Seismic Refraction Tomography (SRT)
The seismic method is based on the principle of recording—as a function of time—the reflections and refractions of acoustic waves that occur at the interfaces between geological formations underground, following the transmission of these waves from a source. By analyzing the resulting variations in seismic velocities, it becomes possible to determine or image the velocity distribution within the subsurface. An acoustic wave generated by an energy source (such as an impact, a weight drop, an explosion, or a vibration) propagates through the subsurface in various phases. Initially, the waves travel as body waves (P and S waves), followed by surface waves (such as Love, Rayleigh, or channel waves). The P-wave (primary wave) travels spherically in the direction of propagation and can reach great distances. It is followed by the S-wave (secondary wave), characterized by motion perpendicular to the direction of wave propagation; these waves are polarized in the direction of motion. S-waves do not propagate through liquid media. Rayleigh waves exhibit elliptical motion, which can occur both in the direction of propagation and in the opposite direction, depending on depth. In a subsurface environment consisting of multiple layers, S-waves produce a transverse, horizontal motion parallel to the ground surface. As seismic waves travel subsurface, they are reflected or refracted due to differences in acoustic impedance; these waves return to the surface, where they are captured by receivers known as geophones and recorded by a recording system. The principle of the method involves recording a signal that originates from a source, travels through the subsurface, and reaches a receiver; the velocity of the medium is then determined based on the travel time (in milliseconds or seconds) of the signal. This process is based on the fundamental physical relationship: velocity = distance/time. Consequently, seismic methods enable the characterization of various subsurface environments, ranging from layers near the surface to geological structures at great depths. Since the method aims to determine seismic velocity, it also allows for the identification of lateral and vertical discontinuities. When applied tomographically, it can reveal a detailed model of the subsurface. The method can be used both on land and at sea, which constitutes a significant advantage. Its utility and widespread application—particularly in hydrocarbon exploration—have facilitated the rapid development of this technique.
Seismic studies can be conducted using either reflection or refraction methods. Reflection seismology, in particular, is widely used in petroleum exploration, though it is a very costly technique. While it can also be applied to shallow-depth investigations, its high expense often leads to it being bypassed in favour of ground-penetrating radar (GPR) surveys. For shallow investigations, the seismic refraction method is more commonly employed. In general, this method relies on the principle that seismic waves—generated by a source positioned at a specific offset—refract off subsurface layer boundaries and return to the surface, where they are captured by geophones placed at regular intervals and recorded by a data acquisition system. These recordings are then used to determine the seismic velocities of the subsurface layers. To ensure reliable results, the process typically involves multiple “shots” (seismic source activations)—usually at the beginning and end of the survey line, and sometimes at the center—to accurately calculate these layer velocities. However, the subsurface is not always composed of simple horizontal or inclined layers. Given the complex nature of the subsurface, a more sophisticated interpretation is required to account for both horizontal and vertical variations in the velocity structure. Seismic Refraction Tomography (SRT) offers an effective solution for this, as it is highly capable of resolving velocity gradients and lateral velocity variations. Consequently, various features and issues—such as karst formations, faults, and specific soil/ground conditions—can be identified in detail using the SRT method.
Seismic Refraction Tomography (SRT) is an investigative technique that has gained widespread use in recent years for addressing geological and geotechnical challenges. This method enables the two-dimensional modeling of the subsurface based on seismic P-wave velocities. By structuring the solution in terms of discrete cells, it becomes possible to model a wide variety of subsurface conditions. Although the method may not offer ultra-high resolution, it is sensitive to velocity variations within the medium; provided there is sufficient velocity contrast, a reliable solution can be obtained. Depending on the geophone spacing and the energy source employed, the method can be used to investigate various depths. Since shallow-depth issues typically fall within the upper 100 meters, the Seismic Refraction Tomography (SRT) approach is well-suited for identifying geological and geotechnical concerns at these depths. SRT is a significant geophysical diagnostic tool used for a range of applications, including the detection of subsurface voids and karstic features, the location of subsurface clay deposits, the imaging of soil-rock interfaces and fill zones, the identification of shallow aquifers and fault zones, the determination of bedrock depth, the assessment of landslide geometry, the mapping of artificial fill, tunnel investigations, and the development of P-wave velocity (Vp) models for the ground beneath high-rise structures.
