Geophysical Methods for Compulsory Question
Types of gravity Surveys -Relative vs absolute gravity
GRAVITY PROFILING o 2-D information o Easily interpreted o Readily modelled Gravity mapping o 3-D information o Interpretation is more complicated o Modelling is more complicated and usually computer assisted.
What are the limitations?
High frequency limits depth. Higher conductivity limits depth e.g damp clay or sand saturated with salt water will be SHIT . Limestone, dry sand, sand saturated with fresh water are GOOD. Need to know *****DIELECTRIC CONSTANT***** Affected by cellphones and 2 way radio. NEEDS FLAT SURFACE.
Limitations?
Limited to nature of subsurface material e.g DRY sand, Ice and Snow. Shape of subsurface material. Depth of target and design/ orientation of coil (Receiver & Transmitter).
What are the draw backs of a gravity survey
Lots of time needed to carry out corrections. Equipment is really sensitive. Hungarian chick may play shit music during transit from site to house and smoke lots of durries.
Why is GPR good?
**NON INVASIVE** Most popular shallow geophysical method used to pinpoint locations of anomalies buried in the subsurface. Can locate metal and non metal objects. *Data produced on screen in real time which leads to fast interpretation in the field.
What targets can it find?
*Buried metal objects, lateral changes in lithology, locating water produced fractures (Dams), Locating Cavities (mineshaft) & investigating landfills.
What is the first thing you do before you conduct any field study?
*Define a suitable transect and parameters prior to site arrival. *Produce GIS field maps or aerial photos for each transect. *Survey / Recon *know the hazards *Select the geophysical surveys you can conduct given your circumstances. *GPS each site you conduct a measurement at.
What can gravity surveys be used for?
*Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects. *The information provided can build a picture of subsurface anomalies which can then be used to more accurately target oil, gas and mineral deposits. *Detection of sub-surface voids, e.g., caves, mines *Detection of buried structures, e.g., foundations, storage tanks *Mapping bedrock lithology *Mineral exploration
***Gravity Survey****
*NON INVASIVE TECHNIQUE *Based on measuring localised variations in the Earth's gravitational field, caused by the presence of materials of different densities or voids in the subsurface. *The presence of an anomalously high (or low) density body in the subsurface causes a localised high (or low) anomaly in the measured gravitational field.
****Electromagnetic Survey****
*NON INVASIVE TECHNIQUE: *Uses MAGNETIC INDUCTION to measure electrical conductivity of the subsurface. (Dry sand, ice, snow). *No ground contact required
Types of Electromagnetic Surveys
*SLINGRAM SYSTEM (TRUCK & PLANE mounted) *TURAM SYSTEM (Deeper targets more powerful) *MAGNETOTELLURIC *GROUND PENETRATING RADAR
How would you conduct the survey
*Set up base station nearby where you can refer back to in order to correct data. *Conduct the survey using Worden gravimeter. GPS site. *Record the data and minus base station data then times by 0.0850mGal to get gravity anomaly at site. * Correct the data with appropriate method.
Why is it good?
*Unlike conventional resistivity surveys, NO ground contact is needed = rapid data acquisition. *USED ON PLANES. *Used to identify anomalies for GREATER DETAILING. *Can cover large area quickly.
*** Seismic Survey ***
- CAN BE INEXPENSIVE NON INVASIVE TECHNIQUE. - Commonly used to determine site geology, stratigraphy, and rock quality. - Reflection and refraction are the most common seismic techniques. - Seismic energy is either refracted or reflected off boundaries giving different seismic velocities or impedances.
Seismic Refraction
- Depths to approximately horizontal seismic interfaces on scales from local environmental targets to continental crust. - Velocities of layers between interfaces - Uses Snells Law - Direct Ray, Refracted Ray (critical angle) & Reflected Ray
What data reduction and corrections are required for Gravity based surveys in the industry?
- Instrument Unit Conversion (0.0850mGal) & Drift (caused by moon) *Eotvous correction (required for sea and moving surveys) *The latitude correction: The gravity value that would be observed if Earth were a perfect, rotating ellipsoid is referred to as the normal gravity. *The free-air correction accounts for gravity variations caused by elevation differences in the observation locations. *The Bouguer correction is a first-order correction to account for excess mass underlying observation points located at elevations higher than the elevation datum (sea level) *The Terrain correction accounts for variations in the observed gravitational acceleration caused by variations in topography near each observation point.
How does it work? (INCLUDE SAND DUNE DIAGRAM)
-***PULSED**beams energy into ground from transmitter in form of electromagnetic waves. Uses frequency up to 1200MHz -Some energy reflected back to antenna along a boundary in subsurface via SEISMIC REFLECTION (LATERALLY CONTINUOUS e.g soil horizon, Sulphide vein, WATER TABLE) and REFRACTION EVENTS (HYPERBOLIC CURVE e.g void, buried pipes, edge of feature). -EM waves travel at a velocity unique to the material. when velocities are known, TWO WAVE TRAVEL PATHS can be converted to depth measurements. -Penetration & resolution depend on ground electric conductivity and dielectric constant.
How does it work? (DRAW DIAGRAM)
-Electrical current through transmitter creates PRIMARY MAGNETIC FIELD (PMF). - PMF cause current to flow through conductive materials in sub surface. -SECONDARY MAGNETIC FIELD (SMF) is created from current traveling through conductive material. -Receiver coil detects the SMF
What can it be applied to?
Mining, Archaeology, Landmines, Geotechnical (Dams, landslides) & Glaciology.
GROUND PENETRATING RADAR. (GPR)
PulseEKKO system
How is gravity measured? What does the Worden Gravimeter look like?
mGal Draw Diagram