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Ground-penetrating radar, also known as ground-probing radar, georadar or GPR, has been

successfully used as a geophysical tool for over 40 years. In the past two dec-ades, GPR’s popularity in mining applica-tions has been growing as a result of technological advancements, the ability to obtain high-resolution data easily and efficiently, and the overall perceived high return on investment.

Accuracy, survey speed and real-time results of GPR systems provide quick and valuable information for mining professionals to make informed decisions that optimise various applications, such as mine safety, mineral exploration, mapping geological features and determining rock quality.

HOW GPR WORKSGPR works by sending a pulse of energy from an antenna into a material and recording the strength and the travel time required for the return of any reflected signals. A series of pulses over a single area make up what is called a single scan.

Reflections are produced whenever the velocity of the energy pulse changes, caused by transitioning into a material with different electrical conductive properties or dielectric permittivity. The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. For example, a pulse that transitioned from salt (having low dielectric permittivity) into shale (with high dielectric permittivity) will produce a very strong reflection, while one moving from dry sand (low dielectric) to limestone (with a similar low dielectric) will produce a very weak reflection.

While some GPR energy is reflected

back to the antenna, energy also continues to travel through the material until the pulse signal is completely attenuated or blocked by a metallic substance. The rate of signal attenuation varies widely and is dependent on the dielectric properties of the material through which the pulse is passing.

THE GPR SYSTEMEvery GPR system has three main components: a control unit, a GPR antenna and a power supply. The control unit displays and stores the real-time data and contains the electronics which trigger the pulse of radar energy that the antenna sends into the ground. It may have a built-in computer and hard disk/solid-state memory to store data for examination after fieldwork.

Some systems can be controlled remotely by an external laptop computer, which can be beneficial for mounting of the GPR system on vehicles. The antenna receives the electrical pulse produced by the control unit, amplifies it and transmits it into the ground or other medium at a particular frequency.

Antenna frequency is the major factor in resolution and depth penetration; the higher the frequency of the antenna, the shallower into the ground it will penetrate. However, a higher-frequency antenna can resolve smaller targets or thinner layers. The lower the frequency of the antenna, the deeper into the ground it will penetrate. A lower-fre-quency antenna will penetrate deeper, but cannot provide the same resolution

as a high-frequency antenna. Antenna selection is one of the most important factors in survey design. GPR antenna frequencies typically range from 16MHz to 2GHz.

GPR equipment can be operated with a variety of power supplies ranging from small rechargeable batteries to vehicle batteries and normal 110/220V supplies. Connectors and adapters are available for each power-source type so that the GPR equipment can be used in a wide range of setups – mounted on small portable survey carts, used on lifts or special trailers, or located on vehicles.

GPR FOR MINING APPLICATIONSSalt/potash miningGPR is used by a number of leading salt and potash mining companies in the US and Canada. Salt (halite) and potash are excellent candidates for the application of GPR technology because they have low dielectric permittivity and the equipment can image deeply while still offering high resolution.

GSSI has found that its most common equipment used at these types of mines include an SIR-3000, SIR-20 or SIR-30 control unit, a 2GHz air-launched horn antenna and a 400MHz ground-coupled antenna.

The 2GHz air-launched horn antenna offers superior resolution for determining the overhead thickness of the salt/potash to shale markers, as well as locating separations at the shale boundary and other potential safety concerns, such as fractures or thinning/thickening of shale

Exploration… and beyondJami Harmon and Brian Jones look at the application of ground-penetrating radar in the mining industry

Underground mines often use

GPR for overhead safety

mapping

GPR is often used for

mapping at potash and salt

operations

“Accuracy, survey

speed and real-time results of

GPR systems provide

quick and valuable

information for mining

profess-ionals to

make informed

decisions”

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MAPPING & SURVEYING

layers (see Figure 1). The lower-frequency 400MHz ground-coupled antenna can be used for layer or anomaly mapping at depths greater than 15m.

Data is often collected using common mining vehicles with a trailer for floor mapping, or a lift setup so that the GPR antenna can be elevated near the back; however, simple setups pulled (or pushed) along by an individual are used too.

The use of drilling and blasting techniques or continuous miners is often done somewhat blindly with limited geological knowledge, which can lead to the removal of too much material (causing safety or impurity issues), or not enough material to optimise profits. The use of GPR not only helps salt and potash mining professionals determine current and potentially future safety concerns, but it can also be used to aid in maximising extraction.

Mine planning GPR is a highly effective mapping tool for mine planning and exploration applications, such as locating problem-atic areas to avoid before they are exposed, or mapping features of interest to head towards, such as mineralised veins. The GPR profile shown in Figure 2 is from a limestone-mining operation in Iowa, US. Data was collected using a GSSI SIR-3000 control unit and 400MHz antenna.

The GPR technology was used at this mine to quickly and accurately determine limestone thickness as it relates to vertical inclusions, or simply to map the presence of shale inclusions prior to the mining process. This allowed the company to plan where to continue mining, as proceeding towards the inclusion would be a costly waste of time, money and resources when the aim is to produce a high-grade limestone product.

GPR data used for planning purposes is typically correlated to visible exposures or test-drill locations and then used to identify further areas of interest throughout the mine. This technique can save considerable time and money that might have otherwise been used to drill numerous, unnecessary exploratory holes.

GPR can map hundreds of metres a day and can give a continuous profile of what is beyond the surface, whereas core or drill holes offer much more limited information. Also, a destructive approach such as drilling or coring can often lead to other unwanted conse-quences, such as exposing water.

Hazard assessment Mining in general offers countless hazards that need to be managed and avoided on a daily basis. GPR can offer mining engineers another tool to help tackle these hazards and effectively remediate or eliminate them before a costly issue arises.

For example, a leading mining company in Chile requested a demonstration from GSSI to identify problem areas that could negatively affect mine safety.

GSSI employed a SIR-3000 control unit with a 200MHz antenna to survey a terrace flight in an open-pit mine to check for voids and/or unconsolidated material, the presence of which could imperil large trucks. Problems such as these could go unknown for years until a major issue occurs that can lead to damage to vehicles or worse.

Another example from the under-ground coal-mining industry is what is commonly referred to as ‘rock bursts’, or the violent fracture of rock under pressure.

GPR can be used to locate potential rock-burst locations by mapping fractures and anomalies within the bedded material. Figure 3 shows a 200MHz GPR profile of such potential zones. This information could be used to plan support structures or preventa-tive measures to ensure the safety of workers.

Mineral explorationMining professionals can use GPR for mineral exploration in igneous and metamorphic rock formations. The technology can be used to map hydrothermal veins and pegmatites, as well as locate cavities and vugs housing crystals that could otherwise be difficult and time-consuming to pinpoint with traditional drilling and blasting techniques.

Using 2-D or even 3-D imagery the features of interest can be precisely detected. For instance, GPR was used to locate a cavity that ended up as home to one of the largest emerald crystals found in North America.

Jami Harmon is marketing communications manager and Brian Jones is an application specialist, both at Geophysical Survey Systems Inc (GSSI), a world leader in the development and manufacture of subsurface imaging products. See www.geophysical.com

Figure 1: data profile from a 2GHz horn antenna showing separations at the salt/shale interface, as well as an intermittent shale seam

Figure 2: GPR data illustrates a divide in material layers (defined by a red line). Above the line is limestone, below is the shale/sandstone inclusion mix

Figure 3: 200MHz antenna data profile showing potential rock-burst zones within the layered material

“Antenna selection is one of the most important factors in survey design”

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