The instrument described in this document is a general purpose Proton Precession Magnetometer, intended for use in archaeological or geological survey work. Editing an ASCII-coded file on a SD-card changes the instrument configuration. Measurement data taken in the field is stored in ASCII code as one line per measurement on the same SD-card. Its DOS format and file structure is directly readable on a PC.

The system is also supplied with a program which runs under Windows (Post-processor) to subsequently process the captured field data stored on the SD-card and to plot the results with colored contour in 2D.

· The instrument can be configured to operate as a mobile Real-Time Measuring Station: this allows the display of a graphical bar chart showing the field gradients while recording the data on a file on the SD Card.

· The instrument will also operate as a fixed Data Logger, which allows the recording of magnetometer data at fixed pre-programmed intervals.

· The instrument can be configured to operate as a Differential Magnetometer where there is a single mobile instrument and a second fixed instrument located nearby. Measurements are made simultaneously with the two instruments; this allows one to compensate for the large-scale field variations caused by various geomagnetic processes generated by modern human activities.

· It is also possible to use two instruments as a Gradiometer. In this mode, the two instruments are operated simultaneously but with the sensors set at a fixed distance from each other. Thus, the difference between the two captured values may be expressed in nT/m and truly measures the gradient of the magnetic field at the location of the two sensors.

The instrument has a built-in mode designed to make a series of measurements over the surface of a rectangular survey area.

When operated as a differential magnetometer or as a Gradiometer, data are stored separately in the two parts of the instrument (Base Station and Survey Station). In these cases, additional time markers are embedded in each data record to allow a post-processing program to make the subsequent time alignment of the separate data files after upload to a PC.

1.1. Instrument Types

This PPM instrument system can be dynamically configured as the following types:

Real-Time Graphical Station: Geophysical mobile instrument with automatic polarizations regularly made at configurable data capture intervals. The magnetic field gradient values are displayed as a graphical bar chart in real-time and data captured on an SD-card file.

Single Station data logger Geophysical instrument with automatic polarizations at a configurable data capture intervals. All the data values are captured on an SD-card file with a time stamp accurate to 0.1 second.

Differential magnetometer (or Gradiometer) This mode assumes that two separate instruments are being used. In the case of the differential magnetometer, the Base Station is put at a fixed location while, in the case of a gradiometer; it is carried with its sensor set at a fixed distance from the sensor of the survey instrument.

The two instruments play the following distinct roles:

· The survey instrument captures its measurements throughout the grid points under control of an operator and stores the results on its local SD-card storage.

· The Base Station triggers its measurement repetitively at a configurable time interval and also stores the results on its local SD-card storage

On both stations, each measurement is stored together with a Time Stamp generated from their RTC (real time clock). After uploading both survey grids to a PC, a post-processing program merges the two data streams based on their time stamps and calculates the difference between the B fields captured at the same time during the survey session.

1.2. Introduction to the Instrument Operations

1.2.1. Real-time Graphical Instrument

This is a geophysical magnetic field measurement mode in which the measurements, in real-time, are shown as a shifting graphical bar chart. This mode may be used to locate buried ferrous objects like barrels, pipes, cisterns, various types of tanks or metallic meteorites but also ferrous objects from sunken ships like cannons, anchors, cannon balls, etc. Those objects generate such high field gradients that their large signature is not obscured by the background geomagnetic variations of a few nT’s. This mode is also useful in locating magnetite ore known to be the potential indicators of gold veins.

The instrument starts its repetitive measurements when a ‘Start Session’ menu command is entered by the operator. During its operation, the measurement results are displayed in real-time on its LCD as a bar chart whose scale can be dynamically adjusted. This chart shifts its bars to show the 20 latest field gradients with bars of maximum 32 pixels high. The repetitive measurements are stopped with a menu key.

1.2.2. Single Station Data Logger

This is a geophysical magnetic field measurement mode for observing the external field variations over long-term periods. The interval between consecutive captures is configurable in the system set-up (see ‘Set Repeat Interval’ command). It can vary from one second to a few minutes depending on the goal of the data analysis.

The station starts its automatic data capture when the operator enters a ‘Start Session’ menu command. During its operation, the measurement results are displayed in real-time on its LCD. The repetitive data capture is stopped with a menu key.

The Single Station stores its data on a local SD-card storage to be uploaded later onto long-term storage, for example, a PC file.

1.2.3. Differential Magnetometer

This is a PPM system made of two separate stations running independently. Time stamps generated by their local RTC synchronize the measurement results of both stations. The two RTC’s are synchronized by entering a ‘Start Session’ menu command on both stations at the same time before starting the survey.

1.2.3.1. Survey Station

The station starts its automatic data capture when the operator enters a ‘Start Session’ menu command and after pushing on the RESUME key. During its operation, the measurement results are captured, along with signal quality assessments, displayed in real-time on its LCD and stored on the local SD-card storage together with a time stamp. The repetitive data capture is permanently stopped or temporarily paused with a menu key.

The survey is made over a grid, one row at a time, and at least one measure is taken at each point of the row. The operator indicates new rows through a menu command.

The survey session is terminated by a specific command on the menu system.

1.2.3.2. Base Station

After the session has been started, this station immediately starts its automatic data capture. During its operation, the measurement results are displayed in real-time on its LCD. It stops data capture with a menu key.

1.2.4. Gradiometer

This is a PPM system made of two separate stations. An operator carrying both the Survey Station and the Base Station synchronizes the operations of the two stations. The two sensors are fixed together and separated by a fixed distance. This configuration is named a ‘Gradiometer’ because the difference between the B fields captured by each station is a real field gradient that may be expressed in nT/m.

1.2.4.1. Survey Station

Same as in 1.2.2.1.

