Gravity, Magnetic, and Seismic Geophysical Methods

Gravity Method

The weight of any body depends on the force of gravity at that location.

The force of gravity varies with elevation, rock density, latitude, and topography.

When a mass is suspended from a spring, the amount of spring stretching is proportional to the force of gravity.

F=m.g

Where:

• g: Acceleration of gravity.

Since mass is constant, then stretch variations determine the variations in the acceleration of gravity (g).

A gravimeter is an instrument used to measure (g) at stations. The readings are corrected for elevation, latitude, and topography. The normal value of (g) is subtracted from the corrected readings to compute the residual gravity.

The values of residual gravity are plotted at the measuring stations to produce a contour map of equal residuals.

In a normal case, the stretching of the spring is normal. If the basement rocks (denser) are deeper and sedimentary rocks (low density) are thicker, the stretching of the spring is less than normal. If the basement rocks are near the surface, the spring stretching is greater than normal.

The value of g at sea level = 980 cm/s²

Closed contours represent gravity anomalies and may denote a subsurface geological structure. Residual gravity is measured in Gal (Galileo) = 1 cm/s²

1 Gal = 1000 milliGal (mGal).

Magnetic Method

The local magnetic variation in the magnetic fields occurs when the basement complexes are near the surface and where concentrations of ferromagnetic minerals exist.

Magnetometers are the instruments used to measure the Earth's magnetic field.

These instruments measure the intensity or field strength of the Earth’s field. This is measured in Tesla (T) and Nano Tesla (nT) or (ϒ)

1ϒ= 10^-9 T

Nowadays, most magnetic surveys are made from airplanes.

The magnetic intensity is recorded.

The readings are corrected (for annual magnetic and other variations).

Residual field = corrected magnetometer reading - theoretical values

The residuals are plotted on a map, and contours of equal gammas are drawn.

Closed contours indicate magnetic anomalies.

Seismic Method

The concept of the seismic method is:

• An energy source (dynamite, vibrator, air gun) to produce seismic waves.
• Detectors (geophones, hydrophones) to register the motion or pressure and convert it to electrical signals.
• Interpretation and conversion of the electrical signals by computer programs to seismic sections (2D or 3D) to reveal the subsurface structures in the surveying area.

There are two paths for the waves between the source and receivers: reflection and refraction.

Instead of one detector, an array of detectors for each energy source is used to gather data for the subsurface layers.

These data are displayed as a seismic record containing several seismic traces. A seismic trace, presented as a wiggly line, represents the response of a single detector (or connected group of detectors) to the seismic arrival energy.

In the figure: (seismic reflection record or shot record because all traces represent energy from a single source or shot)

Event one (0.165-0.78s)

Event two: (1.9-1.99s)

Seismic Waves

Three types of seismic waves:

• Compressional or longitudinal P-waves
  • Propagate in solids, liquids, and gases.
• Shear Waves S-Waves
  • Propagate in solids only.
• Surface Waves
  • Exist at the boundary of the propagation medium.
  • Rayleigh waves
  • Love waves