Geophysical field methods

Density and precision of surveys.

Gravitational anomalies occur only if the rocks that comprise the earth's crust have heterogeneities. In gravimetric problems, the concept of excess density is often used: this is the difference between the density of the enclosing rocks and the density of the structures that create the anomaly. Excessive density can be either positive or negative. In general, rocks consist of a substance in three phases: solid, liquid and gaseous. Density is determined by the ratio of these three phases, as well as the state of the physical characteristics of the natural occurrence: pressure, temperature, humidity, etc.

The density of the rock depends on the material composition of its skeleton, porosity, humidity and other factors. For example, igneous and metamorphic rocks have low porosity (1–2%), and their density is mainly determined by the chemical and mineral composition of the rock-forming minerals. Sedimentary rocks, as a rule, are characterized by a large range of changes in porosity; therefore, their density also varies widely. The average density of the earth's crust is 2.67 g/cm³. In general, the Earth - 5.52 g/cm³.

As a rule, the density of the same sedimentary rocks increases with increasing depth. Reliable density values ​​can only be obtained by measuring it under conditions of natural occurrence of rocks. Most often, the density of the rocks is determined by the samples extracted to the surface. In this case, amendments must be introduced that bring the density values ​​to the physical conditions in which the rocks lie.

So, gravitational anomalies arise only if the rocks composing the earth's crust differ in density and the boundaries between them are not horizontal.

Differentiation of section rocks by density values ​​determines the possibility of using and efficiency of gravity exploration: the more different the density of the rocks composing the structure and the containing, the greater the gravitational anomaly created by this structure. The difference in the densities of the inner rocks and the rocks of the structure creating the anomaly is called excess density.

In table below, we can see density values of magmatic, metamorphic and sedimentary rocks. Density of igneous rocks increases from acidic differences to basic and ultrabasic. In metamorphic rocks, density is determined by the source material, as well as by the degree of metamorphism. The range of variation in the density of sedimentary rocks is very wide. Therefore, the density values ​​given in the table are approximate. As a rule, the density of the same sedimentary rocks increases with increasing depth. The density of the rocks can also be determined by the results of measuring the gravitational field created by the masses in the volume. However, with such definitions it is difficult to take into account the influence of masses that are outside the limits of our known volume. The density determined by the gravitational prospecting is assumed to be called the apparent density.

Rock densities, g/sm^3