1.2.4.2. Base Station

Same as in 1.2.2.2.

2. Survey Procedures

A survey must always start with a ’Start Session’ Command. This command saves the current RTC time to be used as time offset in the grid store.

· Bargraph Display mode or Real-Time Graphical Instrument

This mode is used for random walk type of surveys where there exists no other information about the whereabouts of the targets. When the audible alarm sounds, the location is marked on the ground with a flag and the survey goes on. After that, the precise location of the marked spots are defined by coming back to those spots and make a 90° crossed track over each flag. This will give the precise location of the target.

· The measurements start just after the ‘Start Session’ command and automatically repeat according to a pre-programmed ‘repeat interval’ that is specified in milliseconds.

· At the end of each measurement cycle,

1. The time-based gradient value is calculated as the difference between the last B value and the average B value of the real-time session taken as a reference.

2. This gradient value is appended at the end of the current bar chart and shifts all the previous values to the left.

3. A gradient threshold may be specified and, if it is exceeded, the instrument may be programmed to give an audible alarm.

4. The results are stored

· The operator may clear the reference B field value and the bar chart with the DOWN key.

· The LEFT/RIGHT keys are used to increase/decrease the scale of the bar chart. Each depression of the RIGHT key divides by 2 the value of a step in the bars (a pixel high) while depressing the LEFT key multiplies this step by 2. The gradient threshold is also updated to become 8 times the updated scale.

· The operator terminates the survey session with the OK key.

· Multi-line Survey Station:

This mode of survey is used when it is possible to define a limited rectangular grid over an area where potential targets can be detected.

· The session starts just after the ‘Start Session’ command but the measurements only start after pushing a first time on the OK key. It automatically repeats according to a programmed ‘repeat interval’ in milliseconds. At the end of each measurement cycle,

1. The magnitude and absolute B values are displayed,

2. The various signal quality thresholds are verified with possible audible alarm.

3. The results are stored at the currently displayed row of the grid

· The operator can stop/resume the automatic polarization cycles with the OK key. Resuming the cycles automatically increments the row number by one.

· The operator can pause/resume the automatic polarization cycles with the LEFT key without incrementing the row number.

· The operator switches to the bargraph display mode with the UP key and switches back to the survey station mode with the UP key again.

· DOWN key displays the current grid name for one second.

· UP key while automatic polarizations is stopped or paused = Ends Survey Session and returns to upper level menu. This key is only effective if the OK or LEFT key had stopped the automatic polarizations before.

· Base or Single line stations:

· The measurements start just after the ‘Start Session’ command and automatically repeat according to a programmed ‘repeat interval’ in milliseconds. In these modes, with no operator there, the alarms are still active (sounded and displayed) and the storage of the measurements are automatic and continuous.

· The operator switches to the bargraph display mode with the UP key and switches back to the single line station mode with the UP key again.

· The operator terminates the survey session with the OK key while the system is in pause.

3. User Interface for the BASIC version of PPM-MarkIV

The operation of the station is controlled by a command menu structure. The menu is displayed on an alphanumeric 20x4 LCD display and the function selection is controlled using a 5 key multifunction keypad or a small directional joystick.

3.4. Menu Structure

When the menu is displayed, the first LCD 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.

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 post-processed grid file (GGxxxxxx.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.

3.4.1. Individual Menu Entries

3.4.2. Start Session

Saves the current RTC time stamp value to be used as offset in order to synchronize the time stamps between two stations and generates a grid file header on the SD-card. If a grid file was already found, its data will be erased and a new grid file is created.

The first line of the recording of each survey session is a header as follows:

‘SESSION:GDddmmxx,ddd,pppp,g.g,bbbbb,pppp CR/LF’

with GDddmmxx = Grid file ID

with ddd = measurement delay (msec) used during the whole session

with pppp = polarization duration (msec) used during the whole session

with g.g = amplifier chain gain

with bbbbb = Start B field used to set the f0 of the digital band-pass filter

with pppp = polarization period in msec

· Single Station

o Immediately starts repetitive polarizations, measurements and storage.

o Sequentially writes results as an ASCII line on the current grid file on the SD-card.

o OK key = Ends Survey Session and returns to upper level menu.

o UP key = switches back and forth between Survey station mode of display and Real-time Graphical display mode.

· Real-Time Graphical mode

o Immediately starts repetitive polarizations, measurements and storage.

o Sequentially writes results as an ASCII line on the current grid file on the SD-card.

o OK key = Ends Survey Session and returns to upper level menu.

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

· RIGHT key to zoom in the bars (scale/2) and set the gradient warning threshold to 8*scale.

· If Gradient of the rightmost (latest) reading point of the barchart goes over the gradient warning threshold, then an aural alarm is started.

· Base Station

o Immediately start repetitive polarizations, measurements and storage. However, the system is initialized in pause mode.

o Sequentially writes results as an ASCII line on the current grid file on the SD-card.

· OK key = Resumes/Pauses automatic polarizations.

o UP key while Paused = Ends Survey Session and returns to upper level menu.

· Survey Station

o Immediately start repetitive polarizations, measurements and storage. However, the system is initialized in pause mode.

o Records the current Row

o Writes results as an ASCII line on the current grid file on the SD-card.

o The row of the current point is displayed on the LCD. It is the row where the next measurement will be stored in the survey grid.

o DOWN key displays the current grid name for one second.

o OK key = Resumes/Stops automatic polarizations. The operator increases the current row with the RIGHT key. The next measurement results will be stored on this new row.

o LEFT key = Resumes/Pauses automatic polarizations without incrementing the row number.

o UP key while automatic polarization = switches back and forth between Survey station mode of display and Real-time Graphical display mode.

3.4.3. Grid Control

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)

Review Grid

· 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)

· 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