Mechanical wave measurement techniques are designed to map the underground conditions of a proposed trenchless project. They can also be used for oil and mineral deposit exploration.

Detailed knowledge of the soil conditions in terms of clay, subsurface water, rock and general porosity help engineers to choose the best route for an underground pipe and the best trenchless method of construction.

The better the understanding of the ground conditions, the higher the quality decisions and the less wastage in terms of time and cost.

Obviously the most accurate information can be obtained by digging a core sample and analyzing the layers of soil. However, this process is time consuming and costly. In addition, it is only accurate in the area where the core is dug and may not reflect the soil conditions a few meters away.

The alternative is to use mechanical waves to map the underground conditions based on the way the waves behave as they travel through the earth. Nondestructive test methods like mechanical wave measurement provide engineers with detailed information at low cost.

A balance must be drawn between physical measurements and mechanical wave measurements to predict the ground conditions across the area. Many engineers use core samples to verify the conclusions drawn from wave measurement techniques.

Mechanical Waves Overview

Mechanical waves are introduced into the ground using a source. These waves travel through the different layers of the ground or along the surface of different layers. By measuring the waves, engineers can determine what the underground conditions are.

Both seismic sources and receivers are needed to perform a mechanical wave analysis of a geological area. Dynamite provides a high-power source in some applications, but vibrators are also commonly used.

Receivers use electromagnetic energy to convert mechanical waves into current, which can then be plotted and analyzed. Where multiple receivers are used across an area, the quantity and quality of raw data increases significantly. This helps engineers to make more accurate analysis of ground conditions.

Waves That Travel Through the Ground

Primary waves (P waves) travel in a back and forth motion through the ground and are also known as compressional or longitudinal waves. They have the highest speed of propagation through the earth reaching speeds of around 6 km per second in surface rock and 10 km per second near the earth’s core.

Secondary waves (S waves), also known as shear waves or transverse waves travel through the ground in a motion perpendicular to the direction of motion. These waves travel slower than P waves with speeds of 3.4 km per second in surface rock and 7.2 km per second near the core.

All waves refract when they pass through materials of different densities and it is these changes in the direction and velocities of mechanical waves that engineers measure and study. Reflection of waves occurs when they strike the surface of a new layer at an acute angle. The wave essentially bounces off the surface rather than travel through it.

Waves That Travel on the Surface

Love waves travel along the surface of the ground without penetrating too deep below the surface. The wave shape is perpendicular to the direction of propagation.

Raleigh waves also travel along the surface, but they have both longitudinal and perpendicular effects. This results in an elliptical motion for particles affected by the wave.

Spectral Analysis of Mechanical Waves

Engineers process raw data from seismic receivers to convert mechanical wave measurements into a map of underground conditions. Some key parameters they look at are:

Shear wave velocity

Shear wave velocity is a measurement of the speed of propagation of shear waves. This measurement is directly related to soil conditions like shear modulus and density. As such, it is a useful measurement of soil conditions.

Compressional wave velocity

Compressional wave velocity indicates how compressible the material is. The higher the compressional wave velocity, the harder the rock is and therefore the lower its compressibility.

Seismic velocity ratio

The seismic velocity ratio is calculated as the compression wave velocity over the shear wave velocity. This variable gives an indication of fluid in the pores of the soil or rock.

What We've Learned About Mechanical Waves

Engineers use all possible means of determining the condition of the soil when undertaking a new project. This could include anything from a trenchless construction project to an exploration for mineral or oil deposits.

The most accurate way of determining the ground conditions is to take a core sample but this is very costly and time consuming. It also only gives information about the area close to the sample.

Mechanical waves are a non-destructive technique for gaining information about the underground conditions. Using carefully placed seismic sources and receivers, engineers can evaluate the ground conditions. Core samples can then be drilled to confirm the spectral analysis before the project commences.

Besides core sampling and mechanical wave analysis of ground conditions, engineers also use electromagnetic waves.

Because different materials absorb electromagnetic waves at different frequencies, an analysis of electromagnetic data can also be used to determine the type of soil, rock and deposits underground.