Multiscale integrated geophysics

Applying multi-scale integrated geophysical methods to mineral exploration requires understanding the complete process of mineral deposit formation, including its provenance, pathways, enrichment, and preservation. This allows for the creation of multi-attribute geophysical structural imaging at different scales, guiding subsequent specific exploration tasks. According to the definition of metallogenic systems, deep faults, subduction zones, rift valleys, and suture zones represent favorable metallogenic tectonic environments. In these cases, natural earthquakes, artificial deep-source seismic reflections, and long-period magnetotellurics are needed to characterize key structural features such as the large-scale lithospheric floor and crust-mantle boundaries. Detecting and studying concealed structures is a primary task in the regional metallogenic prediction stage. Radon measurement can be used to measure regional faults, and regional gravity, magnetic, electrical, and seismic exploration methods can be used to establish the physical properties within the metallogenic belt. Multi-scale integrated geophysical exploration methods are used to identify the most favorable ore-bearing spaces, further investigating and exploring the enrichment patterns and distribution characteristics of useful components within these spaces. Ultimately, this resolves the issues of the location, occurrence, scale, morphology, and reserves of the metal ore body itself.

With the development of exploration and development, the requirements for exploration are gradually increasing, which can be summarized as follows:

(1) More complex surface conditions

With social development, the surfaces available for direct exploration are mostly deserts, boulders, swamps, exposed limestone areas, yellow-scaled areas, and extremely cold plateau regions. These areas not only have poor construction conditions and high construction difficulty, but also make it difficult to obtain high signal-to-noise ratio data.

(2) More complex underground structures

The superposition and modification of multiple geological structures mean that only a portion of the previous geological processes are preserved, providing very little reference value.

(3) A combination of the aforementioned situations

Extreme surface geological conditions and complex underground geological conditions, such as in basin-mountain junction areas.

(4) Deep-sea basement structure research

Whether it is the gravitational electromagnetic method or natural earthquakes, the signal will attenuate to varying degrees during transmission. This leads to a decrease in the reliability of exploration as the depth increases. Deep Earth research will remain one of the most important methods in the field of geoscience for a long time to come. With the increase in exploration depth, traditional geological methods are subject to varying degrees of limitation. However, with the continuous advancement of geophysical methods and instruments, high-precision, high-resolution, easy-to-operate, and low-cost geophysical exploration methods and techniques are becoming increasingly prominent.

Based on the differences in density, magnetic susceptibility, resistivity, and wave impedance between the ore body and the surrounding rock, as well as the specific characteristics of ore-controlling structures, gravity, magnetic, electromagnetic, and seismic methods are selected to delineate ore-forming target areas in the shallow part of the deposit. In the deep part, geological elements such as strata, rock masses, mineralization alteration, and structures related to ore formation are explored for indirect mineral exploration. However, a single method will exhibit certain limitations. Incorporating other methods to constrain or supplement the exploration can significantly reduce the problem of multiple solutions in inversion, improve the applicability to complex geological conditions, and to a certain extent, improve exploration accuracy and enhance the understanding of geological conditions. By using multiple geophysical methods at different scales, the genesis of ore deposits at depth is studied, and the distribution characteristics of ore deposits are determined; the spatial range of geological elements controlling ore deposit formation and the ore deposit itself is understood, laying the foundation for the study of deep mineral resources.

Seismic reflections utilize the impedance differences of subsurface geological bodies to accurately interpret stratigraphic sequences. Seismic surface waves can be used for near-surface exploration. Combined reflection and refraction seismic surveys can reveal velocity interface fluctuations in the subsurface medium, including deep crustal boundaries, the asthenosphere, and the crust-mantle boundary. Seismic exploration offers high precision but is expensive and requires significant human and material resources. While non-seismic methods have lower precision than seismic methods, they each have their own unique characteristics. Utilizing density differences in rocks and minerals can comprehensively reflect different density bodies within the lithosphere under gravity. The co-source nature of gravity and magnetic data allows for the detection of magnetic susceptibility differences in the middle and upper crust using magnetic anomalies. Processing gravity and magnetic data using various methods can distinguish source depths, depict the spatial distribution of faults, and characterize the three-dimensional physical structure of the lithosphere. These methods include Moho surface detection and magnetic Curie detection. Gravity and magnetic detection methods are effective at identifying local anomalies and offer high lateral resolution. Compared to gravity and magnetic detection methods, electromagnetic exploration offers stronger vertical resolution and stratification capabilities. However, as the electric field decays exponentially with depth, its identification ability becomes inversely proportional to exploration depth within a certain depth range. Low-frequency signals can penetrate significantly deeper underground, but this method is greatly affected by high-resistivity shielding and has limited effectiveness in identifying low-resistivity bodies. Nevertheless, it still possesses unique advantages in deep structural detection, making it an important tool in deep exploration. Therefore, by integrating various geophysical methods to study the same geological object from different perspectives, a more comprehensive understanding of reality can be obtained, reducing ambiguity. This forms the principle of integrated geophysical interpretation.

With increasing exploration depth, new detection methods and technologies are constantly evolving, not only improving detection efficiency and reducing costs but also expanding the applicability of integrated methods to some extent. Improvements in geophysical equipment and advancements in methods and technologies have significantly expanded the application and problem-solving capabilities of integrated geophysical exploration. The number of available methods across different scales is increasing, as is the selectivity of integrated approaches. Using basic geological data as constraints for integrated geophysical exploration allows interpretations to more closely approximate the true state of geological bodies. Integrated geophysical schemes are being used to gradually refine the geological evolution from large-scale metallogenic domains to the spatial distribution of small-scale ore deposits and ore bodies, which in turn guides integrated geophysical exploration procedures and plans.