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The GEEP development project at Geomatrix Earth Science
Now available for hire!
Geomatrix Earth Science partnered The University of Leicester in
a KTP project, the aim of which was to further develop the MSP,
or newly re-named GEEP system (Geophysical
Exploration Equipment
Platform) into a fully commercial product which
can be operated by a third parties
The KTP project came to a conclusion
in December 2006, with the GEEP available for commercial surveys
by 3rd parties in 2007
GEEP Data sheet (700 kb
PDF)
Geomatrix' partners in the FIESTA project were Glebe Mines, The
University of Leicester, British Geological Survey, Geosoft who were
all under the project management of MIRO and which came to a conclusion in
2003.
Geomatrix main involvement in this project was supplier of field geophysical
instrumentation and data acquisition software, and to aid University of
Leicester in the concept and building of the Multi-Sensor Platform or GEEP.
The underlying principals of the GEEP are similar to that or an airborne
platform, which allow multiple data sets to be acquired simultaneously and
rapidly with as little degradation of signal quality as is possible. This
allows large areas of land to be surveyed at a resolution that would not be
otherwise possible and in a time that is far less than when acquiring the
data by conventional techniques. Applications include Archaeological
prospection, industrial mineral exploration and large area site
investigation and environmental surveys.
A resume
of the GEEP its applications and limitations associated with it are to be found below.
Survey techniques
The GEEP allows surveys to be conducted quickly and efficiently. By using a
DGPS for location there is no need to set up a local coordinate survey grid,
as all locations can be tied back to national grid coordinates. The platform
can be towed across survey areas in an almost random pattern, although
thought must be given to the position of survey lines to avoid spatial
aliasing of data.
The survey technique that was found to be the most efficient in the field
was to drive around the perimeter of the survey areas in ‘ever decreasing
circles’. This allowed the platform to be towed continuously without
worrying about tight corners etc. Then as the loops became tighter switch
the survey pattern to parallel lines across the survey area. This allowed a
complete, dense data coverage with multiple tie lines.
Instrumentation
Geophysical instruments that have been mounted onto the
GEEP during the
FIESTA project include Geometrics G858 Caesium magnetometers, Geonics EM38,
Geonics EM31, Exploranium GR256 and an OMNI VLF system..
GEEP Data flow
Combining
multiple instrument data files
Although the GEEP is collecting several different geophysical
data sets, due to the nature of the data storage these data sets
are not completely processed separately. As some processes would
have to be performed repeatedly on each individual data set if
they were processed on their own, such as a positional
correction, it is more efficient to combine all of the data sets
into one large data file.
As the data sets are being collected simultaneously, the common
factor between each data set is the time. Thus the way to
produce a single data file is to combine the data records that
were collected at the same time. This is not a trivial
procedure.
How does the user know the time that a reading has been taken?
There are few instruments that fix an accurate time stamp to the serial
output string. Even when this is the case, the time stamp will
only refer to the individual instrument’s internal clock, and
have no relation to data collected on another instrument at the
same time.
In addition to this, instruments produce an output signal at
different rates, for example some at ten times a second – others
only once a second. Other instruments only output a signal at
approximate rates. In a 1 second period an instrument may
produce ten readings, in another 1 second period there may only
be 8 readings output.
More complication is added with the fact that the instruments
are not ‘synchronised’ – two instruments may output data once a
second; the first instrument may output a signal at the second,
but the second instrument may be producing a signal half a
second behind the first instrument.
The problem is how to combine signals such as these that may be
out of phase or irregularly timed and have no common time
identifier.
The key to combining the different data sets with the
GEEP system
lies with the GEEPLog logging software.
GEEPLog collects a transmitted data
string from the WLAN signal and adds a GPS
time stamp to the end of the string at the time the data was
sampled. Each data type is then saved in a separate file. The
time stamp is derived from the clock provided
as part of a GPS data string,
which may not be correct relative to GMT. In this manner the
time stamp is independent of the recording instruments, but it
is common to all of them.
GEEPLog removes
the problem of knowing when an item of data was recorded – but
it does not remove the other problems mentioned concerning
sampling rates. One way of doing this is to determine the sample
rate of the output file, and combine the input data files to
match that rate from a set starting time.
The instrument records are unlikely to lie exactly on the sample
interval for the reasons mentioned above, so a search window is
employed to take the nearest recorded value for the sample time
to ensure that the records that are combined are as close as
possible.
If the sample rate of the combined file is set to that of the
most frequently sampled instrument, then the less frequent
sampled instruments will be missing data in the combined file
records. Whereas if the sample rate is at the same rate as the
slowest sampled instrument then every record in the combined
file will be filled.
Although the GPS time field is the key
field with regard to combining the data sets together, the other
common factor that the individual data files share is location.
