How To Determine Depths of Utilities Using Ground Penetrating Radar?
How To Determine Depths of Utilities Using Ground Penetrating Radar?
A depth assessment in utility locating refers to the process of determining the depth of underground utility lines, such as water pipes, gas lines, electrical cables, communication lines, among others. Depth assessments are important for excavation projects, construction, or any activity that involves digging or drilling into the ground.
The depth assessment process typically involves using specialized equipment, such as ground penetrating radar (GPR) or electromagnetic locators, to detect the presence of underground utilities and measure their depth. This information is then marked on the ground with spray paint, flags or any other method the client suggests to indicate the utilities’ location and estimated depth.
GPR is a geophysical procedure that uses high-frequency radio waves to image and locates subsurface features, including underground utilities. Certified technicians at C-N-I Locates LTD know the effective ways GPR can be used for depth assessment during utility locating by measuring the time it takes for the radar waves to bounce back from underground objects, such as pipes or cables.
How To Determine Ground Penetrating Radars Max Depth
Ground penetrating radar solely gauges the reflection’s time and amplitude. Ground Penetrating radar is incapable of determining the signal’s travel distance, but it can determine the time duration from the transmission to reception. Time alone falls short in computing depth since distance equals time multiplied by speed. However, GPR moves at varying speeds depending on the material encountered, which can skew depth accuracy as it relies on the dielectric property of the medium. Thus, it is essential to choose professionals from reputable companies like C-N-I Locates LTD to ensure accuracy while measuring the depth.
GPR relies on time and speed to calculate distance. Ground penetrating radar signals travel at different speeds in different dielectrics. GPR depths are only as accurate as the dielectric that is entered into the system.
A dielectric value serves as a speed indicator for waves, influencing depth accuracy but not data quality. By altering the dielectric, the depth can significantly vary while keeping the data on the O-Scope unchanged. Changing the data’s appearance on the O-Scope is a mere optical illusion, similar to zooming in or out. Note that initializing the antenna only configures the surface and gain, not the dielectric.
The speed that radar waves travel through the air, which is as fast as the speed of light, is remarkably distinct from its more sluggish pace through water, approximately nine times slower or only 1/9th of the speed of light. Other natural materials fall somewhere between air and water in terms of radar wave speed (besides metal).
Determining the maximum depths of utilities using ground penetrating radar (GPR) involves analyzing the radar data to identify the location and depth of subsurface features, including buried utility lines. Here are the basic steps for using GPR to determine the maximum depths of utilities.
How To Interpret Dielectric Values?
A dielectric value is a ratio number that indicates the velocity of a material compared to the speed of light, which is a convenient approach. For instance, instead of expressing the velocity of light in the air as 299,792,458 meters per second, we can represent it as a dielectric value of 1. Similarly, the velocity of light in water can be indicated as 33,310,273 meters per second or as a dielectric value of 81, which corresponds to 1/9th the speed of light. All-dielectric values operate on the same principle (speed of light divided by the square root of the dielectric constant), or speed = c/√k where c is the speed of light and k is the dielectric constant. Other examples include a dielectric value of 4, which represents 1/2 the speed of light, a dielectric value of 9 for 1/3 the speed of light, a dielectric value of 16 for 1/4 the speed of light, and so on. These examples demonstrate that the GPR signal can travel at varying speeds, regardless of the mathematical understanding. Because ground penetrating radar services cannot determine the material being scanned, the speed cannot be ascertained unless the user inputs the dielectric value.
Most materials will have dielectric values between 3 and 12, and as moisture content increases, the dielectric value will also increase. It’s essential to note that GPR waves cannot penetrate metals and instead get entirely reflected. Thus, the dielectric value of metals is considered infinite (∞).
Therefore, it is vital to select professionals from renowned companies like C-N-I Locates LTD for the correct values of the dielectric.
- Dielectric Values for Underground Materials: Air has a dielectric value of 1 and GPR travels through it at 300 mm/ns, asphalt has a dielectric value of 3-5 and GPR travels through it at 134-173 mm/ns, sand/dry clay has a dielectric value of 4 and GPR travels through it at 150 mm/ns, rock has a dielectric value of 5-9 and GPR travels through it at 100-120 mm/ns, wet soil and agricultural land have a dielectric value of 12-15 and GPR travels through them at 77-86 mm/ns, and saturated soils have a dielectric value of 20-30 and GPR travels through them at 55-67 mm/ns. The purpose of these examples is to provide a concise guide to commonly encountered values. However, it should be noted that, as with any chart relating to dielectric properties, these values may vary due to factors such as moisture content and therefore are only approximate.
