Cavity Autoscanning Laser System (C-ALS)

Product number: 8000

Imagine what would happen if you could measure a void easily!

CALS

Introduction Abandoned mine workings are an environmental hazard for the general public, government agencies, developers & mining companies alike.

The most serious threat relates to the effects of void collapse, subsidence & migration of voids to the surface due to natural & man-made causes. To pre-empt problems, accurate surveys of these cavities / voids is imperative.

However, conventional survey techniques are costly, dangerous to undertake & time consuming and in many cases not practical or possible.

Description of C-ALS, Cavity Monitoring System
Measurement Devices Ltd (MDL) have developed a ‘Robotic Laser Surveying System’ capable of being inserted into voids & cavities via predrilled boreholes to quickly and safely survey old workings. This system is known as the Cavity-Autoscanning Laser System (C-ALS).

MDL’s C-ALS cavity monitoring system is a self-navigating, ‘motorised’ robotic probe designed to enter & survey dry abandoned mine workings or cavities. With a diameter of just 50mm, the unit can be deployed down pre-drilled boreholes of 65mm or more, from where it employs a miniaturised laser scanner to measure the three-dimensional shape of the void together with its surface reflectivity.

The C-ALS, cavity monitoring system may be deployed down hole to a depth of 300m, up hole or horizontally to a distance of 100m.


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The C-ALS cavity monitoring system
A complete C-ALS, cavity monitoring system is composed of the following parts:
1). A motorised, high rep rate laser scanning head. The pan & tilt range is 360º in both axes ensuring an unrestricted view of the void. Angle encoders provide angle measurement to 0.1º resolution with scan increments to 0.1°.

The measurement principle is ‘time of flight’.

Laser (Class 1 Eyesafe to FDA and IEC Standards) pulses are generated & transmitted out to the target & back. The time is multiplied by the speed of light divided by two (two way distance measurement) to give the distance to the target.

Measurement Absolute accuracy is 5cm & repeatability/resolution is 1cm. Maximum range is 150m. Scan rate is up to 650,000 readings per hour/ 200 readings per second.


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2). A custom-designed 3D ‘navigation’ module integrated in the C-ALS, cavity monitoring system, tracks the position & orientation of the C-ALS probe during borehole deployment & determines the final x,y,z coordinates & attitude of the C-ALS probe.

The navigation module contains both a 3-axis fluxgate magnetometer & a 3-axis accelerometer. The combination of these two sensor systems enables the inclination, roll & azimuth angles of the C-ALS probe head to be determined. Inclination & roll angles are determined from the accelerometer subsystem, which measures the pull of gravity. After inclination & roll are known, the magnetometer subsystem is used to determine system azimuth angle. Knowledge of the inclination & roll angles enable determination of the horizontal components of the earth’s local magnetic field; this information defines the azimuth angle.

The probe is one metre long & is available with either a stainless steel or lightweight aluminium casing.

3). A ‘Nosecone’ camera & infra-red lighting system is also integrated into the probe. This serves two purposes:
3a). To gauge exactly the distance from the collar to the ‘breakout’ point of the borehole into the cavity. The scan head & the rotating horizontal shaft must be deployed far enough into the cavity to be clear of the borehole and to ensure unrestricted motion. However, the C-ALS must not be so far in to the cavity that its full length ‘free falls’ into the void. If this occurs it may be difficult to retrieve the unit without damage.

3b). To check the condition of the borehole. The camera will pick up any obstructions, blockages or ‘cave-ins’ & thus avoid damage to the C-ALS, cavity monitoring system.

4). A probe extension piece is provided to attach to the C-ALS probe. It incorporates a connector for use with Boretrak rods. The extension piece also acts as a stabiliser, for example within a borehole, to lessen the chance that the C-ALS twists along its axis during the survey.

5). A Kevlar, multi-core C-ALS cable delivers power to the probe and also carries data to and from the instrument. The cable and connectors are highly durable and have a breaking strain of 500kg. The cable can be used without the Boretrak rods to deploy the C-ALS in areas where there is no magnetic interference.

The cable is marked at one-metre intervals by alternate red and black rubber nodules to facilitate accurate Boretrak readings during deployment of the probe.

6). The C-ALS cable is interfaced to a control box located on the surface. Housed in the rugged, watertight case, is an internal battery pack, power regulator & a video interface for the nosecone camera. An ethernet or WiFi link is also enclosed, allowing connection to the PC & remote control of the C-ALS probe.

7). A ruggedised tablet PC computer is used to control, operate & record probe readings & pictures. This will run the C-ALS Control software & can also run MDL’s ModelAce® software to enable the data point cloud to be processed & viewed (on site) within minutes of finishing the scan.

8). A set of carbon-fibre Boretrak rods (1m or 2m in length) are provided to push the C-ALS up or along the hole and to provide probe orientation in high magnetic field environments (where a compass is not reliable.) Boretrak® rods are flexible & hinged but do not twist, so the azimuth of the probe will remain constant. They may be aligned along a known direction to determine the probe orientation at the start of a scanning operation. The rods are lightweight (900g per 2m length). Each full rack has a quick release joint to connect to additional racks of rods.

A heavy duty wire, winch & tripod may be provided to lower & recover the probe during ‘extended’ downhole deployment.

For further information call

  • Anthony Comber on Tel: +44 (0) 1904 791 139 (UK)
  • Bradley Husack on Tel: +1 (281) 646 0050 (Houston, TX)
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