PPM IV (LCD) Manual Procedures

1. Power up procedure

1.1. Connecting a Battery

The PPM system is not supplied with a battery. Thus, you have to select and provide your own battery. It must be a non-magnetic type of battery, usually, a lead-acid 12V battery of a capacity of around 5AH. You must connect its red pins on the red wire of the system cable and the blue pins on the blue wire.

1.2. Inserting the SD Card

Normally, the PPM is supplied with a SD Card already inserted in its slot. Open the cover of the box and you will see its socket on the controller board attached to the cover. The SD Card is removed by pushing and releasing it to unlock it from its socket. In order to re-insert it, just push it up to the top of the socket; it will then stay locked in.

1.3. Connecting the Sensor

The sensor is supplied with a cable and a connector. Insert the connector of the cable into the corresponding connector of the box.

1.4. Power up

Now, you can power up the system by flipping its power switch. You will hear a small beep and a window on the screen will show the message 'Checking SD Card...' for a few seconds followed by the HOME screen.

2. Exploring Menu Structure

Menu Structure

When the menu is displayed on the LCD, the first line shows the current menu entry that will be selected when the OK key will be pressed. In general, pressing OK or DOWN will enter the next menu level, pressing LEFT/RIGHT will scroll on the list of available entries at the same level and pressing UP will go back to the next upper level menu entry or exit from the current status.

The main level menu entries are:

• Set Parameters: Normally, one sets the system parameters by editing the parameter file on SD-card and plugging it into the instrument at home before going to the survey area. However, using the menu keys may change most of these. Changing the parameters, one digit at a time by pressing the keys is a very slow and awkward process and so this is normally only done if absolutely necessary; pre-programming the SD-card is a much more convenient. Under this menu tree is also the FFT process used to measure the local precession frequency at a new earth location.
• File Control: Selects the identification of the grid for the following survey(s) or review any captured grid file (GDxxxxxx.TXT)
• Start Session: Initializes a new survey session. When entering this menu entry, the noise level is repetitively measured and displayed on the LCD expressed in mV every seconds.

Individual Menu Entries

System parameter menu

The System parameter setup menu allows setting the Operation Mode, the Local B field value and the amplifier Gain. All these parameters are set at a default value in factory. At this point, do not change any of them.

In the 'Amplifier Gain' sub-menu, you will see a changing value 'NL'. This is the Ambient Noise Level expressed in mV as captured by the sensor without any polarization and measured by the system. This gives an indication of the local human-generated noise. It must be below 50mV in normal site conditions. We shall use this indication later in the procedure.

The Polarization Period gives the reading rhythm expressed in milli-seconds; it is set by default to one second (maximum rhythm of one reading per second).

The Polarization Duration' gives the duration of the polarization current. Its default value of 650ms is to be left as-is.

The 'Delay' is the delay inserted after the end of the polarization and the start of the measurement period. Keep it with that value.

The ''Gradient Threshold' is the threshold of field gradient over which an aural warning is sounded.

The 'Local B measure' menu is used in 3. Tuning the sensor.

Grid Control menu

• Set Grid ID

This is the grid file ID given as grid file name used during all the following grid control and measurement storage operations. The grid file name is formatted as: ‘GDmmddxx.TXT’ with mmdd being the current date and xx the current survey index on the same day.

(LEFT/RIGHT keys for selecting character positions, UP/DOWN keys to INC/DEC digit value, OK key = confirm)

3. Tuning the Sensor

3.1. Introduction

The sensor tuning process sets the sensor to resonate at the average local precession frequency by connecting proper valued capacitors in parallel to the coils of the sensor.

A tuned sensor gives two important advantages to the whole PPM system.

It acts as a band pass filter filtering out the ambient noise of different frequencies and amplifies the signals with the center (tuned) frequency.

Before doing the tuning, we must first measure or evaluate the average local precession frequency.

There are some Web sites which give the approximate precession frequency given the GPS fix of the local region where the surveys have to be executed. However, we have provided an frequency measurement system integrated in the PPM and based on an FFT algorithm which give a much more precise local frequency value.

Knowing that frequency value (from one method or the other) and the inductance of the sensor, we can then calculate the capacitance to be put in parallel to the coil in order to make the sensor to resonate at that frequency.