- Dielectric Values for Concrete: Very dry concrete has an approximate dielectric value of 4.59 and GPR travels through it at 140 mm/ns, moderately dry concrete has an approximate dielectric value of 6.25 and GPR travels through it at 120 mm/ns, moist concrete has an approximate dielectric value of 9 and GPR travels through it at 100 mm/ns, and wet concrete has an approximate dielectric value of 14.06 and GPR travels through it at 80 mm/ns. These examples help outline simplified dielectric values for concrete that are worth noting. Dielectrics should not be confused with signal attenuation from conductivity. As with any dielectric chart, the values can fluctuate based on moisture content and other factors, and the figures listed here are mere approximations. The examples provided include very dry concrete, with a range of 4-6, for an elevated slab located inside. Mod Dry, which has a range of 6-8, is found in slabs located on grade or elevated slabs between 6 months to 1 year old. Moist concrete, with a range of 8-12, can be found in slabs between 1 and 6 months old. Once fully cured, slabs-on-grade typically have higher values than elevated slabs, usually around 7-10, regardless of age. Exterior slabs or slabs with constant moisture exposure can remain in the Moist range. Wet concrete, which has a range of 12-15, can be found in recently poured slabs less than one-month-old or in slabs exposed to consistent moisture, such as those found in water tanks or reservoirs.
It should be noted that all of these values are subject to change and may even be higher than 15 due to various factors. There needs to be more than the age of the concrete to determine the dielectric, and a potential rule of thumb is to begin with, a value of 14 and subtract one point from every month for six months. The speed of the ground penetrating radar signal through a material is affected by its dielectric. The travel time is measured in nanoseconds (ns). To determine accurate dielectric values, we recommend calculations are made by a certified professional like an expert technician from C-N-I Locates LTD.
Conductivity Compared to Dielectrics
Signal attenuation from conductivity should not be confused with dielectrics. The primary cause of signal attenuation or diminishment is the conductivity of materials. These two different factors can be easily confused. Signals travel slower through higher dielectric values but that doesn’t always mean signals can’t travel as deep. The penetration of a GPR pulse is mainly limited from electrical conductivity. When there is more water in the soil conditions it can raise the amount of electrical conductivity and dielectric value, so they commonly go hand in hand but that is not always the case. Due to its molecular structure clay tends to be conductive even when it is dry. Water can even be puzzling because pure fresh water is not as conductive as water that contains minerals. Most conductivity from water in soils is a result of the minerals from the soil being combined with the water. Ground penetrating radar can even penetrate ice and snow very well because they have a dielectric of 3-4. But generally, any water that is added to soil or concrete is a foe of ground penetrating radar.
Here are some examples of different materials with similar dielectrics and how conductivity effects them. Dry sand, concrete, and dry clay can all have a dielectric value of 5 but because of the conductivity in the materials GPR will travel the farthest in dry sand, second farthest in concrete, and the least amount of distance in dry clay. In the examples provided, the signal speed remains constant as the materials share the same dielectric value. Nevertheless, the signal’s depth penetration differs because of varying conductivities between the materials. Consequently, there are differences in the achieved depths, which are further affected by a materials’ conductivity.
Impact Of Wave Speed and Reflections
The visibility of objects can only occur when a reflection of the signal is received. Reflections are only produced when there is a variation in the speed (dielectric) of the material. When the transition is from a higher to a lower dielectric, it is a negative reflection (black). In comparison, a change from a lower to a higher dielectric produces a positive reflection (white). A low dielectric translates to high speed. The dielectric value is crucial in identifying whether the reflection is positive or negative, while the contrast in speed determines the amplitude or strength of the reflection.
Ground penetrating radar reflections occur as a result of alterations in wave speed, specifically changes in the dielectric. It is not due to a change in material or density. Even if two materials with very different properties have the same dielectric, it will not produce any reflection and will not be detectable by GPR. Similarly, a change in density between two materials with the same dielectric will not produce any reflection or be visible to GPR.
When two materials with the same dielectric value are compared, there is no change in wave speed or reflection. It is the case even if the materials are very different from each other, like sand with a little moisture (dielectric of 8) and granite (dielectric of 8). On the other hand, conductivity and dielectric values are often related, but not always. For instance, sand and clay may have the same dielectric value of 10 due to enough moisture content, but the clay would still be less penetrable to GPR due to its higher electrical conductivity. Therefore, reflections do not happen because of changes in conductivity.
3 Methods for Calculating Dielectric Values
- Educated Guess of Dielectrics: One way to achieve accurate depths in GPR is by using a published reference or making an educated guess. It is essential to be familiar with dielectric values for standard materials and select an appropriate dielectric. The estimated depth accuracy using this method is between 10% and 20%.
- Hyperbola Match: The second approach to adjusting the dielectric is to attain precise depths by conducting a hyperbola match. This method is possible on all GSSI systems except the SIR 3000. In the event that SIR 3000 is the only available option, the raw data files can be transferred to someone with Radan software to conduct a hyperbola match. The depth accuracy for this method is estimated to be between 5% to 15%.
