Geophex - Model GEM-2 Ski - Handheld Geophysical Research Instrument - Manual

MARUM – Center for Marine Environmental Sciences (University of Bremen, Germany)Email: hendrik.mueller@uni-bremen.deSEAFLOOR CHARACTERIZATION BY ELECTROMAGNETIC BENTHIC PROFILINGHendrik Müller Christian Hilgenfeldt Tilo von DobeneckBenjamin Baasch Thomas FrederichsConcept and outline? Relates to the porosity? Discriminates sediment facies (by grain size and sorting ? porosity)? Detects freshwater or brine seepage? Detects conductive (metal) objectsElectric ConductivityMagnetic Susceptibility? Discriminates sediment facies (by magnetite or clay content) ? Maps lateral facies boundaries (infills, sand bars, mud belts)? Detects shallow reductive diagenesis (e.g. magnetite dissolution)? Detects Fe metal objects (UXO/pollution by their ferromagnetic properties)? Pore space and fluid? Solid phase / MineralogyEM-Sensor specificationSensor type: GEM 3 - Frequency domain; concentric – coaxial coilsManufacturer: Aeroquest Sensortech (formerly Geophex) Dimensions (diameter): Transmitter: 0.96 m; Bucking: 0.64 m; Receiver: 0.40 mFrequencies: 25 Hz – 40 000 Hz (25 Hz sampling rate)1-20 kHz ? electric conductivity (quadrature)75 Hz ? magnetic susceptibility (in-phase)Resolution: 0.1% porosity (10-3 S/m), 1 ppm magnetite (10-6 SI)Custom made Fiberglass Epoxy housingSensor electronics on heat sinkSensor installed in the rear part of the sledElectric current MagnetizationEM-Sensor specificationElevation: 20 cm above seafloor Sounding depth (90%): Susceptibility: 0-50 cm Conductivity: 0-90 cm0.00 0.01 0.02 0.03 0.04 0.05Susceptibility sensitivity1.41.21.00.80.60.40.20.0Distance to sensor plane (m)0 20 40 60 80 100Cumulative response0.00 0.01 0.02 0.03 0.04 0.05Conductivity sensitivity0 20 40 60 80 100Cumulative responseQ (5 kHz)Q (1 kHz)Q (10 kHz)Cumulative response1.21.00.80.60.40.20.0-0.2Sediment Depth (m)? ?221 1 00( ) ( ( ) ( )) e 1 ( )2wu hz t t t b b b TEwIH r n R J R n R J R r J r du?? ? ? ???? ? ??,s w w sTEs w w su uru u? ?? ????2 20 0 and w w w s s su i u i? ?? ? ? ? ?? ? ?? ? ? ?with the reflection coefficientTransmitter- Bucking- Scattered- Direct (seawater)-Field:/ :/ :/ :/ :s sw wt tb brIR nR nR? ?? ? electric current sediment permeability / conductivitywater permeability / conductivity transmitter coil diameter / windings bucking coil diameter / windings: :: 2:ihfJ? ? ??th st receiver coil diameter sensor height angular frequency ( ) Bessel function (i order, 1 kind)h?w, ?w?s, ?s6Secondary fieldSensor response (ppm) 10Primary field? ?? ?221 1 00( ) ( ( ) ( )) e 1 ( )2wu hz t t t b b b TEwIH r n R J R n R J R r J r du?? ? ? ???? ? ??,s w w sTEs w w su uru u? ?? ????2 20 0 and w w w s s su i u i? ?? ? ? ? ?? ? ?? ? ? ?with the reflection coefficient2 220 02 21wu hs t tTEp r w r wV R RU G r e d G dV R u R u? ?? ?? ?? ? ?? ? ? ? ?? ?? ?? ?1 1 1( ) ( ) ( )b bt b rt tn RG J R J R J Rn R? ? ?? ?? ?? ?? ?Transmitter- Bucking- Scattered- Direct (seawater)-Field:/ :/ :/ :/ :s sw wt tb brIR nR nR? ?? ? electric current sediment permeability / conductivitywater permeability / conductivity transmitter coil diameter / windings bucking coil diameter / windings: :: 2:ihfJ? ? ??th st receiver coil diameter sensor height angular frequency ( ) Bessel function (i order, 1 kind)h?w, ?w?s, ?s6Secondary fieldSensor response (ppm) 10Primary field? ?Sediment Water? CTD is used to determine seawater conductivity ?wand to calibrate (null) the sensor in the water columnb) Noise level of the conductivity measurementa) Bias of the magnetic susceptibility (k) due to a conductive (?) subsurfaceFrequency (f) <100 Hz can be used for susceptibility measurements? Conductivity is measuredin the kHz - rangeResistive limit and noise levelNERIDIS III setup(Neritic Discoverer)Dimensions (L x W x H): 5,2 m x 1,2 m x 0,8 mMaterial: Fiberglass, POMWeight: 930 Kg (in Air), ~ 400 Kg (in water)Operation depth: 1 000 mTow Speed: 2 – 4 KnotsMission duration: 12 – 16 hElectric power: 12V - 27Ah, 24V - 40Ah (Lithium Ion batteries)NERIDIS III setupEM sensor headBattery unitControl unitCTD probePressure bottlesLift