Introduction

The GSM-19 version 6.0 is an upgraded, high sensitivity Overhauser magnetometer and gradiometer system, providing improved sensitivity and increased memory capacity. Core improvements include a new RISC microprocessor with a 32-bit internal address bus, memory expansion and a GPS engine with optional real-time and/or post processing differential correction. A navigation option consists of lane guidance with programmable lane width, an automatic end of line flag and guidance to the start of the next line. 

The GSM-19 v6.0 has the internal capacity to store a large amount of data and to rapidly transfer that data from field instruments to computer making more time available for the data reduction tasks.

GSM-19 v6.0 Total Field and Stationary Vertical Gradient showing the gradient largely unaffected by diurnal variation

Features

Some of the important benefits of the GSM-19 v6.0 Overhauser magnetometer and gradiometer systems include:

• integrated GPS and real time navigation
• improved sensitivity from 0.02 nT to 0.015 nT/ ÖHz with the same gradient tolerance of 10,000 nT/meter.
• better resolution and absolute accuracy
• up to 16 times more memory (32 Mbytes)
• high speed digital data link and up to four analog outputs available.

	Two GSM-19 Overhauser Magnetometers synchronized with Reduction/Synchronization Cable

GSM-19 v6.0 Advantages

• Sensitivity - based on a new signal processing algorithm, the sensitivity of the measurements has been improved by 25 percent from previous versions. This advantage will increase resilience against background noise, producing better data and faster operation.

• Integrated GPS and real time navigation - we have reached a milestone with real time navigation in ground geophysical survey instrumentation. The effect of this option will increase productivity and, in many cases, lower the cost of establishing expensive grid systems.

• Resolution and absolute accuracy - an order of magnitude improvement in the resolution of precession frequency (from 0.01 to 0.001 nT) improves the quality and repeatability of the measurements. As well, the improved absolute accuracy (a variance of only ± 0.1 nT between any GSM-19 V6.0 sensors) makes this model an ideal choice for gradiometer installations.

• Increased memory capacity - the internal storage capacity of the GSM-19 v6.0 with minimum memory (4 Mbytes) using a standard magnetometer configuration recording time, X&Y and field value, is approximately 262,000 readings. With optional increments, the storage capacity can be increased to over 2 million readings (32 Mbytes).

• Digital and analog data links - a high speed RS232 data link (up to 115200 bps) provides for fast downloading of digital data and up to four analog outputs can be accessed, each with 1000 voltage steps for fine chart resolution. 

GSM-19 v6.0 Integrated GPS

We have tested the GPS board in the GSM-19 V6.0 in all survey modes. Provided both survey and base antennas are seeing an adequate number of satellites, traverse closures average about +/- 5.0 meters or less. A test on uncorrected GPS shows the station to station variation about a rolling mean average is shown in the plot below. This indication of precision of a stand alone system is improved substantially when subjected to the DGPS correction procedure.

A global positioning system (GPS) utilizes a constellation of 24 US satellites operating in 12 hour orbits at an altitude of approximately 20,000 Km. At any point on the Earth, there are from 6 to 11 satellites at 5 degrees or more above the horizon. A minimum of four satellites are required to solve a 3D fix (X,Y,Z and clock error). The "pseudo ranges" are calculated from the length of time signals take to travel from the satellite to the ground receiving station. Ephemeris data is also transmitted periodically to fine tune each satellite’s orbit so that the pseudo ranges can be adjusted when determining final corrections.

Because satellites use high frequency radio transmission (1.227 and 1.575 GHz) to send information, the signals can be blocked in the line of sight to the receiving antenna by heavy deciduous vegetation or solid objects for periods of time. This, together with atmospheric variations in radio transmission, normally require the data to be post processed with data from a fixed base station in the survey area. This is referred to as differential GPS or (DGPS) and we offer a choice of "real time" correction via a radio modem/beacon or by the post processing correction option. 