3.2. Tuning Caps & DIP switch

The tuning caps are given as a binary series of sequentially increasing cap values set in or out of circuit by a 8 position DIP switch.

The binary series is as follows:

Pos1 Pos2 Pos3 Pos4 Pos5 Pos6 Pos7 Pos8

328nF 164nF 82nF 47nF 22nF 10nF 4.7nF 830nF

The FFT process will give the values to be set on each position like for instance ‘01000110’. This means setting pos 2, pos 6 and pos 7 to ON (switch position pushed down to the left) and all the rest to OFF (pushed down to the right).

Position 1 is the switch closer to the border of the board.

3.3. Starting the Tuning Process

• Set all the DIP switch positions to OFF (pushed down to the right)
• Go to a spot where you think there are not too much human-generated electro-magnetic activities, certainly out of any building with power supplies. That can be a back garden or a recreation park.
• Put the sensor in horizontal position and oriented to the East/West axis.
• Clean your body from any object made of magnetic material (iron, steel, nickel).
• Power on the PPM
• Go to the Gain setup menu.
• The NL value displayed and refreshed every second on the System screen should not be much higher than 30mV. If it is not, then, this spot is probably not well selected. In that case, you should try another spot.
• Go to the 'Local B Measure' menu and push OK.
• You must hear a sound three times in a row; the FFT process is actually doing three successive polarization and FFT measurements.
• After this period, the screen displays three precession frequencies expressed in nT with the corresponding tuning capacitance and the average capacitance to be set.
• If the three values are almost or completely equal, push OK to display a list of up to eight DIP switch setting, otherwise push UP to cancel the action.
• One of the settings of the list should be the best value to tune the sensor. Take a note of those but there is also an ASCII-coded file recorded on the SD Card which gives the same indications. Its file name starts with FR and its file extension is TXT. You could take the card out and look at it on a PC.
• This is an example of such a file (FR080302.TXT)

$TB 48384nT=326nF

$TB 48384nT=326nF

$TB 48384nT=326nF

$TC SET:326nF

$TD 01110110

$TD 01111000

$TD 01111010

$TD 01111100

$TD 01111110

$TD 10000000

$TD 10000010

$TD 10000100

$END

• You can see that the three FFT attempts gave the same result, this means that the spot was well selected.

3.4. Setting the DIP switch positions

In principle, the DIP switch position which will probably be the best will be one in the middle of the list.

• Power OFF the system.
• Try first this one (on the above example file, ‘01111100’) and proceed to the next point of the procedure to execute a first static session.

4. Executing a Static Session

4.1. Starting Session

• Goto the 'Operation Mode' setup menu and set the Single Static mode.
• Go back to the 'Start Survey' menu and push OK
• The system starts polarizing, measuring and displaying the local B field values every second.
• The B value is the absolute precession frequency expressed in nT.
• The M value is the signal magnitude expressed in mV. Higher is better.
• The S value is the Sigma (Standard Deviation) giving an indication of the quality of the result. Lower is better.
• The V value is the battery voltage at the end of each polarization period (thus, under stress).
• The G value is the field gradient in nT (signed difference between consecutive readings)
• Look more specifically at the S and M values.
• After a few readings, temporarily halt the repetitive readings by pressing the OK key .
• Take note of the S and M values you have observed.
• Open the box cover and set another DIP switch setting.
• Push on the OK key and look again on the S and M values.
• Loop on these last 5 points of the procedure until you have detected the position which gives you the lowest S value (usually around 1 or 2 and certainly less than 10) and an M value over 100. This is your best tuning setting.
• Stop the session by pressing the OK key then, UP key.
• Now, with the best tuning setting, start a new session on a new grid file (its index will automatically be increased by one) and let it run for a few minutes. During this test, avoid going close to the sensor. This static test will confirm your tuning and will give you the dynamic behavior of the local magnetic field over a period of a few minutes.

