Avon Valley
Archaeological
Society
DISCOVERING THE AVON VALLEYS PAST
Geophysical Survey Manuals
Garry Brisdion
Jul 2010
Contents
Contents............................................................................................................................ ii
List of Figures .................................................................................................................. iii
Section 1 – LAYING OUT THE SURVEY AREA ............................................................1
Laying out the 20 x 20 – Standard Layout ......................................................................1
Increased Accuracy Technique .......................................................................................2
Sampling Path ................................................................................................................3
Standard Layout .........................................................................................................3
Increased Accuracy Layout. .......................................................................................4
Section 2 - RESISTIVITY LOGGER .................................................................................5
Background Theory........................................................................................................5
Logger Overview ...........................................................................................................5
LCD Displays.............................................................................................................6
Resistivity Logger Switch Explanation ...........................................................................7
Output Resistance and Gain Switches .........................................................................7
Test Switch ................................................................................................................7
Mode, Up and Down ..................................................................................................7
Download...................................................................................................................7
Save switch ................................................................................................................8
Clearing the Memory .....................................................................................................8
Setting Up Prior to Survey .............................................................................................9
Data Storage Strategy .................................................................................................9
Cable and Probe Connection.......................................................................................9
Conducting the Survey ................................................................................................. 10
Potential Errors ............................................................................................................ 11
Saving Data to the PC .................................................................................................. 11
Section 3 – SAVING AND PROCESSING DATA .......................................................... 13
Saving Data to the PC .................................................................................................. 13
Data Processing ............................................................................................................ 13
ii
List of Figures
Figure 1 – Basic Survey Area Geometry ............................................................................1
Figure 2 – Layout of Sample lanes .....................................................................................2
Figure 3 - Increased Accuracy Sampling Rate ....................................................................3
Figure 4 - Sampling Route for Standard Layout .................................................................4
Figure 5 - Sampling Route for Increased Accuracy Layout .................................................4
Figure 6 - Layout of the Resistivity Logger ........................................................................5
Figure 7 - Suggested Survey Recording Layout ..................................................................9
Figure 8 - Position of Remote Pegs .................................................................................. 10
iii
Section 1 – LAYING OUT THE SURVEY AREA
1.
The standard area for survey is 20m x 20m but any shape or area size can be used.
However, the size must be carefully planned in advance as this will affect how the data is
stored in the Resistivity and Magnetometer Loggers. This section will cover the process
for laying out and surveying a standard 20 x 20 area followed by specific instructions for
recording variation in each section below.
Laying out the 20 x 20 – Standard Layout
2.
As per any site layout a base line should be established and used for all survey work.
However, for Magnetometry this should ideally run magnetic east-west. This is to
reduce the variations due to crossing lines of magnetic field. In practice small variations
have little impact upon the recorded measurements.
3.
Once the baseline is established, mark out the 20 x 20 using the diagonal measurement
of 28.28m to give a true right angle as Figure 1 below:
28.28m
20m
Baseline
Reference
Corner
20m
Figure 1 – Basic Survey Area Geometry
4.
Ideally the whole site should now be marked out using this baseline and a 20x20m grid.
5.
Once the Basic Survey Area (BSA) is marked out it must be divided into columns to
guide the sampler.
6.
Both horizontal sides of the BSA should be marked out with pegs at 2m. These are
joined across the BSA by a Guide Line which effectively divides the BSA into 1m wide
strips referred to as Guide Columns. The Guide Columns are marked off at 0.5m and
then at every 1m intervals as shown in Figure 2 below. These markers are referred to as
Sample Markers and if laid out correctly will create two hundred 2m x 1m Sample
Boxes. Two separate readings are taken from within each sample block as described
below to give a total of 400 samples per a single BSA. This will equate to 1 sample for
every 1m x 1m square within the BSA.
7.
It is feasible to increase the sample rate to every 0.5m x 0.5m block by increasing both
the number of vertical Guide Lines and the sample markers. However, this would
1
increase the number of samples to 1600 per BSA and would increase the difficulties of
later processing for very little gain. This is only recommended for small areas where
increased resolution is required over areas of specific interest.
B
C
F
G
K
L
O
P
S
T
W
Peg
Line Marker
Guide Column
Sample Block
Sample Location
Guide Line
A
D
E
H
J
M
N
Q
R
T
V
Figure 2 – Layout of Sample lanes
Increased Accuracy Technique
8.
The technique described above is the standard layout used universally across the
industry and will provide a perfectly adequate guide to sampling location and rate.
However, the inexperienced user may experience some difficulty monitoring the logger
and keeping a track of the guide line to the side at the same time. When using the
magnetometer it is very easy to loose track, resulting in less samples taken per column
that is required. Additionally, placing the resistivity probes or holding the
magnetometer over the centre of the 1m Sample Block is subject to the obvious errors
9.
