Introduction

GSMP-30 is a high sensitivity potassium magnetometer and gradiometer system, providing unmatched sensitivity and absolute accuracy, in addition to near zero orientation or heading error effect.

The GSMP-30 operates on principles similar to other alkali vapour magnetometers such as the Cesium magnetometer, but benefits from the unique nuclear properties of Potassium.

The unmatched sensitivity of GSMP-30 systems, allow the magnetometers and gradiometers to be used for applications that demand more sensitivity than other alkali vapour systems can deliver.

Features

Some of the important benefits of the GSMP-30 optically pumped magnetometer and gradiometer systems include:

• almost no orientation errors
• highest sensitivity available in a portable magnetometer
• best absolute accuracy
• fast start-up and recovery - only seconds for the GSMP-30 to re-establish oscillator lock after an orientation change from a dead zone into the active zone
• fast tracking - the GSMP-30 measures precise Larmor frequency changes in response to field changes of several thousand nT within a sample period

GSMP-30 Advantages

• insensitivity to orientation - virtually zero heading error result from the nuclear properties of potassium
• sensitivity - inherent potential sensitivity results from the nuclear properties of potassium. The very narrow spectral lines of potassium provide sensitivities that are the highest among all optically pumped magnetometers
• absolute accuracy - the GSMP-30’s unmatched absolute accuracy (a variance of only ± 0.1 nT between any GSMP-30 sensors) makes this sensor the best choice for all gradiometer installations. Cesium's broad spectral lines severely limit the determination of the absolute accuracy of these systems
• no regular calibrations, GSMP-30 sensors do not require periodic calibration as the relative position of the RF exciting coil and the optical axis, does not impact performance

Because of Cesium’s spectral line configuration, the relative position of the RF coil and optical axis is critical, impacting performance. Mechanical vibrations alter the relative positions, resulting in the need for regular calibrations or balancing of the sensor.

GSMP-30 console

GSMP-30 Orientation Indifference Advantage

Variations in heading or spin error1 responses between individual sensors, lead to errors in magnetic gradient measurements of the same order of magnitude as gradients of interest. This fault is manifested in magnetometer systems that feature Cesium magnetometer sensor technology.

The GSMP-30 sensor, with virtually no orientation or heading error, does not introduce orientation "noise" into the magnetic or magnetic gradient measurement and as a result, high system sensitivity can be realized in both dynamic and static environments.

Applications

The long term stability, high sensitivity and fast response to the changing magnetic field make the GSMP-30 ideal for a wide variety of applications, such as:

• Ferrous ordnance location - for the detection of ordnance and mines by portable methods
• Ground portable magnetic and magnetic gradient surveying - for archaeological searches, engineering applications for detection of buried drums containing hazardous wastes and mineral and oil exploration
• Base station magnetic monitoring for observing diurnal magnetic activity and disturbances

GSMP-30 System Setup

A GSMP-30 magnetometer system is comprised of:

• staff mounted sensor and RF drive electronics / pre amplifier assembly
• data acquisition / display console
• 24 VDC battery belt and harness

GSMP-30 Sensor Head

The lightweight (less than 2.9 lbs) GSMP-30 sensor head houses the electro-optical detection system. All components of the sensor head, including the outside plastic housing, are made of carefully screened, nonmagnetic materials. The compact plastic housing has a diameter of 3.5" and a length of 6.0", allowing the GSMP-30 sensor to adapt seamlessly into existing airborne, marine, portable and base station installations. The electro-optical detection assembly includes a potassium lamp and absorption cell and photodetector, all housed in an insulated plastic housing to reduce warm-up time and power consumption, during survey operations.

GSMP-30 High Sensitivity Potassium Magnetometer Sensor Assembly

GSMP-30 Data Acquisition Console

The GSMP-30 console is equipped with a graphic display and a 16 key alphanumeric keyboard. The graphic display is an 8 line (30 characters/line) reflective monochrome LCD that can also display 30 x 8 characters. The console contains the magnetometer's Larmor signal processing electronics, in addition to providing data storage and retrieval facilities and functionality to graphically review data in real time and from system memory. All of the GSMP-30 system features, including optional, advanced navigation features, are accessible through an easy to use, interactive menu system.

Universal Gradiometer Staff

The GSMP-30G is supplied with a universal staff (patent pending) that is uniquely constructed, permitting the measurement of both vertical and horizontal magnetic gradients, through the use of the same apparatus. The staff's unique construction assures that the angle between the magnetic field vector and the sensor(s)'s optical axis is constant, regardless of the sensor's elevations above the ground. This important feature, reduces the problems of "drop outs" that plague other alkali vapour magnetometer systems.