4.2. Studying the result file

This is an extract of such a file captured in my own office (not the best place to make this test but you still see that the results are not too bad when the sensor is well tuned)

SESSION:GD080610, 50mS, 650mS,160x,48165nT,1000mS,255

# 13 48177.6 4.1 357 11.04

# 23 48183.3 4.6 328 11.02

# 33 48178.8 4.4 307 11.02

# 43 48174.1 5.1 330 11.01

# 53 48177.3 4.3 365 11.01

# 63 48178.0 3.7 371 11.01

# 73 48171.5 3.4 409 11.01

# 83 48171.1 4.3 441 11.01

# 93 48170.3 3.5 387 11.01

The first line gives the main system parameter values:

• Grid file name
• Delay
• Polarization Duration
• Amplifier Gain
• Base B
• Polarization period in msec (i.e. one per second)
• Gradient Filter

The next lines are the reading results:

• Time stamps expressed in 1/10 second
• Absolute B field in nT
• S value
• M value
• Battery Voltage value

In normal conditions of a magnetically-quiet environment, you should see the B values not varying by more than a few nT between consecutive readings. If a car has passed by at a few tens of meters, you should see a large bump in the plot of B.

5. Executing a Multi-line Survey Session

5.1. Surveying Procedures

5.1.1. Survey Field evaluation Procedure

• If this is the first experiment in that particular region, check the value of the local B field and the noise/Spike/magnetic disturbance levels by running the FFT until you get three consistent B field values. In that case, the $SB parameter has been automatically set to the right value on the SD Card.
• Fine adjustment of the tuning (minimize S and maximize M) at a fixed station in the middle of the area to be surveyed. No problem to flip the DIP switches during the repetitive polarization cycles.
• Set the Single Station operation mode and make a long traverse on the two diagonals of the grid while observing the sigma, magnitude and field gradient. By looking at the whole captured data, you can evaluate the limits of B field values and check that the tuning is not on the 'edge of stability' for the whole area.

5.1.2. Survey Procedure

The surveys are usually made with a grid line separation of one meter and a length of maximum 100 meters. In one hour, one is able to cover about 50 grid lines making a survey area of 5000 square meters without any external help and without much effort.

We measure the exact length of a survey line with a decameter, say L=50m. On each side of the rectangular grid, we pull a line (W1,W2) with a knot every meters or a decameter. We initially plant poles with visible flag F1 and F2 on points 0,1 and L,0

That’s all for the site preparation.

· Starting from one corner of the grid (0,0), we trigger the instrument to start its repetitive readings (menu : Start Session) and we start a slow straight walking at a regular pace while visually aiming at flag F2.

· When arriving at flag F2, we interrupt the readings (OK key).

· We move the flag F2 to point L,2

· We go to point L,1 and resume the readings (OK key) slowly walking straight at a regular pace while aiming at flag F1.

· When arriving at flag F1, we interrupt the readings (OK key).

· We move the flag F1 to point 0,3

· We go to point 0,2 and resume the readings (OK key) slowly walking straight at a regular pace while aiming at flag F2.

And so on.

Note that the walking pace should be as regular as possible for each line but it is not critical that the walking paces are the same for all the lines.

The result of this survey is a sequence of survey line readings going in zigzag mode.

5.2. Reviewing Grid file

· Goto the 'File Control' menu, 'Grid Review' sub-menu

· Shows the first grid file name starting with GD

(OK key to select this file to be reviewed, DOWN key to display the next file name starting with GD)

· It starts a local post-processing routine which re-reads the grid file ‘GDmmddxx.TXT’, regularly spreads the reading points over each survey line and stores the grid file under the name ‘‘GGmmddxx.TXT’. The survey lines can be given a specific length. By default, the average number of points per line is evaluated and this is used as line length

· The whole file is first completely read and the average absolute B field value is calculated over the whole survey duration.

· Starts reading the grid file and displaying its Gradient values as a running barchart. The Gradient value is the difference between the absolute B value and the average B value of the session.

· While reading is running,

· If Gradient of the reading point in the middle of the barchart goes over the gradient warning threshold, then an aural alarm is started and the reading is stopped showing the detailed values of that reading.

· OK key to stop the reading and show the detailed values of the reading located in the middle of the barchart,

· UP key to stop the reading and Select between:

· UP key to exit from the Review process

· RIGHT key to Zoom OUT the bars (scale*2) and set the gradient warning threshold to 8*scale.

