Sonic drilling offers distinct benefits:

Superior information: Since the sonic cutting action sheers the soil cleanly and no fluid is present to dilute the sample, this method results in a relatively undisturbed sample through any type of soil. You get at or near 100 percent core recovery, continuous to the depth, which means superior subsurface information for decision making.

Speed: The combination of vibration and slow rotation allows the casing to quickly advance through unconsolidated formations, providing a penetration rate up to three times faster than traditional overburden drilling.

Reduced waste: With efficient cutting and no fluid involved, drill spoils and waste are reduced by up to 80 percent compared with conventional drilling.

Accurate to depth transitions

Uranium formations

These advantages give miners more control in planning projects and managing resources. The use of sonic drilling minimizes the risk of a project failing or being delayed due to unknown or difficult subsurface conditions, and decreases overall time and costs. It also lessens the impact to the environment.

Because of these benefits, sonic drilling works well for a variety of mining applications. Drillers can sample unconsolidated ore piles and tabulate reserves from legacy stockpiles, whose medium- to low-grade ores can be efficiently processed today. They can also study leaching effectiveness, pinpointing dry zones and optimizing the leaching process of desired minerals.

These features also make sonic rigs ideal for shallow drilling in overburden or precollaring before a conventional drill takes over to drill deeper into bedrock.

Sonic drilling is well suited for geotechnical studies for mine construction and other building projects. Additionally, because some sonic rigs are smaller and have very low ground pressure, they can operate successfully in environmentally sensitive areas; for example, they can drill without ruining asphalt or landscaped areas or sinking into marshy, swampy land.

While sonic drilling has been employed by the mining industry in the United States since the late 1980s, it is still an emerging technology in many countries. Boart Longyear has been a forerunner in sonic technology and has the world’s largest selection of advanced sonic rigs and highly trained drillers. Time and again we’ve experienced the exceptional subsurface information and other benefits this specialized technology can offer drillers, especially in the difficult formations that often frustrate the search for minerals.

Heap leach pads are large piles of rock waste from a mine operation that receive further processing through a leaching solution to remove the mineral ore from the rock. The leaching solution is applied via an irrigation or sprinkler system to the top of the heap leach pad. The solution penetrates the heap leach pad and separates the mineral ore from the rocks. At the bottom of the heap leach pad is an impermeable liner that is angled to allow the solution to be collected in a reservoir and then be sent for further processing.

Using one of their sonic rigs, the LS600, Boart Longyear can collect highly representative samples and achieve excellent sample recovery. Leach pads are made up mostly of unconsolidated material and the LS600 has proven effective in these conditions by producing 100 percent accurate in-situ core samples. The rig is also used to install wells in the heap leach pad.

Boart Longyear was contracted to sample and install solution wells for the largest gold heap leach pad in the USA, located in Northwest Nevada. The heap leach pad is over 400 feet tall. To achieve that height the mine operation built up the heap leach pad via 50 foot lifts.

When constructing each level, the haulers unavoidably crush and compress the previous top layer of the pad while adding material to the top of the pile. This creates a dense layer that causes a dam effect and does not allow the leaching solution to penetrate through the heap leach pad properly. The heap leach pad can then develop dry pockets where no leaching activity takes place — lowering the efficiency and return on investment.

Boart Longyear was hired to detect dry zones in the leach pad and to reactivate the zones via solution enhancement wells. First the Boart Longyear team drilled test holes to determine the depths of the dry zones and the specific lithological (physical characteristics of the ground) information.

These boreholes were 107 meters (350 ft) deep and were 15 to 23 meters (50 to 75 ft) above the heap leach pad liner. A continuous core sample was obtained by using the LS600 sonic rig. This allowed the Boart Longyear team to determine moisture concentration at accurate depths and to identify exposed ore.

Once depths of ore and moisture levels of the dry zones were indicated, Boart Longyear installed solution enhancement wells using an 11.5-centimeter (4.5 in) flush threaded well casing. The casings allowed the borehole to stay intact as an upper and lower k-packer were installed at specific depths. A 100-foot injection screen was placed between the k-packers to direct the leaching solution into targeted dry zones.

This process was repeated on an eight-week cycle with four weeks of solution and four weeks of rest. Once the cycle was completed, the k-packers and screen were moved to a new depth. The well would receive the solution at three heights, spanning 100 feet each time — bottom, middle, and top.

Over a three-year period, the team, using the LS600 sonic rig, drilled 55 gravity flow solution wells to alleviate the fine unprocessed gold from the heap leach pad. This resulted in 6,100 ounces of gold that equated to $8 million ($1,300 per ounce average). The project paid for itself within the first 70 days and the client received a 400 percent internal rate of return (IRR).