A fluxgate magnetometry survey was undertaken on the public green at
Seagrange, Baldoyle, Co. Dublin with the hope of evaluating the archaeological
potential of the site and determining whether or not archaeological deposits that
may relate to the cropmark enclosure D015-018 were still present, buried
beneath the soil. The wider landscape had formerly comprised open green fields, before the
construction of the Seagrange estate in 1973. Evidence from the 1st edition
Ordnance Survey map showed the area of the green had been divided by two perpendicular
field boundaries, one running north-south, and the other east-west. The east
west field boundary once formed the property line to the rear of numbers 37-48,
Marian Park. The grainy aerial photograph from the Cambridge aerial series
(1970-CUCAP AIG 95-c) suggested that there may also have been another
curving linear feature leading north-eastward from the cropmark enclosure.
Maybe it still survived under the green?
Fig 1. Topograhic model of the Survey Area
It was clear from previous visual inspection, that the green had undergone heavy landscaping through the years, and the chances of subtle archaeological features surviving the construction of a modern housing estate were slim. The natural ground-level was difficult to discern. The southern and eastern parts of the green
were clearly built up with over-burden. But perhaps our archaeological features
still lay beneath? Could we find them with our survey equipment through the
great depth of modern deposits and debris?
Our survey comprised two parts. First of all, we wanted build a 3D topographical
model of the green to examine the topography and see if it could tell us anything
about what lay beneath. For the purposes of accurate recording, a controlled
measurement of the natural and artificial landscape features was carried out. A
Trimble VRS system was used in conjunction with a GPS Rover unit. It allowed for
surveying to an accuracy of +/‐ 2cm.
Fig 2. Topograhic model overlain with 2009 aerial photograph
During the topographical survey, our eagle-eyed survey team noticed a trickle of
water which appeared to come from the approximate location of the eastern end
of the curving linear earthwork which was evident in our aerial photograph. It
was known locally that a natural spring once flowed from this spot. Could this be
a natural spring which once fed a curving leat that led to the enclosure? We
walked the green, paying particular attention to the bumps and undulations,
plotting the ground surface as we went. As the sun went down, the shadows
lengthened, and a clear linear dip became apparent, running east-west through
the site. It looked to be in the right location to represent the east-west field
boundary marked on our 1st edition OS map. It seemed that the green was not as
disturbed as we had feared. The surface had given us some clues as to the
nature of what lay beneath and what might once have existed there. It was time
for a little fluxgate magnetometry to see what lay under the soil.
Fig 3. Evidence of a natural spring in the east of the green
Fluxgate Magnetometry is a form of geophysical survey. Geophysical survey is
used to create maps of subsurface archaeological features by measuring and
plotting fluctuations in the properties of the soil. Geophysical instruments can detect buried features when their physical properties contrast measurably with
their surroundings. A magnetometer is a measuring instrument used to measure
the strength and, in some cases, the direction of magnetic fields. Every kind of
material has unique magnetic properties, even those that we do not think of as
being magnetic. Different materials below the ground can cause local
disturbances in the Earth’s magnetic field that are detectable with sensitive
magnetometers. Magnetometers react very strongly to iron and steel, brick,
burned soil, and many types of rock, and archaeological features composed of
these materials are very detectable. Where these highly magnetic materials do
not occur, it is often possible to detect very subtle anomalies caused by
disturbed soils or decayed organic materials. The chief limitation of
magnetometer survey is that subtle features of interest may be obscured by
highly magnetic geologic or modern materials. This was a worry for our survey,
as it was believed that the green had been heavily disturbed during the
construction of the Seagrange estate, and so rubble and other debris could
mask possible buried archaeological features.
Survey shots: The GPS units (above) and the cart in action (below)
We conducted our survey using a magnetometer cart, kindly loaned to us by
Rubicon Heritage in Cork. A Differential Global Positioning System (dGPS),
capable of Real Time Kinematic (RTK) navigation, was fixed to one end of a
custom-built cart and the magnetic gradiometer was fixed to the other end. This
allowed each data point to be collected with centimetre- accurate GPS
coordinates. The system utilises a Bartington Grad 601-2 dual magnetic
gradiometer (magnetometer), capable of surveying to an accuracy of 0.1nT. This
unit is comprised of two horizontally aligned sensors, separated by 1m. The
objective of the geophysical survey was to establish the presence or absence of
any archaeological sub-surface features or deposits which may be associated
After several hours of pushing the cart across the green to gather data, we had
covered the entire area and it was time to download and process the results….
The green had indeed been heavily disturbed in the past. It is known locally that the area was used as a rumble dump during the construction of Seagrange estate and frequent dipole magnetic responses scattered throughout the site probably reflect the nature of such subsoil conditions. ,
Dipolar responses typical of metallic debris, littered the survey area, making it difficult to pick out
the archaeology which might lie beneath. Nonetheless, two linear responses
(broken at their junction) could be identified amongst the rubble and metal, one
running north-south, the other east west. These corresponded with the
approximate location of our field boundaries. It seems, in all likelihood, that our
former field divisions are still beneath the surface of the green.
Fig 4. Geophysics results with interpretations: H01 and H02=east/west field boundary; H03 and H04= north/south field boundary; M01= circular feature; M02-possible leat
The disturbed nature of the area made interpretation difficult. A low-pass filter*
was applied to the data, and a very faint shadow of a possible linear feature
could be made out, curving from our natural spring, south-west toward the northeast corner of D015-018.
Fig 5. Low Pass Magnetometry Results with the suggestion of a leat in the south east of the image
Although difficult to discern, we hope that this response
represents the buried remains of a leat associated with the cropmark enclosure.
To the north-west of the intersection of the field boundaries, the faint traces of a
circular feature were identified. A surprise bonus perhaps? And so it seems likely, that after years of modern disturbance, below tonnes of
overburden and debris, D015-018 may still survive to some degree. As a season
of excavation is planned, time will tell if the survey was accurate and the remains
of a former human settlement still exist beneath the green at Seagrange Road .
Fig 6. Overlay of the 1970 aerial photograph over the First Edition Ordnance Survey Map showing the field boundaries and their relation to the enclosure and leat.
* High/Low Pass Filters – The low pass filter, when applied to a ’noisy’ data set i.e. a data
set displaying too much variation that it obscures detail, will smooth the data, removing
the noise and revealing anomalies that were obscured. Conversely, high pass filtering
accentuates differences in subtle data sets and can increase the visibility in data set,
often uncovering very weakly magnetic anomalies.
Enda O’Flaherty April 2013