bagLift bagTurbidity sensorCamera(Neritic Discoverer)Control unitDimensions (L x D): 0.7 m x 0.3 mMaterial: POM, aluminumWeight: 48 Kg (in Air), ~ 3 Kg (in water)Telemetry: 2 Mbit/s (via 10 km standard coaxial tow-cable)EM systemSMBus communicationboardsMotion sensorControl PCEthernet and transponder unitsHeat sinkbulkhead connectorBattery unitDimensions (L x D): 0.7 m x 0.3 mMaterial: POM, aluminumWeight: 68 Kg (in Air), ~ 5 Kg (in water)bulkhead connector6x 24V, 6750 mAhLithium IonenBattery system3x 12V, 9000 mAhLithium IonenBattery systemHeat sinkNERIDIS III operationMultibeam Echosounder EM data Sled control RecoveryDeploymentControl rack DeploymentRecoveryEM004 Landing1st 5 min of bottom surveying(Mud belt after Diaz et al. 2002)(Facies after Lantzsch et al. 2010)Case study: NW Iberian ShelfWhy study shelves? Sedimentary pathway coast – deep sea? Storage area for terrigenous material? Ecological, economical importanceThe NW Iberian continental shelf? High energy, low accumulation regime? Known mud belt structure (= modern sediment) but unknown sediment pathways to the deep sea? Sedimentologically well characterized facies units(Dias et al., 2002; Lantzsch, 2009)? Mud belt (mainly silt - fine sand) ? high porosity and susceptibility? Quartz sands on outer shelf? low porosity and susceptibility? Asymmetric transition zonesShelf Profile I (42°02‘ N)Müller et al. 2011 (in rev.)Shelf Profile II (42°10‘ N)? Mud belt (inner shelf) ? Glauconite sand (mid shelf)? Quarz sand (outer shelf)? Mega ripples (undulations)Wavelength 0.6 - 1.5 kmGenerally lower porosityand higher susceptibilityon ridgesMudQuarz sand Glauconite sandMüller et al. 2011 (in rev.)? Bivariate susceptibility vs. conductivity plots depict distributions and transitions of different shelf and slope facies? After a sample-based laboratory calibration, transfer functions for other specific sediment properties of interest (mineralogy, stability) can be derived Sediment classificationMüller et al. 2011 (in rev.)Sediment distributionELECTRIC CONDUCTIVITYMAGNETIC SUSCEPTIBILITYEM profiles? Match well with mud distribution? Identify sediment wave patterns? Extend the knowledge and precisely map sediment distribution on the NW Iberian Shelf? Conductivity lows at pockmark structures and slopes delineate localized freshwater intrusion? Susceptibility lows mark magnetite reduction by anoxic freshwater Relative Magnetization (Am²)Case II: Freshwater seepageEckernförde Bay? Pockmarks (morphological depressions) mark freshwater (or methane?) seeps? Deep towed magnetometer maps the glacial substrataMüller et al. 2011 (Geo-Marine Letters)• CTD bottom water conductivity depicts recent freshwater seepage• Pockmarks show up as 1-2 m deep morphologic depressions• Holocene Mud-cover is thinned in pockmark• Glacial sand-subsurface bulges below pockmarks• Sediment conductivity and susceptibility are reduced within the pockmark• Magnetometer images the glacial Subsurface only-30-29-28-27-26-25-24-23-22Depth (mbsl)5060708090100110120130Apparent Susceptibility (10-6 SI)0.00.20.40.60.81.0Apparent Conductivity (S/m)2.42.52.62.7Bottom-Water Conductivity (S/m)121620242832Relative Magnetization565900 565950 566000 566050 566100 566150Easting (m)Glacial SandHolocene MudWGS 84 / UTM zone 32NMetalObjectsabcdefV.E. = 10Freshwater seepageBivariate k vs. ? Plot? Facies transitions from sand (slope) to silt (basin)? Seeps vary in conductivity due to flow rates; susceptibility lowered by magnetite sulfidizationCase II: Freshwater seepageMüller et al. 2011 (Geo-Marine Letters)Conclusions? We demonstrate the eminent potential of EM profiling for assessing the complex distribution of shallow marine surficial sediments? Electric conductivity primarily relates to the fluid-filled pore space and detects salinity, porosity and grain-size variations? Magnetic susceptibility mostly assesses solid particle characteristics such as terrigenous or iron mineral content, redox state and contamination level? EM profiling reveals climatic, hydrodynamic, diagenetic and anthropogenic factors governing the sediments formationEND