- Ground Facts and Truth: To achieve accurate depths, the most precise method for adjusting the dielectric is to perform a ground truth, which confirms the depth of an object. This method can be utilized on all GSSI systems employed. The depth accuracy obtained through ground truth is estimated to be between 5% to 10%.
What Factors Can Change the Dielectric Properties Found With GPR?
There are several factors that can change the dielectric properties found with GPR.
- Adding Water: Water is one of the most common ways to change the dielectric properties of a material. Adding water to the soil, for example, can increase the dielectric constant and reduce the GPR signal speed.
- Applying Heat: Heating a material can also change its dielectric properties. This method is often used to dry out materials with a high moisture content, which can reduce the dielectric constant and increase the ground penetrating radar signal speed.
- Chemical Treatment: Chemical treatment can alter the dielectric properties of materials by changing their molecular structure. For example, adding salt to water can increase its conductivity and reduce its dielectric constant.
- Changing Material: Changing the material itself can also change its dielectric properties. For example, replacing a high-density material with a low-density one can decrease the dielectric constant and increase the GPR signal speed.
- Changing Temperature: Temperature can also affect the dielectric properties of materials. As the temperature increases, the dielectric constant of some materials decreases, which can increase the GPR signal speed. Also, some materials dielectric constant will decrease when frozen like water increasing the GPR signal speed.
Things to Remember When Adjusting Dielectric Values
When using ground penetrating radar, only one dielectric value can be entered into the system at a time. This value represents the average dielectric for all the materials in the scanned area. It’s crucial to note that the system cannot account for changes in dielectric values. Therefore, it’s crucial to choose the appropriate dielectric value for the majority of the area being scanned.
When adjusting the dielectric to achieve accurate depths, performing a hyperbola match or ground truth should be done on the deepest item possible if it’s within the desired medium. This is because soil can change both vertically and horizontally, and the deepest item is likely to be the best representation of the overall medium.
On the other hand, when scanning concrete, it can be assumed that the material is consistent, and a ground truth can be done on any location within the concrete structure. However, it’s important to note that the dielectric value of concrete can vary depending on factors such as its age, moisture content, and composition.
General Procedures to Determine Max Depth With GPR
In addition to identifying the location and depth of underground utilities, ground penetrating radar scanning may also help evaluate the condition of the utilities, such as whether they are damaged or in need of repair. The information can further be used to plan and execute excavation or construction projects safely and efficiently while minimizing the risk of damage to utility lines and the surrounding infrastructure. Read below to know the general procedure followed by professional locating services to calculate the max depth GPR can get a visual on.
- Collect GPR Data: The first step is to collect ground penetrating radar scanning data by scanning the area where the utilities are suspected to be located. GPR equipment emits radar signals that penetrate the ground and bounce back when they encounter subsurface features with a change in dielectric, including buried utilities. Further, the strength and time of these signals can be recorded to create an image of the subsurface.
- Identify Utilities: The next step is to identify any utilities visible in the GPR data. This can be done by looking for hyperbolic signals in the ground penetrating radar data that correspond to the shape and size of utility lines. Furthermore, the depth of these signals can be measured to estimate the depth of the utilities. Therefore, it is essential to take professional services from C-N-I Locates Ltd expert technicians for better results.
- Evaluate Soil Conditions: The depth at which GPR signals can penetrate the ground depends on the soil conditions, including factors such as soil type, moisture content, and the presence of rocks or other obstructions. To accurately determine the depth of utilities, it is important to evaluate the soil conditions and adjust the GPR settings accordingly.
- Interpret GPR Data: After collecting and analyzing the ground penetrating radar data, the next step is to interpret the results to determine the maximum depth of the utilities. This involves identifying the deepest point at which the utilities are visible in the GPR data and measuring the depth at that point.
- Verify Results: Finally, when possible, it is important to verify the results of the ground penetrating radar survey by digging test holes or trenches to confirm the location and depth of the utilities. This can help to ensure that the GPR data is accurate and can be used to safely plan and execute excavation or construction activities.
Contact CNI Locates for GPR Utility Locating Services Today!
Overall, using GPR to determine the maximum depths of utilities requires careful data collection, interpretation, and verification to ensure accurate results. It is also important to work with experienced professionals who are trained in GPR and utility locating to ensure that the survey is conducted safely and effectively.
Do you need ground penetrating radar services to calculate the depth of your utilities correctly? You can rely on the trustworthy services of C-N-I Locates LTD to calculate the depth of your utilities accurately. Further, our qualified professionals in Oregon and Washington State have decades of experience in our industry. Call us or navigate through our website for more details regarding GPR and its capabilities.