T&A owns a wide range of geophysical techniques for the mapping of the subsurface and underground constructions. Based on our knowledge and experience we decide which technique will lead to the best results for each individual case. Important factors in the decision making process are the goal of the survey and details like the desired depth range, location specific circumstances, accuracy and speed of measuring, costs of measurement and data analyses.
Carrying out measurements
The measurements are done by moving the equipment over the location, either by foot or using a quad or a boat. Objects or geological transitions in the subsurface directly or indirectly influence the signal transmitted by the measuring equipment or the naturally present magnetic fields. The geophysical techniques detect this and create a 3D image of the subsurface.
Small, deep-lying objects or thin ground layers with small property differences are very hard or sometimes even impossible to trace. If the mapping of the investigation's target is not possible from the surface, the equipment is brought closer to the target by using a borehole.
In most cases the collected data needs to be analysed and reported at the office. Depending on the goal of the investigation the results are presented in an overview map, a cross-section profile or an outlining map of the subsurface.
Why geophysical research?
- It provides continuous information about the subsurface instead of just local information from drillings and/or probes.
- Due to faster, innovative techniques, the research is more cost-effective.
- It provides solutions to problems regular methods are unable to solve.
- It reduces the risks of unexpected deviations (objects) in the subsurface, which can cause extra costs and delays.
- Due to the non-destructive techniques, it reduces inconvenience to the surroundings. Subsequently, digging activities are carried out in a well defined area.
Geo-electrical measurements (resistance measurements) are an electrical research technique that determines the resistance of the subsoil in a non-destructive way. This technique can be used in dike inspections if the subsoil has to be accurately mapped in order to record geological structures with a high resistance or a resistance that deviates from the surroundings. The depth range of the measurements varies from a few meters to a depth of more than 500 meters.
Drone IR Thermography
T&A Survey Drone Services conducts dike inspections using a drone equipped with an Infrared (IR) thermography camera. The IR thermography camera provides a thermogram, a visual representation of the temperatures on the object of measurement, with different colors indicating different temperatures. Weak spots in dikes are often not visible to the eye, but can easily be recognized on thermography images. It is therefore important to have the GPS coordinates of each recording, in order to accurately locate weak spots.
With a LiDAR (Laser Imaging Detection And Ranging) sensor attached to a drone, T&A Survey Drone Services can register the geometry of dike bodies in detail. The LiDAR sensor measures the distance to point in the surroundings using a laser pulse. This results in a point cloud of millions of points, each with a position (x, y, z) and a reflection value. The end result is a very accurate 3D image of the surface of the dike.
Photogrammetry is the creation of maps using digital aerial photographs made by a special, high-resolution photo camera. By attaching this lightweight camera to a drone, high-resolution digital 3D aerial photographs are generated which are then processed with special software.
By placing measuring equipment in or behind a survey boat, the water bottom can be surveyed in order to identify foreign objects and sediment layers.
Possible applications of geophysical techniques in water bottom investigation are:
- detection of objects like Unexploded Ordnance (UXO), cables and pipelines on or under the water bottom,
- mapping of the thickness of sediment layers,
- investigating sediment erosion, for example underneath bridges
Geophysical survey can be used to detect foreign objects in the ground, for example in infrastuctural and archaeological projects or landfill investigations.
Foreign objects in the subsurface can usually be detected because they have different properties than the surrounding material.
Possible applications of geophysical techniques in the research of foreign objects in the subsurface are:
- detection of cables and pipelines,
- detection of foundation piles,
- detection of metal objects such as oil barrels and tanks,
- archaeological research of roads, foundations, tools and canals or ditches,
- forensic investigation,
- mapping of the thickness of rubble and waste layers and embankments,
- detection of cavities or basements,
- detection of Unexploded Ordnance (UXO),
- dertermine the position of conductive horizonal and vertical boreholes or boreholes with metal casings,
- dertermine the position of non-conductive horizontal and vertical boreholes or boreholes without metal casings.
T&A does borehole surveys using logging equipment, for example for water distribution companies. The logging tools are lowered into the borehole to determine the physical properties of the direct borehole surroundings.
Possible applications of geophysical techniques in borehole investigation and logging are:
- scanning the borehole wall, for example to detect micro fractures,
- determining salinity and other physical properties of the soil surrounding the borehole
- dermination of physical properties of subsurface material
- detecting aquifers
- detecting impermeable layers
- detecting of geological transitions
- detecting of deeper laying (foreign) objects
- detecting pollution
Possible applications of geophysical techniques in pollution investigation are:
- mapping of chloride contamination
- mapping the depth of rubble, landfill and fill material layers.
Possible applications of geophysical techniques in road contstruction are:
- determination of thickness of asphalt or concrete on roads and constructions,
- mapping of cracks and cavities,
- detection of mesh tracks and reinforcement bars,
- mapping of the complete road foundation.
Possible applications of geophysical techniques in geological investigation are:
- mapping the geological structure of the subsurface,
- mapping dike composition,
- mapping of impermeable (clay)layers,
- mapping of aquifers,
- determination of pleistocene-holocene transitions,
- detection of salt domes or caverns,
- detection of deep oil and gas reservoirs,
- Detection of conductive structures as for example mineral ores,
- surveying of salty seepage water.