Multi-Channel Surface Wave Analysis (MASW)
The Multichannel Analysis of Surface Waves (MASW) method, developed by Park et al. (1999), is a seismic technique currently in widespread use, particularly in geotechnical and microzonation studies. Following the data acquisition principle illustrated in Figure 1, the MASW method aims to determine a dispersion curve from field-collected data and subsequently derive the subsurface shear-wave velocity (Vs) distribution through an inversion process.
Figure 1: Illustration showing the systematic data acquisition stages of the Multichannel Analysis of Surface Waves (MASW) method (adapted from Park et al., 2007).
In the field, data are collected using 4.5 Hz geophones—the number of which depends on the equipment—and are displayed along a time-distance axis. Performing impacts (seismic source excitations) at specific offsets from either end of the geophone array yields a one-dimensional result. However, by conducting impacts at various locations along the entire array of geophones, it is possible to generate a two-dimensional subsurface shear-wave velocity model. Once the collected data have undergone preliminary processing, dispersion curves are extracted. Through an inversion process applied to these dispersion curves, Vs velocity values are determined as a function of depth and the number of subsurface layers. In the case of two-dimensional data acquisition, the result is a two-dimensional cross-section of the subsurface Vs velocity distribution.
MASW (Multichannel Analysis of Surface Waves) surveys involve recording surface waves to determine the shear-wave velocities (Vs) of subsurface layers. This data is then used to calculate dynamic and elastic parameters for geotechnical purposes, including density (ρ), maximum shear modulus (Gmax), Young’s modulus (Ed), Poisson’s ratio (ν), bulk modulus (K), the seismic velocity ratio (Vp/Vs), and Vs30 (m/s).
Refraction Microtremor (ReMi)
Refraction Microtremor (ReMi) is a significant technique used for the geotechnical characterization of the subsurface by utilizing shear waves. The ReMi method is conducted using the same geophone array employed for seismic refraction (Vp) surveys. The ReMi technique records ambient background noise (microtremors), and the data is subsequently analyzed using methods similar to those applied in MASW. This approach offers two primary advantages: it enables linear subsurface profiling using the same geophone configuration as seismic refraction surveys, and it utilizes ambient background noise as its energy source. Consequently, the use of such microtremor measurements makes the method highly effective in noisy environments, such as urban areas. This allows for a two-dimensional analysis of the subsurface by leveraging shear waves that originate from ambient sources—referred to as microtremors or seismic noise—and propagate through the medium. In addition to being fast and reliable, this method allows for the acquisition of subsurface data at depths of up to approximately 100 meters. Consequently, it provides significant contributions when integrated with active seismic methods—such as seismic profiling—enabling joint interpretation (Louie, 2001; Clement, 2019).
In single-station MHVSR (Microtremor Horizontal-to-Vertical Spectral Ratio) analyses—a technique introduced by Nakamura (1989)—the horizontal-to-vertical spectral ratio is determined using ambient seismic energy recordings. This approach is widely employed in regional microtremor studies, particularly in areas characterized by surficial sedimentary layers, to estimate the depth to the bedrock. The method utilizes a three-component accelerometer deployed in the field to record ground motion; the spectral ratio of the horizontal and vertical components is then derived through the analysis of these recordings across various frequencies. These calculations are typically performed using 30-to-60-second time windows. Subsequently, the frequency component corresponding to the peak amplitude of the ratio is identified, which serves as a basis for estimating the thickness of the subsurface sedimentary layer. In this process, the shear-wave velocity of the medium is empirically estimated by leveraging an experimental relationship between the resonance frequency and the thickness of the layer.