Binzhong Zhou

Electromagnetic (EM) methods, both airborne and ground, are some of the most widely used geophysical techniques in mineral exploration, in which natural or controlled sources are used to transmit EM waves to the Earth and measure the reflected EM signal

Instead of dismissing seismic diffractions as interfering noise and ignoring or suppressing them, they can be used to identify small geological discontinuities such as faults, dykes, and other features that act as small scatters in the subsurface. We describe a simple, but fast and effective, structure-oriented, moving average error filter (MAEF) applied to the neighboring traces to extract diffractions from time-stack reflection seismic data or diffractors from migrated seismic data. The filter estimates the reflections with the average values of the neighboring traces along the reflection dip direction, which can be computed by the gradients of seismic data. The difference between the input data (unmigrated or migrated) and the estimated reflections yields the diffractions. By identifying diffractions, small faults and other minor features that are difficult to detect using conventional seismic reflection processing can be enhanced and detected. The effectiveness of the proposed method for diffraction extraction is demonstrated by numerical models and by applying it to a 3D seismic survey data over a partially mined-out coal deposit.

Borehole radar (BHR) waves will be severely attenuated when a borehole is filled with high-conductivity (HC) media. As a result, BHR side-reflections are significantly weakened, i.e., the capability to detect the targets far from the borehole is restricted. However, V-shaped reflected guided waves (RGWs) are commonly observed in BHR data from such HC boreholes. They are often regarded as unwanted contamination to conventional BHR surveys and should be avoided or suppressed. However, we advocate that the RGWs, as a type of reflected wave traveling along the borehole, contain geological information about the surroundings of the borehole and can be used to locate and recognize geological boundaries, such as lithological interfaces and fractures that intersect the borehole. Unlike conventional BHR surveys mainly imaging the structures near-parallel to the borehole, the RGWs can effectively sense near-normal ones. In addition, we can use RGWs to distinguish between echoes from fractures and those from bedding planes. This is achieved by recognizing different phase characteristics of RGWs for these structures: the RGWs from both sides of a fracture have the same phase, whereas the ones from a stratigraphic interface have opposite phases. Synthetic and real data are used to demonstrate the feasibility of detecting subhorizontal structures by RWGs. RGWs extend BHR measurements and allow a better understanding of subsurface structures. They help to predict and reduce mining risks and monitor potential geological hazards.

Slim and short borehole radars can be used to profile blastholes in South African chromitite reef mines, in order to investigate the integrity of layers that will, after blasting, form the ceiling of tomorrow’s stope. The profiles are run without disrupting production, in the interval between withdrawing boom-mounted percussion drills from a stope and charging the blastholes. These holes are typically 3m deep and 5cm wide. The fact that they are drilled with precision in a standard, repeated grid eases profile geo-registration and speeds up the synthesis of 3D images of near-horizontal interfacial blemishes such as domes which may be stimulated by blasting to drop roof rock into the stope. Slimline blasthole radars can also be used underground to induce electromagnetic pulses onto cable bolts in order to verify that anchors are well-set, and the cables are holding.

SUMMARY: Borehole radar (BHR) is an effective imaging tool. It can be used to detect and map faults, fractures, folds, domes, partings and mine workings. Most BHRs have azimuthally omnidirectional radiation patterns. The echoes sensed by such BHRs may come from any direction. Considering a radar in a straight borehole that passes through a stack of flat reflection planes, V-shaped events or crosses appear on the time section. One of the arms of each cross is a real image while the other is an ambiguity of known origin. Directional ambiguities such as these obstruct efforts to interpret the data. In this paper, we address this difficulty by using a modified f–k migration algorithm to translate crosses into lines on the final section that are consistent with a priori information about for example bedding. Compared with conventional strategies, for example migration + f–k dip filter, this approach integrates the two separated processes into one and is straightforward, computationally effective and simple to implement. The method is demonstrated using a synthetic model and a real BHR field data set. It allows the interpreter to use a priori information about fault swarms or plausible bedding planes at an early stage. The reconstructed BHR image helps the search for geological anomalies such as fractures, partings, domes and rolls that could be a hazard for mining.