As a result, we have developed a method for laying out which gives greater accuracy
without the need to constantly monitor the sample position relative to the adjacent Line
Markers. However, this method does require twice the number of guide lines or the
need to constantly move lines across the BSR as they become free.
10.
Firstly, the BSR Baseline is pegged out at 0.5m then every 1m. Secondly, the Guide
Lines Markers are also located at 0.5m and then every 1m. If laid out correctly the BSR
should now be broken down into 1m x 1m sample areas with the Line Marker located in
the centre of that Bample Block
2
Resistivity
Probe position
Magnetometer
position
1m
1m x 1m
Sample Block
1m
0.5m
A
D
E
H
Figure 3 - Increased Accuracy Sampling Rate
Sampling Path
11.
This section describes the path taken through the BSA in order to cover the ground in a
systematic way that can be understood by the Data Loggers. Both the Magnetometer
and Resistivity Meter are very specific in the way data must be recorded and so this
section, which applies to both loggers, should be strictly adhered to. Specific instruction
on how to take the recordings will be given in the sections on the respective
instruments.
12.
To ensure clarity the sample path will be described on a smaller sample area as shown
in Figure 4 and 5 below.
Standard Layout.
13.
Figure 4 below represents the sample path to be followed if using the Standard Layout.
The lines A-B, C-D, E-F etc represent the Guide Lines that should be marked at 0.5m
and then every metre as described above. In order to cover the area in a systematic way,
the operator will use these lines as a guide to their LEFT at all times so the walk would
follow the numerical route through the area but keeping the guide line on the left at all
times.
14.
Each sample should be taken adjacent to the Sample Marker on the on the Guide Line.
The Sample Location marker on the diagram represents the position of the
Magnetometer and the mid-point between the two Resistivity probes. The exact location
has to be judged by eye. If the path is followed strictly, two samples are taken from
each 2m x 1m sample block, one on the way up and one on the way back.
NOTE: Whilst it is possible to rotate the Resistivity Logger when changing direction it is
essential that the Magnetometer is held in the same orientation with the handler moving
3
around the Logger accordingly. The reasons for this are explained in the Magnetometer
section.
B
C
2
F
3
6
7
Line Marker
Sample
Location
Direction
of Walk
1
4
A
5
8
D
E
Figure 4 - Sampling Route for Standard Layout
Increased Accuracy Layout.
15.
Figure 5 below show the sampling route to follow if the increase accuracy layout is
followed. It can clearly be seen from the diagram that there is no doubt as to where the
sample position is located
16.
B
C
F
G
Line Marker
Sample
Location
Direction
of Walk
A
D
E
H
Figure 5 - Sampling Route for Increased Accuracy Layout
4
Section 2 - RESISTIVITY LOGGER
Background Theory
17.
When two conductors are place in the soil and a voltage applied across them a current
will flow between them. The amount of current that flows will depend on the resistance
of the soil between the two electrodes. The level of resistance is caused by a number of
factors: the soil’s water content, the chemical makeup of the soil and the presence or
absence of conductive material such as iron or magnetite.
18.
The Resistivity Logger uses a four probe system; 2 transmitters (Tx) and 2 receivers
(Rx). One pair of Tx/Rx (Reference Probes) is placed in the ground approximately 2m
apart and 15-20m away from the closest edge of the sample area. This pair acts as a
standard reference and should not be moved during the survey.
19.
The second Tx/Rx pair (fixed to the Sample Frame) is inserted into the sample area and
a reading taken. The Logger records the difference between the first and second pair of
probes as the relative value at the sample location. This is the figure stored for further
analysis.
Logger Overview
PC Connector
RESISTIVITY LOGGER
510B
14C*
Probe
Connectors
490A
20#
16R = 49
GAIN
OUTPUT
RESISTANCE
OFF
ON
TEST
UP
DOWN
MODE
DOWNLOAD
SAVE
Figure 6 - Layout of the Resistivity Logger
5
LCD Displays
20.
The LCD has two modes: Test and Sample Mode.
On switch on - the LCD will display the following:
21.
The words Soil Resistivity will be quickly replaced by alphanumeric values which will
differ depending upon the mode being used.
22.
The bottom line refers to the sample value (extreme right) and the location in the
memory to which that value is stored. C is the column and R is the row being sampled.
The * indicated which is active ie which one will be changed by using the UP and
DOWN switches. The MODE switch changes to active cell between C and R. NOTE:
Only 128 (0 to 127) rows or columns can be sampled before the value roll over to 0 and
start over-writing the original values. Surveys should be designed accordingly.
In Test Mode the LCD will display:
23.
During Test Mode the top line of the LCD shows the values of the probe pairs at the
output. A and B refer to the two amplifiers within the Logger, one is attached to each
TX/RX pair of probes. The number to the right of the top line indicates the relative
difference and is for information only. This should not be saved to memory.
