This article is the second in a three part series about 2D and 3D ground penetrating radar methods. With the first article we dove into 2D GPR real-time linescanning. In this article we will do the same with 3D GPR scanning. For the third, we will compare and contrast the two methods, and present a few case studies highlighting each.
3D GPR Overview
With a 3D grid scan, an entire area can be analyzed at at the same time. The process is broken down into data collection and data processing. To collect the data, a precise series of individual data files are acquired over the area of concern. After collection, the files are combined to create a three dimensional data model of the area. This 3D GPR model can now be evaluated as a complete unit. We can slice into it and look at a plan view for any depth, or elevation and section views at any position. Through this model we can clearly observe complex interactions of many different subsurface objects. Our post-processing tracks the depth and position of non-linear targets with extreme precision. The findings are documented in a concise report and delivered for permanent record. We can even export them into CADD ready 3D DXF files for direct import into other projects, such as BIM models.
3D GPR Strengths
3D GPR doesn’t make it easy to mark out findings immediately in the field, so it’s a great approach when the findings should be delivered later in a written report. The results become part of the permanent record, and they can always be reviewed later if new questions arise.
The task separation of data collection from data interpetation allows efficient and predictable use of field time. With 3D GPR, the field technicians are able to focus on collecting the best quality (and quantity) of data grids as efficiently as possible. The time required for collection becomes a factor of the physical site characteristics, such as surface conditions and obstructions. Field time is not dependent on the unknown complexity of the subsurface data. Therefore, this time requirement becomes much more predictable while estimating costs.
The data grids are processed later, in the office. At this point the data complexity does become a major factor in the time required, but it is much more manageable than dealing with it in the field. State of the art analysis software eases the tracking of position and depth for many different objects at the same time. Names and color codes can be assigned to every item. These intuitive interpretation tools allow extremely complex areas to be evaluated reliably.
The division of labor arising from the data collection data processing methodology also allows concurrent work on large or remote projects. The field crew can collect a handful of data grids and send them back to the office. The office crew can immediately begin processing them while the field crew has continued on to the collection of additional grids. As the field crew approaches the end of their collection scope, deliverable products are beginning to come out of the office, and the field crew can help to clarify any ambiguity or misunderstanding.
Perhaps the greatest strength of 3D GPR is the opportunity for peer review of findings. Typically, the accuracy of ground penetrating radar service is critical to a project, and mistakes can be costly or dangerous. With 3D GPR data grids, all findings can be peer reviewed prior to delivery and action. As a bonus, a data grid can always be evaluated again if you need to find something different a year or two down the road.
3D GPR Strengths:
- Permanent written record of findings
- Predictable field time requirements
- Complex data sets are manageable
- Large projects can be executed concurrently
- Peer review of findings
3D GPR Weaknesses
Regardless of data complexity, there is a fixed cost of time required to setup, collect, build, process, and report each 3D GPR data grid. When the data sets are complex, these fixed time costs are usually offset by the time savings in processing and interpretation. When the data sets are simple, that may not be the case. There’s often no absolute way to know beforehand whether a data set will be simple or complex, so it becomes a bit of an educated guess.
Once a 3D GPR data grid is collected and sent to the office for processing, it becomes detached from the real world. It’s something like Grid 21A, rather than a 20′ section of slab between columns D and E on the 7 line. This abstraction away from the real world can lead to communication issues during analysis, delivery of findings, and field reproduction. Overcoming these challenges requires a lot of thorough and precise field documentation. On a complex project, field documentation can account for as much as 30% of the total field time.
The 3D GPR method requires a very wide range of technical skills from personnel. Although the essential field mechanics are industry-standard and relatively straightforward, there is a tremendous learning curve when applying those mechanics to real world projects. Expert technicians are as comfortable designing and implementing a multi-grid project covering three acres of a manufacturing facility, as they would be with a simple 2′ x 2′ grid on a concrete slab. The best field technicians are closely familiar with office processing, and tailor the project design for efficient analysis. Likewise, the best office teams remain highly skilled with technical field mechanics, avoiding misunderstandings which could compromise the findings.
3D GPR Weaknesses:
- Fixed time costs for each GPR data grid
- Abstraction from the real world
- Reliance on thorough field documentation
- Requires a wide range of technical skills
3D GPR Summary
3D GPR methods allow qualified field technicians to rapidly collect a lot of high quality data without slowing down to safely interpret complex areas. Time requirements are somewhat predictable. Findings are delivered in a permanent written format. Shifting the post-processing and analysis to the office allows an opportunity for peer review and facilitates the accurate interpretation of complex data sets. However, these 3D GPR methods also require crew of technicians with high-level technical expertise both in the field and in the office.