GEOPHYSICAL EXPLORATION

Geophysical exploration consists of measuring from the surface certain physical properties of the underlying material and of interpreting the measurements in terms of different geological lithology as well as structure. It is emphasized at the start that the data of geophysical measurements are valuable to the engineer only when correctly interpreted in geological terms. The properties investigated by the physical measurements are density, elasticity, electrical conductivity, and magnetism.

The four principle methods of geophysical exploration are: gravitational, magnetic, seismic, and electrical. Of these, seismic and electrical methods have the widest range of application in civil engineering practice.

Gravitational Methods

Differences in densities of adjacent rock masses give rise to measurable differences of gravitation. In its outer part at least, the earth is not homogeneous. Hence gravitational measurements often make possible the establishment of boundaries between masses of different density.

Because the earth is a rotating body, slightly flattened at the poles, gravity values vary with latitude. These values also vary according to terrain and elevation. Corrections must therefore be applied to reduce the observed values to a common basis. In reducing observed values to sea level, the influence of rock masses between sea level and the elevation of the station is taken into account. Abnormalities of the gravity, i.e., the differences between the theoretical and the corrected observed values, are represented in plans by contours of equal gravity anomaly values called isogams, and in section by anomaly profiles.

Magnetic methods

Rocks not only vary in density but also in magnetism. Hence, just as gravitational anomalies may be discovered and represented on maps, magnetic anomalies may be determined and used as a basis for interpretation for mixed conditions on the subsurface. Let’s not forget that our mother earth itself acts as a huge magnet, whose poles are located not on the surface but quite a distance below it. The estimated location of the Earth’s magnetic poles are The magnetic poles of our planet Earth are near but certainly not coincident with the poles of the axis of rotation of the planet. A magnetic needle freely suspend will take a definite position in space depending on the lines of magnetic force of the earth’s field at that place and time. A needle perfectly balanced on a vertical axis before it is magnetized will not remain in horizontal position after magnetism except at points on the magnetic equator. North of this equator the needle inclines to the north the inclination steepens with increasing distances from the magnetic equator, and the needle will be perpendicular with respect to the planet’s surface. The dip of the needle is completely opposite while moving over to the magnetic southern hemisphere. A counterweight commonly a silver or brass wire, is adjusted to balance the needle in a horizontal meridian, and the angle which the magnetic meridian makes with the true meridian at that place is the declination. The declination at any given place, however, is not constant. Long time changes and annual, daily, and irregular variations are recognized. Annual changes in declination are small, on the order of one minute. Daily fluctuations are on the order of 3 to 12 minutes. Because of the variations in declination it is necessary to establish control in magnetic work just as in barometric surveying.

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Electrical Methods

The electrical properties of earth materials, consolidated or unconsolidated vary widely. Generally speaking, rocks, with the exception of the metallic ores, are electrically conductive in proportion to volume, size, continuity, and distribution of voids and fluids present. Various types of unconsolidated materials differ significantly in voids and contained fluids; and consolidated rocks, for example, sandstone and granite differ in porosity and fluid content. These differences affect the conductivity or its reciprocal resistivity. The contrasts between consolidated and unconsolidated materials are usually of a higher order than differences within the two groups; hence, electrical exploration for depths of unconsolidated materials is generally successful.

Whereas some rock masses, as sulfide ore bodies, give rise to spontaneous electric currents, most electrical exploration for engineering ends is carried on by artificially energizing the ground.

Seismic Methods

The elastic properties of earth materials vary widely. Seismic methods of geophysical exploration are based on the variation of elastic properties. Differences in the elastic coefficients of different layers give rise to reflection and refractions of seismic waves, which are treated in the same manner as are the comparable phenomena of geometrical optics. The instruments are designed to measure and record the speed of propagation of such waves in earth materials. The velocity measurements make possible inferences as to the attitude, nature, distribution and structure of subsurface materials. The speed of the seismic waves in rock is influenced in large measure by the degree of consolidation.

Two methods are of common use in seismic exploration. These are known as reflection shooting and refraction shooting. Reflection shooting is generally used for deep exploration, commonly greater than 2000 feet. Refraction methods are better adapted to lesser depth. In refraction determination, a blasting cap or small charge of dynamite is exploded at or near the surface at a point known as the shot-point. The disturbances, commonly amplified, are recorded photographically on moving film. Time intervals are recorded on the film strip by time lines which are obtained from a tuning-fork device, electrically driven. The lines have peep slits which coincide when the lines are in the neutral position; hence two lines are photographically recorded on the films for each complete cycle. The instant of detonation electrically transmitted from the shot-point to receivers and is indicated on the film strip. In one method a wire on the blasting cap is hooked up with the galvanometers of the receptors in such a way that when it is broken by the detonation a kick is given to the galvanometers, and the time of explosion is simultaneously recorded on the curves for each receptor.
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