Geophysics

Gravity anomaly

Arises due to lateral perturbations in rock density, magnetic ones are caused by lateral changes in rock magnetization, and seismic anomalies indicate variations in acoustic impedance.

Elasticity

Deformation that vanish upon removal of stresses.

Magnetic susceptibility

Degree to which a substance may be magnetized.

Contaminant studies

Detection and flow direction

Electrical resistivity method

Earth resistance; electrical conductivity

Self-potential method

Electrical potentials; electrical conductivity

10 gu

Equivalent of 1 mgal= 10^-3 gal= 10^-3 cm/s^2

Sedimentary rocks

Exhibit the greatest range of density variation (mineral composition, cementation, porosity, pore fluid type). Greater density, greater anomaly.

Magnetic field strength

Gauss= 10^-4 T; Gamma= nT

Tidal correction

Gravity measured at a fixed location varies with time because of periodic variation in the gravitational effects of the Sun and Moon associated with their orbital motions, and correction must be made for this variation in a high precision survey.

Clairaut

He formulated the International Gravity Formula in 1743 to account for the variation of gravity with latitude.

Gravimeter

It directly measures small differences in the strength of gravity.

Airborne

It is a mode of applicability for large or heavily forested areas.

Marine

It is a mode of applicability for offshore investigations.

Ground

It is a mode of applicability which is best applied to flat or gently undulating terrain.

Borehole

It is a mode of applicability which records one or more physical properties as a function of depth.

Eotvos correction

It is applied to gravity measurements taken on a vehicle ride such as a ship or an aircraft. Depending on the direction of the travel, vehicular motion will generate a centripetal acceleration which either reinforces or opposes gravity.

Drift correction

It is based on repeated readings at a base station at recorded times throughout the day.

Geophysical anomaly

It is detected when a survey encounters some geometric perturbation in the distribution of a particular physical property in rocks.

Geologic noise

It is due to the variable thickness of overburden, presence of non-economical minerals in the overburden or overlying layers.

Signal

It is the objective of the survey and is due to the causative or target body.

S/N ratio

It is the utmost concern in geophysical surveys and an important factor for measurement precision.

Silica content

It primarily controls the density of igneous rocks. Greater silica content, lower density.

Bouguer correction

It removes the effect that free-air correction neglects by approximating the rock layer beneath the observation point to an infinite horizontal slab with a thickness equal to the elevation of the observation above datum.

5.10

Magnetite

Density

Mass per unit volume

3 gu

Maximum amplitude of gravity variations.

Magnetic method

Spatial variations in the strength of the geomagnetic field; magnetic susceptibility and remanence

Gravity method

Spatial variations in the strength of the gravitational field of the Earth; density

4.00

Sphalerite

Mining geophysics

Studies the detection of ore bodies.

Global geophysics

Studies the structure of the Earth.

Petroleum geophysics

Study the structures that trap oil.

Gravity surveying

Subsurface geology is investigated on the basis of variations in the Earth's gravitational field arising from differences of density between subsurface rocks.

Conductivity

The ability of a material to conduct electrical current.

Underground water supplies

The exploration of these, has the appropriate survey methods such as Electrical resistivity, Seismic, (Gravity), and (Ground-penetrating radar).

Metalliferous mineral deposits

The exploration of these, has the appropriate survey methods such as Magnetic, Electromagnetic, Electrical resistivity, Self-potential, Induced-polarization, and Radiometric.

Bulk mineral deposits

The exploration of these, has the appropriate survey methods such as Seismic, (Electrical resistivity), and (Gravity).

Fossil fuels

The exploration of these, has the appropriate survey methods such as Seismic, Gravity, Magnetic, and (Electromagnetic).

Newton's law of gravitation

The force between two point masses whose dimensions are small with respect to the distance between them.

Terrain correction

The free-air and Bouguer corrections are often applied together as the combined elevation correction.

Tidal variations

The periodic gravity variations caused by the combined effects of Sun and Moon.

Natural field noise

These are fluctuating measurements such as lightning strokes, magnetic storms, earthquakes, and typhoons.

Artificial field noise

These are man-made structures such as power lines, buried metal pipes, and metallic objects on the ground.

Engineering/construction site investigations

These have the appropriate survey methods such as Electrical resistivity, Seismic, Ground-penetrating radar, (Gravity), and (Magnetic).

