March 2014 - 17th Century Spanish fortress, Eijsden-Margraten (Netherlands), Voeren (Belgium)

Until the 1970’s the plan of this Spanish fortress was still largely visible as a massive star-shaped complex of earthworks covering c.20ha of land. Since then however, the fort remains have been almost entirely leveled, and are now barely visible.

1st phase test

Geophysical investigation at this site commenced as a 1st phase test using electromagnetic induction (EMI) and magnetometry across a 1ha predefined area. On the basis of the results from the EMI and magnetometry just 1 of these methods would be used for 2nd phase full-scale survey of the site.

17th Century Spanish Fort magnetometry and EMI – test phase

Both techniques identified features detailed on a 17th century construction plan of the fort, including a suspected infilled/landscaped ravelijn and the defensive ditch beyond it. The data quality and feature definition from the magnetometry proved more informative in the case of this project than the EMI. On this basis 2nd phase survey across the remainder of the site proceeded with magnetometry.

2nd phase full-scale survey

The 2nd phase full-scale survey was conducted across c.11ha, covering parts of 7 pasture, recently sown and heavily ploughed fields. 12ha of the total 23ha study area remained unfortunately unavailable due to access constraints.

The greyscale below presents the magnetometry from the northern portion of investigation. Ferrous material is abundant throughout due to the fort’s 17th Century origin, an underground water main to the south of an access road, and extensive landscaping.

17th Century Spanish Fort magnetometry full-scale survey

Remains of the north-western ravelijnn, outer ditch, incorporated structures and associated defences are visible to the north-west and west. To the west extensive disturbance was also recorded, though several linear features and potential structures suggested remains corresponding to a former toll-house in this location. In the 1st phase test zone a curving area of strong magnetic disturbance to the south-west likely represents part of the north-eastern ravelijn infilled with modern building debris after 1970. Beyond the ravelijn, the outer ditch and defensive features recorded during the 1st phase test zone are indicated. Within these further linear remains and suggested structures are indicated by the magnetometry.

The remains of the southern extent of the outer ditch to the fort are visible extending approximately west to east, to the south of the access road and the underground water main.

The results from this survey will be used to assist the ongoing programme of research being undertaken by the client.

Instrumentation and methodology

5 sensor magnetometer cart system with GPS, 8-10cm sample interval, line separation 0.5m.

Geonics EM38 conductivity meter combined with GPS, 8-10cm sample interval, line separation 0.75m


Dry to wet loam soils (Abpy, Adp, Acp, Ahp).

Reporting language



Many thanks to both Tim Vanderbeken of ZOLAD+ (Belgium) and Jef Rutten of Gemeente Eijsden-Margraten (Netherlands) who kindly arranged access to lands and monitored ground conditions for this project.


  • ZOLAD+
  • Gemeente Eisjden-Margraten, Netherlands

Used technology