Equipment & Scanning Methodology
The goal of this section is to enable others to repeat the survey, while collecting good data and interpreting that data correctly to provide the client with usable information. The following applies to each of the 2.6GHz, 2GHz, 1.6GHz, and 900MHz antennas. Although basic, these methods should be sufficient for over 95% of surveys carried out. System calibration is recommended but is not necessary as each antenna comes with standard settings installed which can be recalled.
To start, survey the area for visual clues about the age, content, and composition of the structure. Ensure that no obstructions exist. The antenna must be coupled with the concrete, ensuring no membrane, insulation or standing water greater than ¼” thick exists. Concrete must be adequately cured (3 months for slab on grade and less for suspended slabs). The screen must always be clearly visible in the operator’s line of sight. The oscilloscope display should always be enabled to allow the operator to estimate the maximum depth of signal penetration. Checking the survey area from below for hazards and obstructions is always recommended.
Ground truth is the best way to calibrate the depth scale. Any feature found in the data can be used if its depth is known from an independent source (like drilling it). When the concrete bottom is visible, the concrete thickness can be used if measured at the edge of slab, in an open core, or if known from good documentation. The dielectric constant will automatically change based on the depth input. Failing ground truth, a choice of pre-set dielectric values for concretes can be used (supplying a 20% accuracy). Hyperbolic shape analysis can also be used.
Two-dimensional data is collected, and single profiles of data are interpreted in real-time. The position and number of survey lines needed on a particular structure depends on the complexity of the slab and the operator’s experience. Most of the scanning is performed with the antenna in normal orientation. Add a mark at the midpoint groove on the antenna housing when the backup cursor is at the summit of the hyperbola. For greater accuracy, scanning should be done from both directions perpendicular to each target and the average of both marks should be accepted as the true centre of target.
The 2GHz Palm antenna incorporates an integrated survey wheel which can change position allowing a cross-polarized configuration. Routine scanning using a cross-polarized configuration is critical for detecting targets found beneath other targets. Transverse metal targets can sometimes be so bright on the screen that they can overshadow non-metal targets. By turning the antenna 90 degrees, you can downplay the influence of those metal targets and make them dimmer. Non-metal targets will then be easier to spot. Reflection polarity (positive or negative) will tell you a great deal about the type of material a reflection comes from. The operator can choose the colour table used to show +/- polarity based on preference. Application of features such as display gain, focus, zoom and horizontal cursor will be different depending on the operator.
Added procedures are needed for the following:
- Estimating rebar diameters.
- Figuring out the depths of targets within the T-R offset zone (which is 1.6” for the 2GHz antenna)
- Locating voids below slab on grade.
- Finding the edge of concrete on beams/slab bands.
- Detecting live power in and below slab (using a VM-550 locator with a 50/60Hz passive frequency)