Showing station to station variance in position about a rolling mean for a square, closed loop traverse approximately 250 X 250 meters

Optional GPS Navigation Subsystem

In addition to providing support for a GPS receiver integrated into the GSM-19 v6.0 console, a GSM-19 v6.0 can store the data for subsequent post processing of differential GPS, thus allowing one man DGPS surveys. If the console is fitted with an additional 28.0 Mb of FLASH memory for logging GPS raw ranges, and supplied with optional firmware, the GSM-19 v6.0 data acquisition console offers an advanced level of GPS integration. Similar to standard GSM-19 v6.0 firmware, the GPS X-Y positions are captured and written along with each magnetometer or gradiometer record. However, the advanced GSM-19 GPS integration option provides the ability to write raw ranges to a separate file in GSM-19 v6.0 memory, for post processing. In order to post process the data, a C3NAV software and license are required and supplied as an option.

In addition to filing the raw range outputs of the GPS navigation subsystem, the GSM-19 v6.0 firmware provides the operator the ability to carry out special navigation tasks, including survey grid navigation in real time. Other data that the operator may be interested in, such as the health of the GPS constellation, current position and distance to the next waypoint or end of line is displayed in the GSM-19 v6.0’s navigation window.

Some of the GSM-19 v6.0 GPS 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

An important benefit of the OEM GPS receiver is its low magnetic signature and lower magnetic interference generated by the GPS subsystems.

Applications

The integrated GPS, higher sensitivity, larger memory capacity and fast response to a changing magnetic field make the GSM-19 v6.0 ideal for a wide variety of applications, such as:

• Surveying in remote locations with no grid system prepared - as an example, diamond exploration in the sub arctic.
• High productivity mineral and petroleum exploration -requiring a high standard of magnetic mapping.
• Ferrous ordnance location - for the detection of ordnance and mines using the survey mode and the GSM-19 v6.0 portability.
• Ground portable magnetic and magnetic gradient surveying - for archaeological searches, engineering applications for detection of buried drums containing hazardous wastes.
• Base station magnetic monitoring - for observing diurnal magnetic activity and disturbances with integrated GPS data.

GSM-19 v6.0 Sensor Head

The lightweight (less than 1.3 kg) GSM-19 v6.0 sensor head houses the Overhauser detection system. All components of the sensor head, including the outside plastic housing, are made of carefully screened, nonmagnetic materials. Optional omni-directional sensors are available for operating in regions where the magnetic fields are typically horizontal (equatorial regions). This option maintains the sensitivity specification and prevents loss of signal regardless of sensor-magnetic field geometry.

The compact plastic housing has a diameter of 71 mm and a length of 170 mm (omni-directional sensor 80 mm X 180 mm), allowing two GSM-19 v6.0 sensors to be mounted easily in a vertical gradiometer configuration. The detection assembly of the Overhauser sensor includes dual pickup coils that are connected in series opposition in order to suppress far source electrical interference such as telluric and atmospheric noise. They surround a hydrogen-rich liquid solvent with free electrons (free radicals) added which effectively increases the signal intensity under RF polarization. This assembly is housed in an rugged plastic housing to preserve the mechanism during survey operations and transportation.

GSM-19 v6.0 Data Acquisition Console

The GSM-19 v6.0 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 signal processing electronics, in addition to providing data storage and retrieval facilities and functionality to graphically review data in real time and from system

All of the GSM-19 v6.0 system functions, including optional advanced navigation features, are accessible through an easy to use, interactive menu system.

GEMLink Acquisition/Display software

GEMLink 6.0 is a Windows interactive interface supplied with the GSM-19 v6.0 magnetometer system console. It functions as the console's bi-directional RS-232 terminal. GEMLink 6.0 offers the user the option to save the instrument readings to a disk file, while displaying the incoming data in text, profile and map modes.