In Survey Mode the LCD will display:
6
24.
During Survey Mode the top line letters are changed to H and L referring to the high
and low values of the two probe pairs and the difference shown at the right of the top
line. Ideally this should be kept below a value of 500 for normal operations.
25.
If a sample value has already been stored at a particular memory location, that value
will be displayed on the right of the bottom line. Saving again will over-write the
original data.
Resistivity Logger Switch Explanation
Output Resistance and Gain Switches
26.
The Gain and Output Resistance rotary switches are used to control the amount of
current passing from the Tx probes into the soil and the sensitivity of the system to the
amount of current detected at the Rx probe. Rotation clockwise increases their values.
Switches set to the twelve o’clock position means the lowest level of Gain and no added
resistance to the output current (full current). The relative setting of these is by trial and
error and ideally should give a sample value as large as possible and ideally in the order
of 500. Moving the Reference Probes closer to the BSA can help to increase the sample
value.
Do not change these settings during the survey.
Test Switch
27.
This switch allows the microprocessor to monitor the output voltages for test purposes
and would not be used under normal sampling other than to periodically check that both
probes remain in connection.
Mode, Up and Down
28.
These control the location within the memory that the Logger stores the sample data to.
The Mode switch toggles between Column and Row and the Up and Down increases or
decreases the Column or Row value. Their operation will be explained later.
Download
29.
This switch is used only during the download of data to the PC for processing and
should not be used at any other time.
7
Save switch
30.
Pressing this switch saves the individual sample value to the memory location set by the
Mode, Up and Down switches. Unfortunately, this has to be done for each sample
taken. The LCD messages will be explained under the section explaining how to save
sampled data.
Clearing the Memory
31.
The memory is of only limited size and should be cleared as frequently as possible to
avoid running out mid survey. Maximum capacity is 128 columns with 128 rows of
samples or 16384 individual sample readings (each sample uses two memory locations).
Each 20x20 sample area will require 400 readings (800 memory locations) therefore
only 40 surveys can be completed before clearing of the memory is necessary. In
practice the data should be saved to PC and memory cleared more frequently as
processing large amounts of data in the PC will be cumbersome and prone to errors.
32.
Clearing the memory takes approximately 3min.
33.
To clear the memory the Logger must be switched OFF. Press and hold down the
SAVE button and then switch power ON. When the LCD shows the CLEARING
EEPROM message on the screen the SAVE button can be released.
34.
The numbers on line 2 show the progress and when complete the LCD will revert to the
start-up SOIL RESISTIVITY screen and start normal operations.
35.
To prevent inadvertent loss of data, clearing of the memory can only be done at start-up.
36.
Saving data to the PC does not clear the memory
8
Setting Up Prior to Survey
37.
To ensure no inadvertent loss or overwriting of data it is good practice to clear the
memory before each survey session (each day and not each sample area). Ensure that
any data is saved to the PC before clearing the memory.
Data Storage Strategy
38.
Plan the survey data storage strategy. Create a plan of the area and record the row and
column numbers to be used for each area. Note: within the Logger the Column and
Rows begin at 0. These settings should not be used as they potentially cause confusion.
The diagram below gives a suggested strategy for recording a number of adjacent
survey grids.
39
Rows
Area 3
Column 1-20
Row 21-39
Area 4
Column 21-39
Row 21-39
Area 1
Column 1-20
Row 1-20
Area 2
Column 21-39
Row 1-20
21
20
1
1
20 21
Columns
39
Figure 7 - Suggested Survey Recording Layout
Cable and Probe Connection
39.
Before switching on the meter, attach the two cables via the appropriate colour probe
connector sockets on the side of the meter.
a. Frame Probes The crocodile clips on the short cable are then connected to the
screw attachments on the probe frame. Ensure the cable is wrapped around the
frame to ensure it is not inadvertently disconnected during surveying.
b. Remote Pegs The crocodile clips on the long cable are connected to two steel
pegs which need to be inserted into the ground in an area adjacent to the survey
area and approximately 20mts away from the closest edge of the area as in the
9
diagram below. This will need to be moved as the survey progresses from Area
1 to Area 3 in Figure 7 above but may be satisfactory for both Area 1 and Area 2
as long as it is located equidistant from each area.
3-4mts
15-20mts
Peg position
for Area 1 only
Peg position for
Area 1 and 2
Area 1
Area 2
Figure 8 - Position of Remote Pegs
Meter Calibration
40.
Before conducting the survey the meter needs to be calibrated. This is done via the Test
Mode operation and by adjusting the position of the Remote Pegs. This stage is a matter
of trial and error as the final setup will vary depending on a number of factors including
soil type, soil moisture content and strength of the battery.
41.
