Increase depth by changing the time window
- The easiest way to increase your GPR's penetration depth is by setting to work at its longer operational time window.
- This effectively signals to the Rx module of your GPR to 'listen' for reflection waves for longer
- The longer time windows are14ns for Concretto,100ns for Scudo, and 150ns for Dipolo.
Make anomalies stand out by using background removal filter
- The background removal filter is a very powerful tool for every surveyer.
- It effectively adapts to the surveyed material, removing patterns that are not of interest to the surveyer - the background.
- A common pitfall of the background removal filter is when doing very long surveys it can cancel out some underground features are very big and gradual in nature.
- Perfect for making features smaller than 5 meters stand out - pipes, buried objects, cavities, walls, etc.
Focus areas of interest with software gain
- The Oerad App allows you to apply an algorithmic amplification of the signal from your GPR in different parts of a single trace.
- This is useful when a surveyed material is rich in anomalies in the shallow subsurface but the surveyer is interested in features at bigger depths.
- There are seven levels of software gain/amplification: x 0.3 (reducing signal strength) x 0.6 (reducing signal strength) x 1.0 (no amplification applied) x 2.0 x 4.0 x 8.0 x 16.0
Increase signal strength with hardware gain
- Amplify signal from GPR via hardware gain.
- Allows differentiation of materials with similar electric properties - i.e. adds contrast to your radargram.
- Useful for focusing on structures at greater depths.
- Works well with a negative software gain filter applied to the upper part of the trace signal.
Minimise distance from the surveyed material
- Air has a low dielectric so EM beams from the GPR travel at almost the speed of light and over a broader area, accounting for signal loss.
- Close coupling with the ground reduces this effect significantly - focuses the beam into the ground.
- Dipolo systems can be recentered via the hardware switch on the Receiver module depending on their height above ground.
Beware of Wi-Fi and other radio devices
- The nature of the GPR architecture and operation by design are such that the receiver antenna listens to all events or signals within its operational bandwidth.
- Cellular towers, WiFi from mobile devices, electrical powerlines or other radio emitting devices sometimes emit signals in the same bandwidth as of the receiver's antenna of a GPR.
- These signals can be picked up and become visible on a radargram as modulated noise
Dipolo or Scudo?
- Two very different systems suitable for different conditions.
- Best way to decide is to ask Wheat type of terrain am I going to survey?.
- Dopolo is suitable for vast open areas, rocky terrains, fields, deserts and other terrains where few big features are present (trees, buildings, etc).
- Dipolo is suitable for very rough terrains where a GPR can only be suspended in air.
- Dipolo is equipped with three sets of antennas that are sensitive to different kinds of structures. For more information, consult our Dipolo product page.
- Scudo is suitable for all types of terrain that allow close coupling with the ground.
- Scudo has excellent performance in cities, construction sites, on roads, inspecting bridges.
- Scudo is more focused and has higher resolution than Dipolo.
Humidity in the ground
- Humidity is the most common reason for signal attenuation (or signal loss). Please refer to the section on how to Spon an area with low to no visibility.
- Even little humidity in a material can greatly skew depth calculations.
- It is recommended to avoid doing surveys after heavy rainfalls.
Spot an area with low to no visibility
- Some soils contain humidity rich or other low penetrability areas that can obstruct surveying.
- These can easily be spotted as seen in the images below.
Depth calculation
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The distance travelled by an EM wave emitted by a GPR or its penetration depth is a function of the GPR's operating time window and the dielectric constant of the surveyed material. It is expressed by the following formula:
where D = depth,
C = speed of light,
έ = dielectric constant,
and t = operating time window.
- Conductor materials such as aluminium, copper and gold have a dielectric constant approaching infinity. That is why it is impossible to 'see' beneath conductive structures.
- It is important to note that although all Oerad GPRs are thoroughly calibrated to cm level precision in laboratory conditions, in outdoor surveys it is rare to find perfectly homogenous materials. This formula is used as a reference for better understanding radargrams.
- Refer to the Dielectric cheat sheet below for common materials and theoretical maximum achievable depths with Oerad GPRs.
Double reflections
- When an EM beam travels through the ground and encounters different materials it can reflect, refract, transmit or attenuate.
- Some object with cavities within them such as pipes reflect can produce double or tripple reflections.
- This occurs when part of the beams reflect off the top of a pipe, others are transmitted and reflect off the bottom of a pipe.
Image taken from Dean Goodman's and Salvatore Piro's GPR Remote Sensing in Archaeology
Minimum distance between objects
- This is defined as the minimum distance between two objects horizontally or vertically for a GPR system to be able to discriminate between them - or show them as separate hyperbolas.
- Dipolo with 100MHz antenna: 1.2m horizontally and 1.5m vertically
- Dipolo with 300MHz antenna: 1.0m horizontally and 1.2m vertically
- Dipolo with 500MHz antenna: 1.0m horizontally and 1.2m vertically
- Scudo: 0.6m horizontally and 0.7m vertically
- Concretto: 0.18m horizontally and 0.02m vertically