Geophysical Mapping: Method Details
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Method Name: Very-high resolution reflection seismics
Method Type:   Seismic Techniques
Assigned Problems:
+ Cavity detection Civil Engineering
+ Depth of Overburden-bedrock interface Civil Engineering
+ Fractures Groundwater
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Ice thickness Natural Hazards
+ Landslides Natural Hazards
0 Earthquakes / paleoseismology Natural Hazards
0 Heat mining Natural Resources
0 Location of buried materials Hazardous Waste
0 Permafrost and ice detection Natural Hazards
0 Porosity / Permeability Groundwater
0 Quality / Thickness of aquifer/aquitard Groundwater
0 Quality and thickness (Natural resources) Natural Resources
0 Quantity/ Thickness Hazardous Waste
0 Soil / rock quality Civil Engineering
0 Young's / shear modulus, Poisson's ratio Civil Engineering
US Building stability Buildings and Structures
US Quality / Thickness of concrete Buildings and Structures
   '+' = Technique applicable; '0' = Application possible/limited use; 'US' = Ultra-Sonic (different equipment)
Principle:   Measurement of elastic waves reflected at interfaces in the underground (depth range several m to several tens of m)
Keywords:   Seismic Techniques;; ultra-shallow reflection seismics; critical refraction; seismic velocity contrast; velocity-depth functions; subsurface models
Prerequisites:  
  • Target must be characterized by a seismic impedance contrast
  • Significant absorption of seismic energy in shallowest subsurface layers (i.e., heterogeneous materials (moraines)) may result in low quality data
  • Applicability of very high-resolution reflection seismics is limited to sites with homogeneous layers with low seismic velocities
  • Ambient seismic noise (e.g., air blast, traffic, rain, wind) may reduce data quality significantly
  • Areas with rough surface topography can be very problematic for processing
  • Extensive processing is required, typically done by professionals
  • Complex subsurface geology may lead to misinterpretation in 2-D profiles
  • Areas around high-voltage and train power lines should be avoided
  • Permission of land owner and local authorities (permitting)
  • Detailed maps of cables (e.g., electricity and phone) and pipes (e.g., drainage, water, gas)
  • Licences for handling of explosives (contractor)
Resolution:   Vertical and horizontal resolution depend on seismic velocity and the dominant signal frequency. Because seismic velocities generally increase with depth whereas the dominant frequency decreases with depth, seismic resolution decreases with depth.Typical resolutions are:
Depth of investigation (Dry, unconsolidated sediments / Watersaturated, unconsolidatedconsolidated sediments, hard rock): few m / few tens of m
Vertical resolution (Dry, unconsolidated sediments / Watersaturated, unconsolidatedconsolidated sediments, hard rock): several dm / few m
Horizontal resolution (Dry, unconsolidated sediments / Watersaturated, unconsolidatedconsolidated sediments, hard rock): few m / few tens of m
Expected Results:  
  • Measured parameter: velocity of ground motion (in mV) vs. time (in ms)
  • Data Analysis processing of reflection seismic data yields an image of reflectors (either in travel-time or depth: seismic time or depth section). Depth conversion is required for one-to-one correlation with other geophysical data. Pitfalls: identification of reflections, direct, refracted, guided and surface waves. Features dipping greater than 45 on stacked (unmigrated) seismic sections are unlikely to be real reflections.
  • Interpretation seismic interpretation that is not calibrated with borehole data (e.g., synthetic seismograms or VSP), assumes that the resolved reflectors represent true lithological interfaces. In general, geological interpretation is done by correlation with geological core logging combined with borehole logs and - if available - VSP data.
Combination with other Methods:  
  • Required additional information: geological information, necessary for the interpretation
  • Related add-on information: refraction seismic and surface-based tomographic data (may be helpful for processing and interpretation); Sonic-log data (Synthetic seismograms)
  • Independent additional information: if overlapping depth of investigation: ground penetrating Radar (GPR); geoelectrical measurements; electromagnetic measurements
Operating Expense:  
  • Crew size: 1 key person; 2 - 3 assistants
  • Acquisition speed: is given by the number of geophones / channels (number of channels depend on equipment), geophone and shot-point spacing, type of employed source and topographic conditions (terrain, access): ~50 to 200 shot-points per day
  • Processing: requires in general 3 - 4 days per acquisition day
  • Equipment rental costs: high
Parameters to specify:  
  • Seismograph: Channel number (usually 24 - 48 channels or more), dynamic range (20 bit or more dynamic range)
  • Sampling rate (usually around 0.25 ms)
  • Record length depend on target depth and seismic velocity
  • Geophone properties
  • Geophone spacing (usually few dm)
  • Maximum offset (usually between 5 m and 20 m)
  • Source type/ Source parameters (e.g., amount of explosive, sledgehammer) depend on surface condition and depth of investigation (test required)
  • Source-point interval (usually between one and three times the geophone spacing)
QC Documents:  
  • On-site equipment tests (seismograph; geophones; sources)
  • Trigger accuracy
  • Daily checks: Noise level; Impedance of geophones and cables; dynamic range and gain adjustment of seismograph
  • Field notes (e.g., all activities, effective time schedule, present personnel)
  • Coordinates and map of shot and geophone locations
Products:  
  • Measurement of noise level (test measurement)
  • Raw data and geometry files
  • Data after pre-stack processing
  • Seismic time section (stacked data)
  • Migrated section
  • Subsurface models / Interpretation
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