· LEFT key to Zoom IN the bars (scale/2) and set the gradient warning threshold to 8*scale.

· OK key to resume reading

· While reading is stopped,

· LEFT/RIGHT key to show the detailed values of the previous/next reading on the barchart.

· OK key to resume reading

· UP key to Select between:

· UP key to exit from the Review process

· RIGHT key to Zoom OUT the bars (scale*2) and set the gradient warning threshold to 8*scale.

· LEFT key to Zoom IN the bars (scale/2) and set the gradient warning threshold to 8*scale.

· OK key to resume reading

5.3. Post-Processing and Plotting Survey Grid

Review these procedures in Chapter 9 of the operator manual on the web site.

6. Executing a GPS-based Session

The required accuracy of GPS fixes of PPM readings is relative and depends on the work to be achieved:

- For underground surveys in fair and rather flat ground where a rectangular grid of reasonable dimensions (say, less than 50x50m)  made of several parallel survey lines can be clearly delimited for a single survey session, a single GPS is most probably not accurate enough. However, in that particular case, the location of two opposite corners of the grid can be defined by two GPS readings using a commercial GPS unit or relative to a well-known fixed physical reference point (to be able to come back to it later on), then, the survey itself could be made line by line with a fixed separation of around 1 meter.

- For single line surveys (as at sea from a boat or in a deep forest or in a desert looking for meteorites from a vehicle), what is needed is a continuous log of both PPM and GPS readings during the survey followed by a study of the track and a localization of potential targets over the survey line. In that particular case, an accuracy of 5 meters for the GPS fixes is enough to be able to go back later where the potential targets have been detected during the post-processing.

It means that, except for professional applications requiring an ABSOLUTE precision in the GPS fixes, most types of PPM surveys can be made with a single commercial GPS and still give very satisfying practical results.

The GPS chip we integrate in the PPM system is based on the SIRFIII Chip set and it is more sensitive than most of the commercial GPS units. It is able to use up to 20 Satellites and the experience showed that most of the surveys were made with 9 to 10 satellites in view at a time when being in open countryside. This gives a high fix precision evaluated to the meter range(HDOP <= 1).

6.1. Surveying Procedures

• In order to make GPS-based surveys, there must be two conditions met:

1. The system parameter $GO needs to be set either to '$GO Y' or '$GO U'. The first option generates GPS fixes expressed in Decimal Degrees and the second option generates the GPS fixes expressed in UTM units (i.e. in meters). This parameter is not accessible on the GUI menu system, it needs to be set using a text editor on a PC on the GLOBAL_P.TXT file on the SD Card.
2. The Operation Mode must be set to 'GPS'. This is set on the upper left of the System screen as a radio button. After this setting, push on the SET button to record this value on the SD Card and power OFF. At the next Power ON, the GPS will be activated and operational.
• Go to the survey site and power ON the system. If it was left without power for some time, the GPS will make its 'Cold Start' procedure which will take possibly one or two minutes to complete. If you start a session before the GPS is ready, you will get a screen message asking to wait until the first fix is ready and showing the number of Satellites in View (SV:x). In that case, just wait until this message disappears and the normal latitude/Longitude values are displayed. If the system was left without power for a short period of time, the GPS chip will still have kept the knowledge of its last location in its super-cap and will just make a 'Warm Start' which is much shorter in time.
• Observe the H values displayed at the right of the longitudes. This is the HDOP (Horizontal Dilution Of Precision). In order to get the most precise GPS fixes, you must wait until the H values are 1 or less. This will not happen immediately, it will possibly take a few more minutes before the GPS has synchronized to enough satellites. The HDOP will only be small if the number of satellites is high (about 8 or 9) and those are sufficiently separated in the sky to enable the GPS to calculate precise fixes. When visible GPS satellites are close together in the sky, the geometry is said to be weak and the Dilution Of Precision (DOP) value is high; when far apart, the geometry is strong and the DOP value is low. Thus a low DOP value represents a better GPS positional accuracy due to the wider angular separation between the satellites used to calculate a GPS unit's position.

Note: With an HDOP <= 1 and SV >= 7, one can expect an average global fix precision of less than one meter. In full open country or at sea, SIRFIII GPS devices will usually give that quality of fixes or even better. This range of precision is valid for the PPM surveys over wide area where a GPS support is needed (e.g. underwater surveys).