Optional GPS Navigation Subsystem

In addition to providing support for a variety of OEM GPS receivers integrated into the GSMP-30 console, a GSMP-30 can accept data from other GPS receivers in NMEA format, a standard which virtually all receivers support.

All of the survey navigation features provided with the integrated OEM GPS receiver option are supported with external GSP receivers as well.

An important benefit of the OEM GPS receiver is that the magnetic interference generated by the GPS subsystems is shielded from the sensitive magnetometer sensors, by the GSMP-30 chassis.

Some of the GSMP-30 navigation features include:

• real-time coordinate transformation to UTM and local X-Y coordinate rotations
• survey "lane" guidance , with cross-track display and audio indicator
• user selectable (and user defined) datums, such as the commonly utilized WGS-84

Advanced GPS Integration Features

The GSMP-30 data acquisition console offers an unprecedented level of GPS integration, if the console is fitted with an optional 32.0 Mb of FLASH memory for logging GPS raw ranges, and supplied with optional firmware.

Similar to standard GSMP-30 firmware, GPS X-Y positions are captured and written along with each magnetometer or gradiometer record.

However, the advanced GSMP-30 GPS integration option provides the ability to write raw ranges to a separate file in GSMP-30 memory, for DGPS post processing.

GSMP-30 Sensor Specifications

• Sensitivity: 0.001 nT/ÖHz
• Absolute accuracy: ± 0.1 nT
• Dynamic Range: 10,000 to 100,000 nT
• Sample Rate: up to 20 readings per second, programmable
• Heading Error: less than 0.1 nT combined for sensor
• spins on all orientations from 10° to 80°

• Storage Temperature: -70°C to 60°C
• Operating Temperature: -40°C to 55°C

• Humidity: 0 to 100%, splashproof

Power Requirements

• Approx. 25 W at start up dropping to approx. 8 W after warm up
• Power Consumption: 8 Watts typical at 20°C
• Outputs: Continuous signals at the Larmor frequency which is proportional to the magnetic field

"Walking" Magnetometer / Gradiometer

Aegis Instruments pioneered the innovative "Walking" option that enables acquisition of nearly continuous data on survey lines. The "Walking" option is a standard feature of every GSMP-30. Similar to an airborne survey in principle, data is recorded at discrete time intervals (up to 20 readings per second) as the instrument travels along the line. At each major survey picket (fiducial), the operator touches a designated key. The Walking Magnetometer automatically assigns a linearly interpolated coordinate to all intervening readings. A main benefit of the Walking option is that the high sample density improves definition of geologic structures. And because the operator can record data on a near-continuous basis, the Walking Magnetometer increases survey efficiency and minimizes field expenditures -- especially for highly detailed ground-based surveys.

Optional Omnidirectional VLF

With omnidirectional VLF option, up to three stations of VLF data can be acquired without orienting. Moreover, the operator is able to record both magnetic and VLF data with a single stroke on the keypad.

• Frequency Range: 15 - 30.0 kHz
• Parameters Measured: Vertical in-phase & out-of-phase components as % of total field. 2 relative components of horizontal field. Absolute amplitude of total field
• Resolution: 0.1%
• Number of Stations: Up to 3 at a time
• Sensor Dimensions: 140 x 150 x 90 mm(5.5 x 6 x 3")
• Sensor Weight: 1.0 kg (2.2 lb)

Theory of Operation

A typical alkali vapour magnetometer consists of a glass cell containing the evaporated alkali metal, which is exposed to light of a specific wavelength.

According to quantum theory, in every population of alkali atoms, there is a set distribution of valence electrons between two energy levels, represented by 1 and 2 in the figure below.

A wavelength of light is applied to the cell to excite electrons only from level 2 to level 3. This is called polarization. As the number of atoms with electrons at level 2 decreases, the cell will stop absorbing light, and will turn from opaque to transparent.

Electrons at energy level 3 are not stable, and will spontaneously decay back to levels 1 and 2. Eventually, level 1 will become fully populated, and level 2 will become depopulated, and the cell will be completely transparent. That’s where RF depolarization comes into play. RF power of a wavelength that corresponds to the energy difference between levels 1 and 2 is applied to the cell to move electrons from level 1 back to level 2.

The significance of this act is that the energy difference between levels 1 and 2 (the frequency of the RF depolarizing field) is proportional to the magnetic field.

The entire weight of the GSMP-30G staff & sensor assembly
is suspended by 2 attachments to the backpack