Survey Logistics and Site Preparation
The general procedure followed to perform most surveys is to divide
the survey area into a series of square or rectangular survey "grids."
Each grid is surveyed by taking readings at regular intervals along
regularly spaced transects. Ropes marked at regular intervals are
used to control transect spacing and position along each transect.
Successive transects are surveyed in a zigzag pattern until the
grid is completed. The value and position of each data point is
automatically recorded in digital format, and is later downloaded
to a portable computer.
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Workers moving ropes during a magnetic survey. In this case, one rope is flung over the operators head while he surveys along another
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Generally, one or more assistants are required to assist the operator
of each instrument. The number of assistants required will depend
on instrumentation, survey design and field conditions (two per
instrument is typical). The principal task of the assistants is
to move the ropes that are used to control the position of the instrument
during the survey (a more-or-less constant job). Other tasks include
moving equipment, clearing obstacles, pounding stakes, etc. These
assistants may be Archaeo-Physics personnel, or personnel provided
by the client.
The task of staking out the site grid and clearing the site may
be performed either by Archaeo-Physics or by the client. If performed
by the client, it should be done under the direction of, or in consultation
with, Archaeo-Physics personnel. The decision to use personnel employed
by the client versus Archaeo-Physics personnel may be based on budget
and available manpower.
Spatial Control
The usefulness of these survey results is dependent on accurately
locating anomaly sources within the survey area. In general, the
recorded position of the anomaly may be expected to be within one
meter (often much less) of the anomaly source. Avoiding additional
errors in locating recorded anomalies on the ground will increase
the chances of success in identifying anomaly sources, while minimizing
search time and disturbance to the site.
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Good spatial control assures that
anomaly sources can be found with minimal invasive testing,
and that survey results can be located on the landscape
at any time in the future. |
If the stakes marking the corners of the survey grid remain in
place at the time of excavation or ground truthing, tapes may be
used to locate anomalies of interest within individual survey grids.
If the grid is no longer staked, a total station or theodolite may
be used to re-establish the survey grid system or locate specific
anomalies for testing or excavation.
It is strongly recommended that the survey grid system be permanently
referenced using a GPS, permanent datums, or other suitable means.
If a separate site grid system exists or is established during future
investigations, the geophysical survey grid should be accurately
located within the site grid.
Survey of Grid System
Each geophysical survey area will consist of a contiguous series
of 'grids" - squares or rectangles marked on the ground with
stakes. Stakeout of these grids may be performed by Archaeo-Physics
personnel using a total mapping station, or by the client prior
to survey.
If the site is to be gridded prior to survey, it should be done
in consultation with Archaeo-Physics.
Generally, these guidelines should be followed:
- Grids of uniform size, measuring either 30-x-30m or 20-x-20m
grids are preferred. Although other dimensions may be used to
cover irregular spaces, non-standard grids should be laid out
only under the direction of Archaeo-Physics personnel.
- The corners of the grids should be marked with wooden or other
non-ferrous stakes.
- Grid coordinates (e. g., N240/E180) should be marked on the
stakes.
- It is helpful to flag the grid corners, datums, etc., but metal
pin flags should not be used.
- It is preferable that these survey grids correspond with an
established site grid, and must be tied in to permanent datums
or landmarks.
- It may be expedient to orient the survey grid to the landform
or site boundaries, in which case the corners of the survey area
should be located within the existing grid system (if any).
- It is preferable to measure distances as horizontal distance
rather than slope distance. This is particularly important on
large sites, as small errors due to elevation, tape deflection,
etc., may accumulate into errors of many meters over larger distances.
- Published magnetic declination (from topographic maps, older
reports, etc.,) may be in error by several degrees. Archaeo-Physics
can provide an accurate declination (we would need the exact latitude,
longitude, elevation, and date).
Site Preparation
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Metal detection can be useful for clearing the site of modern
trash, or for locating the souce of some magnetic anomalies. |
Physical obstacles such as vegetation, fences, and rough terrain
can greatly affect both survey speed (and therefore cost) and data
quality. Metal (even very small objects), brick, and other materials
may be highly magnetic and can compromise
magnetic survey data. Metal objects may also affect GPR and
EM conductivity survey data. Heavily obstructed areas may not be
practical to survey.
Extensive clearing of the site is sometimes impossible or undesirable,
but the following measures may result in a faster (i. e., less costly)
survey and better results:
- Sparse brush is generally not a problem, but thick brush should
be cleared (or at least thinned to allow easy passage along closely
spaced transects).
- It is often possible to easily survey through tall grasses or
sparse weeds, but tall, thick, or tangled vegetation should be
cleared to below knee height to allow easy passage.
- Larger trees are not a major problem unless they are growing
very thickly or have spreading limbs close to the ground. When
practical, spreading limbs may be trimmed to above shoulder height.
- Other physical obstacles such as rubbish, fallen trees, etc.
should be removed from the survey area.
- Especially for magnetic surveys, it is important to remove as
much ferrous metal as possible from the site (metal pin flags
and spikes as well as rubbish and machinery).
- If the survey area is under cultivation, it may be possible
to schedule fieldwork when field conditions will be favorable
for survey and crop damage costs can be avoided.
- For GPR survey, the instrument is pulled along the surface like
a sled. A smooth ground surface is desired to minimize jostling
of the instrument. It is sometimes better to leave taller vegetation
standing and allow the instrument to push it down, than to create
stubble by clearing it.
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