Geophysical logs can be used not only for qualitative interpretation such as strata correlation but also for geotechnical assessment through quantitative data analysis. In an emerging digital mining age, such a use of geophysical logs helps to establish reliable geological and geotechnical models, which reduces safety and financial risks due to geological and geotechnical uncertainty for new and existing coal mining projects. This paper presents some examples of geological and geotechnical characterizations from geophysical logs at various coal mines in Australia and India. The applications include rock strength and coal quality estimations, automated lithological/geotechnical interpretation and geophysical strata rating, all based on geophysical logs. These derived parameters could provide input to modelling, control, even ‘digital twin’ generation in a form of geological and geotechnical models as part of the future digital mining. The outcomes can be visualized in 3D space and used for identifying the key geotechnical strata units that are responsible for caving behaviors during longwall mining. This could assist site geologists and planning and production engineers predict and manage mining conditions on an ongoing basis. Both conventional logs such as density, natural gamma and sonic and less common logging data, such as full waveform sonic, televiewer and SIROLOG spectrometric natural gamma logging data are examined for their potential applications. The geotechnical strata classification and rock strengths predicted from the geophysical logs match the laboratory tests, drill core geotechnical strata classification, core photos and the mining condition/behavior observed. These illustrate the usefulness and effectiveness of using geophysical logs for geological and geotechnical characterizations.

Reflected guided borehole radar waves can be observed when a borehole radars (BHR) is either suspended by a conductive communication cable or run in a borehole filled with saline water. They are often referred as unwanted contaminations to the conventional BHR surveying and should be avoided or suppressed. However, as a type of reflected waves, they contain geological information about the surroundings of the borehole and can be used to recognise geological boundaries such as lithological interfaces and fractures intersecting the borehole. The guided BHR reflections have different phase characteristics for stratigraphic boundaries and fractures. The reflections from both sides of a fracture have the same phase, whereas the reflections from both sides of a stratigraphic interface have opposite phases. Such characteristics enable us to differentiate fractures from bedding boundaries down the borehole using the borehole guided waves. This is demonstrated by both synthetic and real field data.

Damage to the top of coal seams, caused by incorrect blast stand‐off distances, results in coal losses of up to 10–15% to the Australian open cut coal mining operations. This is a serious issue to be addressed. Here we propose to use a new forward‐looking imaging technique based on the borehole radar technology to predict the coal seam top in real time while drilling blast holes. This is achieved by coupling the conventional borehole radar waves on to a steel drill rod to induce a guided wave along the axial drill rod. The drill rod ahead of the borehole radar behaves as a forward‐looking antenna for the guided waves. Both numerical modelling and field trials simulating a drill rod as an antenna are used to investigate the feasibility of the proposed technique for prediction of the coal top under typical open cut environments. Numerical modelling demonstrated that conductivity of the overburden is the most important factor affecting our ability to see coal seams ahead of the drill bit, the guided borehole radar waves could be used for top coal prediction and a theoretical prediction error less than 10 cm and a forward‐looking capability of 4–6 m can be achieved. Field trials at Australian open cut coal mines also demonstrated that guided borehole radar waves can be observed and used for prediction of coal top ahead of drill bit during blast‐hole drilling in resistive, open cut environments (the average resistivity should be higher than 75 Ωm).

Migration is used to collapse “diffractions,” i.e., to focus hyperbolic events that appear in the space-time of a seismic profile — into spots of finite area in the image space. These usually represent scattering objects. However, there are situations in which some of the energy can be focused by migration, and muted without significantly damaging the remaining echoes. Demigration or forward modeling then restores the remaining data, and the removed signals can be rebuilt by subtracting these restored data from the original records. This process can be classified as migration-based filtering. It is demonstrated by synthetic and field data that this filter can be used for suppressing unwanted coherent signals or separating/extracting wavefields of interest: (1) the suppression of ground roll in seismic shot gathers, (2) the suppression of axially guided arrivals in borehole radar profiles, (3) suppressing the direct arrivals to enhance Stoneley-wave reflections in full-waveform sonic logging data, and (4) separating up- and downgoing waves in vertical seismic profiles.