marine magnetic mineral enrichments receive increasing interest by coastal research and economic geology, creating a need to develop more specific geophysical profiling and mapping techniques. Marine CSEM has great potential to estimate the grade and spatial distribution of iron-titanium sand deposits and to study their formation processes and coupling to bedforms and hydrodynamics.
An environmental survey of some 500 km² in the inner Bay of Plenty (New Zealand) has been performed in 11/2012 with our benthic EM profiler NERIDIS III, hosting a concentric loop EM sensor, a CTD with turbidity meter and a high speed flash camera system. The CSEM method coevally quantifies electric and magnetic properties of the seafloor. Electric conductivity is used to map pore water salinity (seepage), sediment porosity and to deduct grain-size. Magnetic susceptibility is used as an integral measure of iron/heavy mineral content in the topmost 1-2 m of the sediment. Together, both parameters permit to distinguish gradual near-surface facies changes. From 33 CSEM profiles covering water depths from 5-30 m over a total length of 250 km, we identify numerous relict and modern magnetite-enriched sand deposits and modern sediment fluxes.
We also collected continuous optical seafloor macro images and have access to sidescan sonar backscatter data with nearly full area coverage in our working area. This combination of high resolution electromagnetic, hydroacoustic and optical surficial sediment properties delivers detailed insights into the bay’s sedimentology and hydrological and environmental conditions.

Sediment distribution on the NW Iberian Shelf



The NW Iberian continental shelf is a high energy, low accumulation nonglaciated shelf of narrow width (25 to 50 km) and large variety of sediment types and patterns. Extensive EM cross-shelf profiling in this area on a 2008 RV Poseidon and 2011 RV Meteor cruise had the objective to classify the facies and textures of the four major surficial sediment types of the entire Galician shelf and to map their distribution at meter-scale spatial resolution with a special focus on transitions: (1) mud facies of fine-grained, mostly muddy Holocene high-stand sediments from the Duero and Miño rivers deposited in two successive coast-parallel mud belts, (2) mixed sand facies consisting of relict and reworked siliciclastic and carbonatic sands deposited at moderate regimes, (3) glaucony facies of mostly relict late Miocene sands indicative of slow to arrested sediment accumulation on the outer shelf, and (4) gravel facies dominated by bioclasts and siliciclasts representing low-stand shoreface and storm deposits. All facies could be easily discriminated by EM-based crossplots of sediment and matrix susceptibility against porosity, demonstrating the potential of EM-based sediment classification. Transitional environments between the sediment units cover several hundreds of meters within the profiles linking facies end-members in the crossplots. Large bedforms and seabed undulations clearly correlate with porosity and susceptibility profiles, indicating mud deposition in troughs and glauconite or magnetite enrichment on bottom-current exposed ridge crests. Ongoing work is devoted to a stratigraphic 1D inversion of multifrequency EM conductivity data and integration with hydroacoustic and laboratory results.

Electromagnetic mapping of submarine freshwater seepage at Eckernförde Bay


Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for the first time with a high-resolution survey of freshwater seeps in Eckernförde Bay (SW Baltic Sea). EM profiling creates a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations.


For further details see www.geophysik.uni-bremen.de