• When the HDOP value is low enough, start a GPS-based session while leaving the sensor at a fixed position and let the session run for a few minutes. Then, review the file and look at the variations of the B field. The G, S and H values should stay very low.
• Now, you are ready to start your real survey session.
• If you need to survey an area where the human electro-magnetic activities generate large artificial field variations in the underground and you want to detect deep and relatively small targets, you will need to make a differential survey. With two GPS-based PPM instruments, this is easy. You just start a session with a PPM located in the vicinity of the survey area and you leave it there for the whole duration of the survey session. You use a second PPM to make the real survey session walking around this area. At the end of the session, you stop the session on the two PPM's (on the mobile PPM first) and you can then go to the Post-processing step.

6.2. Reviewing Grid file

See 5.2. of this document.

6.3. Post-Processing GPS-based survey files

The GNUPLOT screen displays two GPS-related tabs, 'GPS m+b' and 'GPS m'.

• You push on the 'GPS m' tab to post-process a survey file made with a single mobile PPM. This process will just filter out the bad readings (S > 15) from the file and generate a .GRID file with the same name and same format as the input grid file.
• You push on the 'GPS m+b' tab to post-process a survey file made with a mobile and a fixed PPM units. This process will merge the two data stream based on their UTC values, filter out the bad readings (S > 15) on the fixed and on the mobile stations and generate a .GRID file with the same name as the mobile file. This is a sample of such a file:

$ 4.4461083 50.5075188 15.9 48451.0 3.9 163 48466.9 1.4 275

$ 4.4461088 50.5075264 16.2 48451.0 3.9 163 48467.2 1.3 257

$ 4.4461112 50.5075302 16.7 48451.0 3.9 163 48467.7 1.6 246

$ 4.4461074 50.5075302 9.5 48451.0 3.9 163 48460.5 1.4 244

$ 4.4461098 50.5075340 9.2 48451.0 3.9 163 48460.2 1.2 290

$ 4.4461122 50.5075340 11.4 48451.0 3.9 163 48462.4 1.1 324

$ 4.4461126 50.5075340 13.4 48451.0 3.9 163 48464.4 1.5 288

$ 4.4461126 50.5075302 12.5 48451.0 3.9 163 48463.5 1.6 283

$ 4.4461126 50.5075302 11.4 48451.0 3.9 163 48462.4 1.2 295

The field1 and field 2 are the corrected long/lat,

The field 3 is the delta B between corresponding readings

The fields 4,5 and 6 are the B,S and M of the fixed PPM

The fields 7,8 and 9 are the B,S and M of the mobile PPM

6.4. Plotting Survey Grid

GPS-based sessions may be made as a linear survey or a grid survey:

• Linear Survey.

This type of survey session is made as a single line of readings without attempting to cover a specific and delimited surface on the ground. This type of survey just requires a 1D plotting given either by the grid review bar graph on the PPM itself or the same plotting made by simple tools like Excel. The field gradients generated by potential targets are very easily located from their corresponding long/lat fixes.

• Grid Survey.

This type of survey session is made by making a multi-line grid attempting to cover as much as possible a delimited surface in order to plot it in 2D or 3D. This is equivalent to the multi-line surveys already described in this document but using the long/lat coordinates rather than regularly-spaced row and columns. This type of survey requires a gridding process to spread the reading points over a regular grid. This process is provided by programs like Surfer using various gridding algorithms.

7. Upgrading PPM Firmware

The MarkIV application code now allows to distribute new firmware versions through the Web and allows the end-users to install this firmware on their own system themselves.

The new firmware is made available to the selected users through encrypted binary files and distributed through file attachments on EMail messages directly from the factory or from other distributors.

The firmware files to be loaded in the PPM controller have an extension 'FWP'. This file should first be copied on the SD Card. Then, power is applied WHILE DEPRESSING BOTH LEFT AND RIGHT keys of the keypad.

This power up sequence will trigger the firmware upgrade process. The name of the files with extension 'FWP' will successively be listed (by the DOWN key) and the latest one will be loaded (by the OK key).

At the end of the load, a reset is triggered and the new firmware version is activated.