Chapter 8: Geophysical methods

Caliper logs should be ran ______ casing is put in because the data is helpful for interpreting other types of logs

Before

What is the most common neutron source for neutron logs

Beryllium and americium

What can affect the result of neutron logs

Borehole diameter, thickness of mudcake, salinity of a borehole and interstitial fluids

Electrical resistivity uses

Characterize subsurface hydrology, map vertical extent of certain types of soil and groundwater contamination, estimate landfill thickness, determine the depth to bedrock thickness, map fauls,

When using GPR what material shows the highest resistivity to penetration? 1. Sands ls or granite 2. clays

Clays

A spontaneous potential is mainly used to determine

Correlation of lithology, bed thickness, and salinity of formation water

Downhole geophysical method uses

Determining and correlating lithology, bed thickness, formation water, lithological interpretations, determining shale content, mesuring porosity, recording hole diameter, measuring rock density, correlating units and facies

Resistivity Logging

Electrical Resistivity (R), is the resistance to an electrical flow in the rock.

Difference between GPR and Siesmic refraction

GPR uses electromagnetic energy to record boundaries and subsurface materials while Siesmic refraction uses acoustic energy recording

Sandstone, limestone and dolomites have a __________ resistivity and a _______ porosity

High; low

Fresh water has a _________ resistivity than materials with more total dissolved solids

Higher. Materials with more total dissolved solids tend to be more conductive

Gamma ray logging

Indicates the natural gamma radioactivity present in a formation versus depth

Caliper probe

Is a probe that is sent into a borehole that has 3 arms measuring 120 degrees attached to a potenitometer

When the formation water of a sand-shale sequence is more saline than the drilling mud, the deflection in the sand is to the _________

Left

SP deflections are recorded and negative to the _______ and positive to the _______

Left; Right

Shale line

Line drawn through the extreme positive and negative deflections of the SP log curve

Gravity survey uses

Locate and characterize buried bedrock channels and bedrock features, detect voids and caves, pertoleum coal mineral exploration, geotech studies

Magnetic survey uses

Locate buried steel drums and tanks, locate geologic features such as BR fractures and igneous intrusions along with ore bodies

Shale has ________ resistivity and _________ porosity

Low; High

The greater the porosity of a rock (the amount of water in the rock) the ________ the resistivity is

Lower

Neutron logs

Measures H+ Content in a borehole. The amount of hydrogen can be used to determine the amount of porosity related to the structure.

Gravity surveys

Measures the changes in earths gravitational fiels measured in gals (1 Gal= 1cm²/sec²)

The principle of refraction are based on what law

Snell's Law

Neutron logs can also be used to measure _________ ________ in unconfined aquifers

Specific yield

geoid

The actual shape of Earth, which is rough and oblate, or slightly squashed. Earth's diameter is longer around the equator than along the north-south meridians.

rippability

The ease with which soil or rock can be excavated mechanically.

Electrical resistivity

The measure of a material's ability to conduct electricity. A material with low resistivity will conduct electricity well.

Snell's law

The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for a given frequency.

In general, gamma ray logging is used to determine

The shale content that is found in formations. Because they typically tend to have a higher concentration of radioactive material

Test pits

Trenches dug at the job site that allow visual inspection of the soil strata and direct collection of undisturbed samples.

What can seismic refraction be used for

determining the depth of bedrock, and determine the rippability of earth materials

Ground Penetrating Radar (GPR)

A remote sensing technique in which radar pulses directed into the ground reflect back to the surface when they strike features or interfaces within the ground, showing the presence and depth of possible buried features.

Potentiometer

A sensor used to measure the angular position of the axle or shaft passing through its center.

How do SP logs work

An electrode is placed in the borehole and a fixed electrode is placed at the top. This measures differences in electrical potential between the borehole fluid and the material inside of the hole

Types of rock with increasing radioactivity

Anhydrite/gypsum < Gabbro/Basalt < Coal <Limestone & dolomite < sandstone < Shale < Arkose< Granite

Aerial photographs are useful for what type of project investigations

- Examining historical uses of sites for environmental evaluations - Interpreting landslide features and mapping faults - Examining drainage and erosion characteristics for potential impact on engineering structures - Examining small scale structureal features to determine the presence of traps for petroleum - geologic mapping of lithology character of the rocks - mapping join sets and lineations - delineating soil patterns and distributions - determining general lithology, stucture, and physiography for locating ore deposits - mapping surficial deposits for potential burrow areas

The scale of aerial photographs is dependent on what two things

1. focal length, 2. flying height

Magnetic surveys

Measures variations in the magnetic field of subsurface material

Gravity surveys are useful for what type of work

Mineral and petroleum exploration, geophysical surveys, and measurement of the geoid

On an SP log, a deflection to the left means that the formation water is More/less saline

More saline

What types of rock are usually measured by magnetic surverys

More so igneous rocks. Sedimentary rocks tend not to have any magnetic properties to them

seismic refraction surveying

Most commonly used by engineering geologists. It can indirectly examine the strength of rocks and their suitability for foundations, pressure zones, and discontinuities within the rock

The shape of the gamma ray logs are determined by the concentration of what elements

Potassium, thorium, and uranium

Caliper logs

Record continuous record changes in borehole diameter, which may be a function of drilling technique and lithology

Gamma ray logs can substitute for ____ logs when ftilled with saline or oil based muds

SP logs

A line drawn through the extreme negative of the SP log is called the

Sand line

formula to determine scale of an aerial photograph

Scale = F/H - F= focal length - H= flying height

Seismic refraction uses

engineering geology investigations, mineral exploration, petroleum exploration, hydrology investigations

Ground penetrating radar uses

locate subsurface features, lokate karst related voids in the subsurface, map landfill boundaries, map geologic strata, examin structural integrity of roads bridges and buldings, identify drilling locations, deleniate and map groundwater, profile lake and river bottoms

Salinity can cause the Resistivity to...

lower

Spontaneous potential (SP) logging

this borehole geophysical method of logging is a record of the naturally occurring potentials developed between the borehole fluid, formation water, and the surrounding rock materials with depth. Records the difference in electrical potential of borehole fluids.

What else can surficial electrical resistivity be used for

to locate bulk waste or buried drums, trench limits, and locating buried utilities

Snell's law formula

v₁/v₂ = sinα/sinβ V1= Velocity of layer 1 V2= Velocity of layer 2 α= angle of incidence β= angle of refraction