Archaeological investigations

These have the appropriate survey methods such as Ground-penetrating radar, Electrical resistivity, Electromagnetic, Magnetic, and (Seismic).

Free-air correction

This correction is for the decrease in gravity with height in free-air resulting from increased distance from the center of the Earth, according to Newton's Law.

Radar method

Travel times of reflected radar pulses; dielectric constant

Seismic method

Travel times of reflected/ refracted seismic waves; density and elastic moduli which determine the propagation velocity of seismic waves.

True

True or False: Bouguer correction makes the assumption that the topography around the gravity station is flat.

True

True or False: Free-air correction is positive for an observation point above datum to correct for the decrease in gravity with elevation.

True

True or False: Gravity is greater at the equator than at the poles.

True

True or False: Gravity survey can be used for hydrogeological investigations to determine the geometry of potential aquifers.

True

True or False: Gravity survey can be used for locating sedimentary basins and possible hydrocarbon traps.

True

True or False: Gravity survey is used for the investigation of large-and medium-scale geological structures of ancient suture zones.

True

True or False: In combination, the equatorial bulge and the effects of the surface centrifugal force due to rotation mean that sea level effective gravity increases from about 9.780 m/s^2 at the equator to about 9.832 m/s^2 at the poles, so an object will weigh about 0.5% more at the equator than at the poles.

True

True or False: In marine surveys, Bouguer anomalies are conventionally computed for inshore and shallow water areas as the Bouguer correction removes the local gravitational effects associated with the local changes in water depth.

True

True or False: In mid-latitudes, the Eotvos correction is about +75 gu for each knot of E to W motion so that speed and heading must be accurately known.

True

True or False: Magnetic susceptibility is dimensionless.

True

True or False: On land, the Bouguer correction must be subtracted as the gravitational attraction of the rock between the observation point and datum must be removed from the observed gravity value.

True

True or False: Spatial variations in the Earth's gravitational field are caused by lateral variations in rock density.

True

True or False: Terrain correction is always positive.

True

True or False: The Bouguer anomaly forms the basis for the interpretation of gravity data on land.

True

True or False: The Bouguer anomaly is not appropriate for deeper water surveys.

True

True or False: The Bouguer correction of the sea surface observations is positive to account for the lack of rock between the surface and seabed.

True

True or False: The Earth's ellipsoidal shape, rotation, irregular surface relief and internal mass distribution cause gravity to vary over the surface.

Gravitational field

U=GM/r

Ohm-meters

Unit of resistivity

Latitude correction

Variations with latitude result from the oblate spheroid of the Earth.

Natural nature of field

e.g. gravity, magnetic, and spontaneous potential

Artificial nature of field

e.g. seismic reflection and refraction, electrical resistivity, induced polarization

Causative body

A rock unit of different density from its surrounding. It represents a subsurface zone of anomalous mass and causes a localized perturbation in the gravitational field known as the gravity anomaly.

Geophysics

A subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis.

Free-air correction

Accounts solely for variation in the distance of the observation point from the center of the Earth; no account is taken of the gravitational effect of the rock present between the observation point and datum.

0.1 mgal

Accuracy of gravimeter

2.70-3.20

Amphibolite

Noise

Anything else measured but which may not contain useful information.

2.00-2.65

Shale

Groundwater studies

depths and thicknesses

Gold

19.0

Quartzite

2.60-2.70

Granite

2.65-2.75

Gabbro

2.85-3.10

Pyrite

5.00

Gravitational constant

6.67 x 10^-11 m^3/kg s^2

Site investigation

Mechanical properties and depth

12 hrs

Minimum period of gravity variations

3.30

Olivine

Induced polarization method

Polarization voltages or frequency, dependent ground resistance; electrical capacitance

Lateral density contrast

Prerequisite for gravity anomaly

2.65

Quartz

Electromagnetic method

Response to electromagnetic radiation; electrical conductivity and inductance

2.20-2.70

Sandstone

2.60-3.00

Schist

2.20

Serpentine

Gravitational acceleration

g=GM/r^2

Density

kg/m3; g/cm3

Seismic velocity

m/sec; km/sec; m/msec

Gravitational field strength

mgal= 10^-3 cm/s^2

Conductivity

mho/m= Siemens/m