Once the meter is connected and the remote pegs positioned:
a. Switch on the meter with the Test mode on
b. Check reading and try to maximise the value on the right of the top line of the
LCD by altering the Gain and Output Resistance to ideally achieve a figure of
150-200 but well below 500.
c. Varying the distance between the Remote Pegs can also increase or decrease the
measured value.
Conducting the Survey
42.
Conducting the survey is a routine task and care should be taken to follow the sequence
of activities to ensure the safely record of the sample data. DO NOT press the
DOWNLOAD button during the survey. If you do it is not catastrophic and can be
recovered – see below.
43.
The process involves the following steps:
10
a. Before starting any new days survey activity Plan and record the Layout.
b. Clear the memory
c. At the start of the survey set the Column and Row numbers as per the plan set
out in the previous section or as C1/R1 if this is the first survey conducted. To
make the Column or Row active press the MODE button until the LCD displays
C or R and press the UP button to increase the value.
d. Insert the probes into the ground directly over the sample location. The LCD
will change and then settle down.
e. Press SAVE
f. Move to the next Sample Location and insert the probes.
g. INCREASE the Row location by 1 and press Save
h. Repeat steps d-g until the sample column is complete.
i. Move to the top of the next sample Column.
j. Increase the Column number by 1.
k. Insert the probes and press SAVE
l. Move to the next Sample Location and insert the probes.
m. DECREASE the Row location by 1 and press Save.
n. Etc until the column is complete and R is down to 0 and then repeat.
44.
GOLDEN RULE:
INCREASE for every New Column
INCREASE the ROW going up
DECREASE the ROW going down
Potential Errors
45.
Pressing DOWNLOAD during Survey.
Pressing DOWNLOAD at any time stops
the current activity and instructs the Logger to speak to the PC. As the PC will not be
connected at this stage the Logger will wait until it is connected. The remedy is simple
and there is no risk of loss of data by following these steps:
a. Switch the Logger off then on.
b. Ensure the Column and Row are set to the last known sample location or reset to
the beginning of the Row.
c. Recommence sampling
46.
Failure to SAVE or reset the C/R number. Neither of these is a problem as the data
can be recaptured from the last known location:
a. Examine the values stored in the Row using the MODE and UP and DOWN
buttons.
b. Identify the last location with a value stored.
c. Move back to that location using the Guide Lines and Markers as a guide.
d. Rest the Row number to that location and recommence sampling
Saving Data to the PC
47.
Connect the Logger to the PC using the USB-RS322 cable.
11
48.
On the PC open the Magnetometer software.
49.
Select the data mode button to Resistivity
50.
Click ‘DOWNLOAD DATA’ button. This will start the process and ask for
confirmation. Once confirmed a DOWNLOAD screen will appear. The software is now
awaiting receipt of the data from the Logger. Press the Download S7 button on the
Logger within 30 Seconds to send the data.
51.
A bar graph on the software will show time left before S7 must be pressed otherwise the
process will be aborted.
52.
The LCD on the Logger will show ‘SENDING TO PC’ followed by ‘SENDING
FINISHED’ when complete. Display will then return to SOIL RESISTIVITY screen.
53.
If data is not received check the COM port settings via the screen options and repeat.
12
Section 3 – SAVING AND PROCESSING DATA
Saving Data to the PC
54.
Connect the Logger to the PC using the USB-RS322 cable.
55.
On the PC open the Magnetometer software.
56.
Select the ‘RESISTIVITY, data mode radio button (top right)
57.
Click ‘DOWNLOAD DATA’ button. This will start the process and ask for
confirmation. Once confirmed a DOWNLOAD screen will appear. The software is
now awaiting receipt of the data from the Logger. Press the Download S7 button on
the Logger within 30 Seconds to send the data. A bar graph on the software will
appear showing the download progress
58.
The LCD on the Logger will show ‘SENDING TO PC’ followed by ‘SENDING
FINISHED’ when complete. Display will then return to SOIL RESISTIVITY
screen.
59.
If data is not received check the COM port settings via the screen options. This
should be set to Com 2 for the Resistivity meter
Data Processing
After successful two data files will be placed on the desktop of the computer.
These contain the individual sample readings but must be processed before
they can be imported into the geophysics visualisation data.
Data File 1 - EARTHRESORIG File
Typical value = 149*126
Open the file with a Text Editor
13
Remove the *126 and *127 from each string in the TXT file. The remainder is the sample
value
Remove the __0*__0, (incl the comma) where __ is two spaces. These are Start and Stop
recording markers and are not required.
Also remove the * 0 (the gap is two spaces)
Copy the remainder to Excel or Word
Data File 2 - EARTHRES File
The individual values are the sample value plus 32256. Therefore to obtain the true value
copy the full data table into Excel and subtract 32256 from each value.
As a double check, the resultant value should be the same as the number before the *126 in
the EARTHRESORIG File
14