Today, geophysical methods are recognized as effective techniques for determining geotechnical parameters and the characteristics of subsurface layers. Geophysical tests are conducted at low cost and high speed, and their results are often qualitative. Seismic methods in geophysics are based on the propagation of mechanical waves through an elastic medium. The wave generated by a seismic source (such as a hammer blow) reaches a wave receiver (geophone) and is then amplified and recorded by a seismograph. Based on the particle motion direction within the ground, two types of waves are most commonly used in civil engineering project design: compressional or longitudinal waves, where particle motion is in the direction of wave propagation, and shear or transverse waves, where particle motion is perpendicular to the direction of propagation. By knowing the velocities of compressional and shear waves, as well as the density of the medium, the dynamic moduli of soil layers can be calculated.

The Downhole test is one of the most practical geophysical tests in geotechnical engineering. For construction projects planned on a specific site, the dynamic elastic moduli of the materials at that site are critically important, as the structural design depends on these values. One way to determine these properties is by measuring the velocities of longitudinal and shear waves.

The principle of the Downhole Seismograph Test (DHT) involves measuring the travel time of waves from the surface to various depths. A seismograph, typically equipped with three orthogonal sensors, is placed at different depths via a cable, and seismic waves generated by the source are recorded. Downhole and geophysical testing on different layers, such as soil and rock, is carried out by Sakht-Azma Company.