T&A conducted a GPR survey along a trajectory in South Limburg for TNO Geological Survey of the Netherlands. The research goal was to detect sink holes and cavities under the 1 km trajectory. The cavities are created by local solution of the limestone (marl) in dolines (solution depressions).


The subsurface of the research area consists of a loess cover with clay and flints below. Under the clay layer there is a limestone layer, in which the dolines are found. These dolines gave a maximum diameter of a few tens of meters, but often also a lot smaller. The dolines were created by the dissolving the underlying limestone, which caused the overlying layers to collapse.

GPR survey

During the GPR measurements T&A's data analyst marked details in the radar profiles (in particular repairs to and cracks in the surface or the road and the lowest point of dolines) and provided them with an explanation. The interpretation of the GPR data collected by T&A was then carried out by TNO geologists.

T&A has mapped 22 grout anchor locations in the Buiksloterkanaal in Amsterdam for a client. At the location of these grout anchor locations, 38 pile foundation pairs (76 piles in total) will be placed .

Magnetometer CPT

Based on our knowledge and experience we determine which geophysical techniques can best be used to achieve the research objective. In this investigation we have opted for the use of magnetometer CPT. With the help of a magnetometer integrated in a CPT, it is possible to map the influence of the earth's magnetic field caused by a grout anchor. The influence zone depends on the diameter, length and type of material of the anchor.

Data analysis

Based on the measurement data, the horizontal and vertical position of the grout anchors could be determined. This resulted in the recommendation to take the established position of the existing grout anchors into account during the design phase of the pile foundation.

A lagoon in the Manchester area has been filled with sewage sludge from a nearby water treatment plant for the past 50 years. To enable a good planning of the dredging and soil improvement activities, information about the exact depth of the lagoon was needed. T&A was commissioned to map the transition between the sewage sludge and the actual water bottom, consisting of glacial clay.

ERT measurements

The technology that T&A has used to determine the transition between the different soil layers is called ERT. With ERT it is possible to map the electrical resistance of different soil layers. The estimated survey depth was 13 meters below the water surface. The survey was conducted from a vessel with a special marine measurement system. Because the water depth of the lagoon was between 0 and 0.3 meters, the boat sometimes had to be pulled over the sludge.


The geophysical measurements showed that the sewage sludge had a very good electrical permeability, probably because it was discharged poorly into the lagoon. The clay had a much lower electrical permeability, making it possible to map the depth of the sewage sludge properly.

Project Goal
RSK-NetherlandsDetection of cables and pipes, industrial site on Maasvlakte
Dura Vermeer InfraLocating objects at a sheet piling location, Amsterdam
RijkswaterstaatLocating cabels and pipes near swing bridge, Den Oever
Antea GroupLocating trench deposits in Holocene overburden, De Meern-Meerkerk
McMahon Design & ManagementUXO detection seabed IJmuiden
Antea GroupLocating of obstacles is the subsurface, Groningen
De Boer en De Groot Civiele WerkenObstacle detection for installation of grout anchors, Cruquiusgebied, Amsterdam.
RSK NetherlandsInvestigaron of soil resistivity and obstacle localization, Maasvlakte
Antea GroupLocating obstacles in th subsurface, Eemshaven
Waterschap Hollandse DeltaPreliminary exploration of dike reinforcement, Hekelingen (ZH)
Overhoeks Development Management BV
Locating grout anchor locations, Buiksloterkanaal, Amsterdam
TNO Geological Service
Detection of sinkholes and cavities under a road
Locating underground obstacles, Geleen
BK Ingenieurs
Geofysical survey of remnants of concrete industry, Son en Breugel
Municipality of Haarlem
Dermining the location of anchors and anchor bulkheads in quay wall
BAM infra
Confirming absence of horizontal directional drillings (HHD)
Detecting leakage from drainage lock Bovenland
Regionale Uitvoeringsdienst Drenthe
Determining the continuity of a clay layer, Hoogeveen
Royal Smals
3D delineation of sludge transition, Oldham, UK
Projectbureau BLEIZO
Determining the position of cables and pipes
Determining sheet pile location, Amsterdam Rijnkanaal
Royal Haskoning DHV
Geophysical survey of a landfill, Tiel
Royal Haskoning DHV
Asfalt survey of a dike between Waardenburg and Gorinchem
Pinnacle Consulting Engineers
Radar survey Agriport
Dekker BV
Cables and pipes survey in a canal, Amsterdam
Antea Group
Geophysical suvey  Zevenbergen castle garden
BAM Wegen
Detection of hollow spaces