Shallow seismic exploration is an engineering geological and geophysical exploration that uses the characteristics of seismic wave propagation in different rocks and soils to detect shallow geological structures (usually tens to hundreds of meters) and determine the physical and mechanical parameters of rock and soil. As per different tasks, purposes and different geological conditions, different exploration methods can be applied.
According to propagation modes of seismic waves. shallow seismic exploration can be defined as the refraction wave method, reflection wave method and refraction wave method. According to different modes of excitation and reception, shallow seismic exploration can be defined as S wave exploration and P-wave exploration and according to the position of the lateral line, shallow seismic exploration can be defined to water surface exploration, land exploration, underworkings exploration, PS logging and VSP seismic profile and etc.
The application of the refraction method is based on the premise that the target layer must be the reflected layer and there must be a difference in wave impedance between the top and bottom of the interface of the target layer (Z2≠Z1).
As the reflected wave is located in the continuation zone, it is difficult to compare and trace the original data and make geological interpretations, so the method of multiple coverages is generally adopted for data processing. The reflection wave method is widely used in the following fields: 1) The exploration of geological data such as oil fields, coal fields and metal minerals. 2) The exploration of underground hot water resources. 3) The exploration of active faults. 4) The investigation of geological hazards, such as karst and 5) The distribution of engineering geology.
The application of the refraction method is based on the premise that the target layer must be the refraction layer, that is, the velocity under the interface must be greater than that of all the layers on the interface (V2 >V1.). At present, the refraction method is mainly applied in the following fields:
1) Low speed band measurement for providing travel correction data for reflection and refraction measurement.
2) Engineering Seismic Survey for providing geotechnical elastic parameter data for buildings.
3) Overview: Measure the thickness of the sedimentary layer and the burial depth of the crystalline base.
4) Deep seismic sounding: Determine the velocity distribution of the crust and upper mantle.
5) Special measurements: Determine near-vertical geological bodies, such as faults, intrusions, rock mound boundaries, ore bodies and etc.
Return wave is a method applied in the case of continuous media. The time distance curve of return wave mainly depends on the vertical velocity result of underground media. The return wave method is often used in engineering geological exploration to determine the underground velocity structure.
PS logging generally adopts layer by layer detection method and is mainly used to measure the velocity of P wave and S wave. PS logging is stimulated near the surface wellhead and data is received by geophones or hydrophones placed at different depths in the well. In order to ensure that the first arrival wave is a direct wave or a transmitted wave, the excitation point should be close to the wellhead.
In order to improve the accuracy of seismic wave identification and pickup, the method of bidirectional percussion can be used to accurately identify the various seismic phases of longitudinal wave( P wave) and transverse wave (S wave) after the corresponding data processing, so as to improve the accuracy of data processing and interpretation.
In recent years, shallow seismic exploration technology has made great progress in hydrogeological engineering exploration, environmental geology, mineral geology exploration, engineering foundation test and geothermal exploration. Engineering seismograph has been greatly developed due to low cost and portable use, such as Geometrics Geode 24 and PASI GEA 24 and etc.
Seis Tech is a professional supplier for seismic refraction cable, seismic geophones and 24 channel hydrophone cables for shallow exploration and we have sold these seismic instruments to our clients in more than 30 countries and regions.
After years of exploration, development and practice, we can see that the shallow seismic exploration is playing an very important role in the engineering and construction of foundation exploration, bedrock buried depth detecting, karst subsidence, underground caves, ground fissure, landslide, and the seabed engineering survey. But in practice, it is important to pay special attention to the work method and the selection of acquisition parameters.
In the process of high-precision exploration such as crack and karst, it is not suitable to use too many times of superposition (including vertical superposition and horizontal superposition) for data acquisition, nor strong modification for data processing. At the same time, because of the small scale of shallow exploration, it is difficult to obtain ideal exploration results when facing abnormal bodies with large buried depth.