Backpack Walking Magnetometer/Gradiometer Support

The GSM-19 v6.0 "walking magnetometer" and "walking gradiometer" are supplied with an optional backpack supported sensor configuration that is uniquely constructed, permitting the measurement of the total field (one sensor) or vertical magnetic gradient (two sensors) while having both hands free to maintain balance or to operate the console during travels.

GSM-19 v6.0 Sensor Specifications

• Sensitivity: 0.015 nT/ÖHz
• Absolute accuracy: ± 0.1 nT
• Dynamic Range: 10,000 to 120,000 nT
• Gradient Tolerance: Over 10,000 nT/meter
• Maximum Sample Rate: 1 reading per 3 seconds (Standard); 2 readings/sec Walk option; 5 readings/sec, Fast option
• Console Weight: 2.1 kg
• Console Dimensions: 223 x 69 x 240 mm

Environmental

• Storage Temperature: -70°C to 60°C.
• Operating Temperature: -40°C to 60°C.
• Humidity: 0 to 100%, splash proof.
• Power Requirements: 12 V 2.2 Ah battery will operate continuously for 45 hours on standby.
• Power Consumption: 2 watt-seconds per reading typical at 20°C.
• Sensor Output: Sequential precession signals at frequencies which are proportional to the magnetic field.

"Walking" Magnetometer / Gradiometer

"Walking" option enables acquisition of nearly continuous data on survey lines. The "Walking" option is a popular feature of the GSM-19 v6.0. Similar to an airborne survey in principle, data is recorded at discrete time intervals (up to 5 readings per second) as the instrument is carried along the line. At each survey picket (fiducial), the operator touches a designated key. The Walking Magnetometer automatically assigns a picket coordinate to the last reading and linearly interpolates coordinates of all intervening readings during post processing.

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. 

Near-Continuous Surveys Improve definition of Magnetic Anomalies

Data: Courtesy Val D'or Geophysics Ltd.

Optional Omnidirectional VLF

With GSM-19 Systems’ omni-directional 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: 160x150x150 mm.
• Sensor Weight: 1.3 kg.

Overhauser Theory of Operation

In a typical proton magnetometer, current is passed through a coil wound around a sensor containing a hydrogen rich liquid. The auxiliary field created by the coil (>100 Gauss) polarizes the protons in the liquid which build up to a higher thermal equilibrium with the auxiliary magnetic field. The current and hence the field is abruptly terminated, allowing the polarized protons to precess in the Earth’s field. The scalar component of the Earth’s magnetic field is derived from the precession signal which decays exponentially and lasts till the protons return to steady state. The quality of the measurement can be derived from the signal amplitude and its decay characteristics and is averaged over the sampling period and recorded. Overhauser magnetometers, utilize a more efficient means of proton polarization by using electron-proton coupling and an electron (free radical atom) enhanced liquid to produce an order of magnitude stronger proton precession signals. The unbound electrons in the fluid can be easily and efficiently stimulated by exposure to RF magnetic field radiation that corresponds to a specific energy level transition. Instead of releasing this energy as emitted radiation, the unbound electrons transfer it to the protons in the solvent and the much larger resultant polarization produces signals with strong amplitudes. The RF field is transparent to the Earth’s "DC" magnetic field and the frequency is well out of the bandwidth of the precession signal. The sensor can be polarized in tandem with precession signal measurement making faster sequential measurements possible. In turn, this further enhances the potential for advanced statistical averaging over the sampling period and/or increasing the sampling speed. The proportionality of precession frequency and magnetic flux density is linear and is known to a high degree of absolute accuracy. The high sensitivity and cycling speed (up to 5 readings a second) and exceptionally low power consumption over a wide temperature range and low noise levels combine to make possible a superior magnetic field measuring device. Low field measurement with superior sensitivity near the magnetic equator, where fields as low as 20,000 nT are encountered, is accomplished by creating a small auxiliary magnetic flux density while polarizing.