All of the data points recorded need a location on the ground,
and this is derived from the DGPS signal. As there must be a
location for each data point the DGPS data can be interpolated
across the missing records if the sample rate of the combined
file is greater than that of the DGPS input file. If this is the
case then the DGPS data is linearly interpolated across missing
values. The slowest rate of data output from the
DGPS
instrument is one reading every second, so assuming good DGPS
fixes, the worst interpolation errors are going to occur over a
one second period. This interpolation is carried out for the
DGPS values and not the other data sets because in any
one-second period the DGPS generally moves in a straight line
between two points. The same prediction cannot be confidently or
accurately made for the data from other geophysical instruments.
Due to the complexities of the transfer of the serial data from
the instrument to the logging machine, the output serial string
from some instruments can become distorted. This can involve
omitting characters from the string, or adding extra characters
in the middle of the string. These records can be easily removed
from the instrument data files, as they do not follow the set
format of the instrument record.
GEEP system with 4 Caesium Vapour magnetometers
and EM38 in tail stinger
The Post processor program
The post processor allows the data file
produced to be partitioned into
an individual file for each recording instrument. As recording
instruments have their own discrete output rates, and as they
are all at different positions on the sledge, this is a critical
step in creating an output file that can be read by a data
display package such as Profile Analyst from Encom or Oasis
Montaj.
The files for each instrument have
for each reading, a discrete XY location and time of
sample as provided by the on-board GPS system
DGPS data
The DGPS antenna is situated at the front end of the
GEEP, 0.40m
above the ground. The console unit is located on the towing
vehicle in an attempt to remove all unnecessary magnetic
material from the platform. The signal is transmitted from the
antenna to the console via coaxial cable within the tow cable.
From early work on the accuracy and repeatability of the GBXPro
DGPS data an accuracy of 0.4m has been recorded. With the
introduction of another differential beacon station at
Wormleighton since these findings, the signal to noise ration
and signal strength of the differential signal has been
increased. The positional accuracy investigations have not been
repeated in depth since the introduction of the new beacon, but
the data quality, and reliability appear to have increased with
its introduction.
The DGPS height data is not so well constrained as the XY
positional data. This is due to the position of the satellites
in three-dimensional space around the sensor.
A position in the XY plane can be derived well from 4 or more
satellite signals to the object as the satellites are likely to
be well spaced around the object in the XY planes of 3D space.
To accurately derive a Z position of an object on the surface of
the Earth from orbiting satellites the position is not going to
be so well constrained. This is because the satellites are all
essentially the same large distance away from the object, with
the XY distance differences becoming small in comparison. This
problem is compounded by the fact that the satellites that the
positional fix is derived from keep changing. As the number of
satellites changes in the data often a ‘jump’ in the height data
can be clearly seen. This affect is reduced as the number of
satellite signals received increases. Data repeatability is thus
dependant on the number of satellites.
Processing of the DGPS data involved the removal of data points
that were derived from using less that 4 satellites and a
differential signal latency of greater than 12 seconds. In
addition the data had to be corrected for the height of the
antenna above the ground.
GEEP tow vehicle example showing the
Datem unit and wireless LAN conmmunication system
Compass data
The fluxgate compass is mounted at the front of the platform
along with the majority of the other electronic components of
the system. The compass is mounted in a foam block as a means of
damping out vibration noise.
The data recorded by the fluxgate compass is a very important
component of the GEEP system. The data is used not only to derive
the position of the instruments in relation to the DGPS antenna,
but also gives an additional method of checking the quality of
the DGPS positions. Initial trials with the compass found that
the instrument was very sensitive and large, unstable variations
in the heading, pitch and roll of the instrument were recorded
at the slightest movement of the platform. Vibrations were also
being transmitted from the towing vehicle along the tow cable –
causing the deck to vibrate. Many attempts were made to reduce
the noise that was being recorded, including increasing the
rigidity of the platform ‘deck’ and cushioning the instrument
with foam. The method that was found to be the most successful
was the internal digital damping process as the instrument
recorded data. The damping was set so that the instrument
internally used a rolling average over 20 records (2 seconds),
this was found to leave the sensitivity high enough to detect
the motion of the sledge without being affected by the
vibrations.
Little other processing was needed on the compass data, except
the removal of bad data strings, which was done automatically
using the Interpol program.
From the conclusion of the FIESTA
project, Geomatrix has been involved in further developing the
GEEP as part of a MIST award (Minerals Industry Sustainable
Technology) and subsequently the KTP project along with the University of Leicester, and is
actively looking to further develop the system in applications
such as Archaeological prospection where large tracts of land
need to be rapidly surveyed.
GEEP acquired
magnetometer data at Wroxeter Roman City
The main applications for the GEEP
include mineral exploration, environmental and brown field
investigations and archaeological investigations. For
further information on the use of geophysics in archaeology,
the Journal Archaeological Prospection may be considered,
details can be found at www.interscience.wiley.com/journal/arp
page updated 05
February 2008 |
