Geophysical Mapping: Method Details
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Method Name: BEAM G4 SCAN
Method Type:   Electrical Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ Cavity detection Civil Engineering
+ Fault zones Civil Engineering
+ Fracture zone Civil Engineering
+ Fractures Groundwater
0 Location of buried materials Hazardous Waste
0 Porosity / Permeability Groundwater
0 Quality / Thickness of aquifer/aquitard Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   BEAM G4 SCAN is a technique that detects and records lateral changes in complex impedances within particular depth ranges of rock mass or soft ground through isolated zones of TBM, drilling jumbo, reinforcement of concrete, constructional components or fixtures and separate electrodes.
Keywords:   Geoelectric Techniques; Bore-Tunnelling Electrical Ahead Monitoring; BEAM G4 SCAN; Real-time monitoring and geology prediction; Induced Polarization and focused geoelectrics; Rock mass or soft ground; Tunnel-referenced electrical sections; Prediction models, Geological models.
  • Targets of investigation must be characterized by a sufficient electric impedance contrast
  • 1 counter- and 1 reference electrode (B1, B2) in tunnel
  • Sizes of insulated electrodes for use as A0/A/M/N within a surrounding metal matrix of TBM have to be adjusted to diameter of tunnel
  • For perimeter and face measurements in open tunnel, reinforcement of concrete need to be connected, in other respects, TBM is used as guard electrode A1
  • Ongoing boring signal and chainage information is necessary for automated operation from TBM as well as remote access for system maintenance and file transfer; in case of rotating electrode zones, rotary feedthrough of wirings and incremental encoder for permanent provision of the cutting wheel angle is required
  • Power supply to engine of TBM or drilling jumbo without insulation transformer may reduce data quality significantly
  • Lateral resolution ahead or in perimeter scales with number of electrodes and spacings, further with parameter contrasts and sizes of anomalous volumes; in the course of approach of the tunnel-face towards water-bearing zones, fault-, fracture and cavernous zones with contrasting electrical parameter attributes, measured complex impedances in the area of tunnel face start increasing or decreasing in about three times distance before encountering.
Expected Results:  
  • Measured parameters: Apparent and Formation Resistivities/IP
  • Data analysis: Pre-processing through digital lock-in technology yield complex impedances with according geometry factors from focused or non-focused arrays, sorted whether from boring or non-boring in case of TBM
  • Interpretation: Areas displaying anomalously high or low values, or anomalous patterns can be identified; depth of objects can be roughly estimated, particularly through successive advancement under repetitive measurements; additional geological or geophysical data may be required; comparison of descriptions of passed geology with formerly acquired according geoelectric parameters may contribute to a more reliable interpretation.
Combination with other Methods:  
  • Required additional information: rough estimates of target parameter attributes
  • Related add-on information: surface-based electric resistivity tomography data, vertical electrical soundings, electromagnetic data: lateral extension of existing information about geological constraints on ground water table, fracture and fault zones
  • Independent additional information: seismic and/or georadar data, gravity data, borehole logs
Operating Expense:  
  • TBM-modification and maintenance of specific parts to be used for geoelectrics, costs: on demand from TBM manufacturer or owner
  • Adjustment and taking-into-operation on TBM on-site usually 3 days, no downtime for measurements
  • Downtime for Drill & Blast (scanning of tunnel face) about 30 minutes; perimeter investigations in open tunnel according to area size and capability of field crew
  • Crewsize for Drill & Blast and perimeter investigations: 1 key person; electrode connections should be prepared by tunnelling staff
  • Processing: realtime
  • Service costs: moderate
  • System rental costs: intermediate
  • System purchase costs: low in relation to tunnel project size
Parameters to specify:  
  • Power supply: usually 110/230V from grid
  • Electric guard: type and geometry
  • Electric transmitter(s): true sine wave limited at 42 V (SELV - Safety Extra Low Voltage); current usually limited at 3 Amps, max. accuracy < 0.2 ľA
  • Number of measuring electrodes in investigation direction: usually 3 fixed in cutterhead of TBM, usually 1 for perimeter mapping
  • Frequency: usually 500 Hz for migrating electrodes, lower for perimeter mapping
  • Sampling rate: usually 8 kHz.
QC Documents:  
  • Time of measurement (UTC)
  • Acquisition checks: noise levels; impedance of electrodes and cables; hardware interfaces; dynamic range and gain adjustment of instrument board
  • Verification/Comparison of manual and/or automatic interface inputs: excavation chainage and/or electrode positions, boring signal and cutting wheel angle (TBM)
  • Automatic control of equivalent potentials for focused measurements at points of current inputs A0() and guard A1 through separate wiring
  • Raw data and time records with valid measurements
  • 2D anomaly maps of geoelectric parameters
  • 3D visualization of geoelectric parameter distributions
  • Interpretation
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