Geophysical Survey in Archaeological Field Evaluation - HELM

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2.2 Fieldwork
All fieldwork should be conducted under the
principle of repeatability; in other words, that,
within reason, the data obtained should be
capable of independent duplication. Fieldworkers
must ensure that every effort is made on
site to be courteous and considerate in their
dealings with landowners, local residents and
organisations, respecting all aspects of the
environment. A high level of professionalism
is necessary at all times.
Correct observance should be made of any
legal constraints on site – for instance, the
requirement of a Section 42 Licence for
survey over scheduled monuments and
other protected places, and the licence now
needed for survey on National Trust land
(Part II, 7.3).
2.2.1 The survey grid
This is the network of control points used to
locate the geophysical survey measurements
relative to base mapping and/or absolute position
on the Earth’s surface, (see Part IV, 1.1). Whether
physically marked on the ground or measured
while surveying using a global positioning
system (GPS), these must be located to
survey-grade accuracy (±0.1m). The survey
grid must be independently re-locatable on
the ground by a third party, by measurement
to local permanent features, and/or by the use
of GPS coordinates. All locational information
must be geo-referenced. In certain cases (eg
where permanent features are absent), and with
appropriate permission, it may be acceptable to
emplace permanent survey markers.
Care must be taken to ensure that any survey
markers or other equipment are not a hazard
to people or animals.
2.2.2 Magnetometer survey
Survey must be conducted with a continuously
recording magnetometer of appropriate
sensitivity.
Area survey must be the preferred method
of ground coverage in all instances where this
is practicable.
The maximum acceptable sampling interval
for an area survey is 0.25m on traverses a
maximum of 1m apart.
Magnetometer scanning, as a method of initially
assessing the magnetic response of a site, may
be used at the discretion of surveyors who
are experienced in its application, for devising
(or advising upon) an appropriate evaluation
strategy that will use other methods. The
technique should not otherwise be included
in briefs or specifications.
2.2.3 Earth resistance: area survey
The maximum acceptable sampling interval for
area surveys is 1m along traverses separated
by a maximum of 1m.
Area surveys, using the twin probe (or twin
electrode) probe configuration, are the preferred
method of ground coverage. The square array
(often employed on cart-based systems) is also
acceptable for area surveys. Other methods
require special justification.
For twin probe systems the mobile probe
spacing should usually be 0.5m; wider
separations and/or multiplexed arrays require
explanation. The equivalent spacing for a
square array would typically be 0.75m.
2.2.4 Ground penetrating radar survey
Generally, this technique will be applied for
the detailed investigation of a site by individual
profiles and the visualisation of the data as
time slices. A maximum traverse spacing of
0.5m is recommended with samples taken
at intervals of 0.05m.
Specific site conditions and the aims of the
survey may require an alternative sampling
methodology to be adopted, but this must
be fully justified in any supporting
specification documents.
Determination of an appropriate sampling
interval, centre frequency of antenna(s) used
and sub-surface velocities used for depth
estimation from the resulting data must be
supported through an appropriate survey
design, including field test measurements
where appropriate.
2.2.5 Magnetic susceptibility survey
Magnetic susceptibility survey should not take
precedence over magnetometer survey where
the latter is practicable.
Areas of high topsoil magnetic susceptibility
should be complemented by detailed area
magnetometer survey. Some areas of low
or indifferent magnetic susceptibility values
should also be sampled with detailed
magnetometer coverage, to confirm that
under the prevailing site conditions, changes
in magnetic susceptibility do correlate with
archaeological potential.
The chosen method(s) of magnetic susceptibility
measurement must be appropriate to prevailing
ground surface conditions.
Measurements of topsoil magnetic susceptibility,
for area surveys and transects, must be made
at intervals not exceeding 10m.
Where possible, such topsoil measurements
must be compared and contrasted with
those obtained from subsoil and local
archaeological features.
2.3 Data treatment
Area surveys must be conducted, and
subsequent data treated, so as to result
in a data-set that is as uniform as possible.
Edge-effects between contiguous survey
areas should be minimised.
A copy of unprocessed raw data must be
retained and archived (see below, 6 Archiving).
Raw data collected in the field must be of high
quality. Any data-collection artefacts subsequently
apparent in the survey data should be identified
and removed using appropriate data processing
(Part IV, 2.1). All such processing should be
clearly described. Any data collection artefacts
that cannot be corrected by data processing
should be described and clearly distinguished
from possible archaeological anomalies.
If data has been seriously compromised during
collection, a return to the site to re-survey
the affected area(s) should be considered.
2.4 Data interpretation
The interpretation of survey data must be
undertaken by a competent archaeological
geophysicist who is knowledgeable of the
archaeological and geomorphological conditions
prevailing on site. Consultation must also take
place with other site specialists (eg landscape
archaeologists, aerial photographers) wherever
possible.
The interpretation of magnetometer and
magnetic susceptibility data must endeavour to
distinguish anthropogenic from other causes of
magnetic enhancement on the site(s) concerned.
A clear distinction must always be made
between interpretation that is scientifically
demonstrable, and interpretation based on
informed speculation.
Any reference to ‘negative evidence’ must be
fully qualified and explained. Lack of geophysical
anomalies cannot be taken to imply a lack of
archaeological features, and in such cases an
alternative evaluation procedure – eg trial
trenching, or the use of a different geophysical
technique – should be considered.
2.5 The survey report
All fieldwork must be followed by a report.
This will be a clear and succinct text, supported
by tables, figures, appendices and references as
necessary (see below, 2.5.1). It ought to stand
independent of supporting material and should
combine the qualities of concise technical