Geodetic and Geophysical Survey

Geodetic…
…is the scientific discipline that deals with the measurement and representation of the earth.
Geodetic Survey
•Geodetic survey is the basic form of mapping used by archaeologists to accurately record three dimensional information about landscapes, buildings and monuments.
•Basic equipment that is used includes dumpy levels for recording relative heights and measured surveys (analogue and digital) for spatial information.
•Some laser survey equipment is now available which records in 3-D.
Dumpy Levels and Tapes
•Most basic techniques are ancient – they were known to classical societies.

The height of a known point is taken with a dumpy level.This forms the basis for a contour plan.
ContourMap
•Manual survey of Navan Fort.
•Hand-drawn contour plans based on measured surveys
Various conventions used on plans/maps
•‘Hachures’ used to indicate slopes
Electronic Distance Meter (EDM)
•Records the position digitally

Round Island
Co. Down
•Top is a contour plan created in a programme called Surfer. It is shaded to enhance features on the island.
•The bottom map is an interpretation.
Tara
•Detailed topographic survey
Tara
•Geophysical surveys
What is Geophysics?
•The subsurface site characterization of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth's surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons.
Eh, so, what is Geophysics??
•The investigation of subsurface conditions through measuring, analyzing and interpreting physical fields at ground surface.
•Some studies determine what is directly below the surface (the upper meter or so); others extend to depths of 10's of meters.
Geophysical Techniques
•Two basic types of Geophysical Techniques are employed:
•Passive surveys of the earths magnetic field
•Measurements of an electrical current sent through the soil
•All of these techniques are often referred to as Remote Sensing.
•They include: Metal Detecting, Resistivity, Ground Penetrating Radar and Magnetometry
Metal Detectors
•Metal detectors are not only popular with members the public who regard their use as an innocent hobby, but also with professional treasure-hunters who plunder sites for profit.
•Most types penetrate the soil only to a very limited extent, but they have been used by archaeologists to locate dispersed metal artefacts.
•In Ireland – all detection devices required a license for their use (from Dept of Environment, Heritage and Local Government and National Museum). The use of metal detectors to search for antiquities is illegal.
Metal Detectors
•Extracts from National Monuments
(Amendment) Act, 1994, Section 7
•7.—(1) A member of the Garda Síochána may without warrant seize and detain— a detection device found in, at the site of, or in the vicinity of, a monument (an archaeological area which stands registered in the register by virtue of section 5 of the Act of 1987; a restricted area; a monument of which the Commissioners or a local authority are the owners or the guardians or in respect of which a preservation order is in force).
Resistivity surveying
•When an electric current is passed through the ground between electrodes, the resistance to its flow may be measured. A current will pass relatively easily through damp soil, but drier compact material such as a buried wall or a cobbled road surface creates higher resistance.


RESULTS FROM DATA LOGGER PLOTTED BY COMPUTER
Ground penetrating radar (GPR)

•Electronic signals are transmitted into the soil, and bounce back to a receiver. The signals are altered by the density and position of whatever they encounter underground, and the patterns received from the ground are plotted diagrammatically.

GPR – Two Basic Methods
•Common Offset Antennae (CO)
–easily interpreted
–quick
–can’t be extensively processed
•Common Mid-Point data (CMP)
–provides info for depth conversion
–slower to acquire
Ground Penetrating Radar
•Using CMPs allows for the recording of the variation in the signal pattern across a profile.
Ground Penetrating Radar
•At each point the length of time it takes the signal to return is logged and digitally corrected. Taking measurements along a particular course gives a profile of the signal at each point:

Ground penetrating radar results

Magnetic surveying
•Deviations can be detected in the general background of the subsoil, indicated by variations in its magnetic field. Several aspects of past human occupation cause suitable anomalies.

Magnetometer
•Various types:
•Proton Magnetometer
•Caesium Vapour Magnetometer
•SQUID (opposite) is ultrasensitive and are used as medical devices to measure brain and cardial activity
Proton Magnetometer
•Inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with the newly created field. The field is then interrupted, and as protons are realigned with Earth's magnetic field, spinning protons precess at a specific frequency. This is picked up by the same inductor and the ratio of the two fields is measured.
•Accuracy is okay. Measures in nano-Tesla
•Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.
•Now largely superseded.
Proton Magnetometer
•Both portable and fixed instruments are used.
•This fixed probe is measuring the magnetic activity at the active volcano at Karymsky in Russia
Fluxgate Gradiometer
•Measures the ‘gradient’ of the magnetic susceptibility
•Requires machine to be calibrated each time it used
•Measures susceptibility relative to an arbitrary zero
•Much more accurate than a proton magnetometer
•Data can be downloaded straight to a laptop and viewed almost straightaway
Caesium Vapour Magnetometer

•This is a highly sensitive and accurate device used in archaeological applications.
•The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapour and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.
Magnetometers
•Raw data is plotted, then it is filtered and processed using software to try and iron out any irregularities (e.g. different grids)
An interpretation of the results is then drawn up.Ultimately this needs to be tested by excavation. Some people call this ground-truthing or calibration.

For images of geophysical plots see the www.m3motorway.ie website.