Q. What is the Diurnal correction?

 

A. The Earth’s magnetic field is always changing. This is due primarily to the Sun’s ejection of ionized particles that interact with the Earth’s magnetic field and atmosphere which causes a compression of magnetic field lines and atmospheric currents that give rise to variable magnetic fields. These temporal changes are generally considered noise when doing a magnetic field survey and it must be removed from your mobile magnetometer data before total field data can be processed and interpreted. The removal of the diurnal is completed in post processing where the accurately time stamped base station (stationary magnetometer) data is subtracted from the survey data.

 

Using a proton precession magnetometer as a base station will tend to leave gaps of time, as they typically cannot reliable measure the magnetic field any faster than one reading every 5 seconds due to the need for a polarization and relaxation period to make the measurement. The base station data is interpolated between measured data points to approximate what the field value is between readings so as to more appropriately remove the diurnal from survey readings that are not the exact time as the base station data.

 

Cesium base stations allow for base station readings of 10Hz or even more, which can be time synched using a GPS to ensure that the base station and mobile magnetometers are measuring at the exact same point in time to more accurately remove the diurnal variation. This high degree of synchronization can help to remove more subtle temporal changes in the Earth’s magnetic field.

 

Q. What is the K index?

 

The K index is a quasi-logarithmic scale to quantify the disturbances seen in the horizontal component of the Earth’s magnetic field. This is primarily associated with geomagnetic storms which cause abrupt and significant changes in the background magnetic field in a short period of time. It is unadvisable to survey during a severe geomagnetic storm, especially when not using a base station or using a base station that is not appropriately time disciplined, as the production data will be affected by the sudden changes in the background field. The K index ranges from 1 (being calm) to 9 (very severe geomagnetic storms start around 5).

 

Check nearby observatories and online resources for geomagnetic storm activity before preparing to survey. Magnetic storms are fairly regular so they can be predicted a few days in advance.

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Q. How long should my survey lines be?

 

A. For mineral surveys, your line length should extend at least 3-5 times the estimated depth of the target of interest on either side. This should allow you to safely extend past the magnetic effects of the feature so that you can properly assess the relative extent and location of the geological body. So, for example, if your interested body is 1km deep, you should have 6-10km long lines to be sure that you have fully recorded the target. For UXO and near surface features (archaeology, pipelines, cables), the survey lines should extend another 50% of the zone of interest so that you are sure that you have completely identified the targets of interest. For example, if a shipwreck is expected to cover an area of 1 sq. km, you should have survey lines of at least 2km long.

 

Q. What line spacing should I have?

 

A. This is again a function of what sort of feature you are looking to measure and their depth. It is most important to have your line spacing close enough together to see the magnetic effects of the target over several lines for proper location and extent analysis. For UXO surveys, a line spacing of 1-2 meters may be appropriate to fully cover the survey area and locate small 20mm targets. The standard line spacing for archaeology is 50 cm. When surveying with a horizontal array of magnetometers you can use each sensor to cover multiple lines allowing you to cover the same amount of area in a shorter period of time. Deep geological surveys could have line spacing of 100m or more depending on the depth and extent of the geological body of interest.

 

Q. What should my sample distance be?

 

A. Sample distance is less important when a magnetometer samples continuously at a fixed sample rate, such as 10Hz (10 samples per second). A normal walking pace is about 3 mph (or 1.3 meters per second). So, sampling at 10 Hz will provide about 10 samples per meter. This sample speed and distance is close enough for most magnetometer surveys of small objects. The larger the object is, the fewer samples needed per meter to ensure the entire object is measured. For magnetometers that do not sample continuously, like Proton Precession magnetometers, your sampling distance should be as small as the smallest object of interest. For archaeology surveys, a common sample distance is 10cm. Mineral surveys do not require very close sample spacing because the targets of interest are deep geological features that cover a large area. A 20-100m sampling distance is common for mineral surveys.

 

Q. What is instrument noise?

 

A. Noise by definition is any variation in the measurement not caused by actual variations in the external magnetic field. Instrument noise, or system noise, is the variation in measurement caused by the instrument itself. It also represents the lower limit of the noise in the data; i.e. it is the smallest change in the magnetic field that the magnetometer can measure. Noise and sensitivity often are used interchangeably. It’s important to note that instrument noise does not take into account noise from other sources such as AC electrical fields, mobile platform effects, or Earth’s diurnal field variation. Noise is normally specified as the number of nanoTeslas (10-9T) per square root Hertz (nT/√Hz). Hertz refers to the sample rate. For example a sample rate of 10Hz means that measurements are being taken 10 times a second. Increasing the sample rate by a factor of 4 will increase the noise in the data by a factor of 2 (noise goes up as the square root of the sampling rate increase).

  

Q. How important is absolute accuracy to a survey?

 

A. Absolute accuracy, also referred to as absolute error, is defined as the difference between the average of the readings of the magnetometer and the average of the field it measures. It tells us how closely the measured magnetic field value approaches the real magnetic field value. However, absolute drift is more important than absolute error. Absolute Drift is the change in absolute accuracy with time. All magnetometers will have some absolute error and drift in their measurements. In most cases, the drift will be much less than the absolute error. For most surveys, a drift of less than 0.1 nT per day is acceptable. Values greater than this can cause artifacts in surveys such as survey block edge effects. Because other survey errors (positional accuracies, heading errors due to internal instrument noise and mobile platform effects, diurnal Earth’s field variations) usually dominate and are cumulative, absolute accuracies in the range of 2 to 4 nT are quite acceptable. Absolute accuracy becomes more important when a magnetometer is being used as a primary standard or in an observatory situation (base station).

 

Q. Why use GPS locations for my survey?

 

A. Combining GPS locations with magnetic field measurements helps you locate targets more easily. For surveys covering very large areas, it is recommended to use a GPS. For marine and airborne surveys, GPS is critical to keeping track of your positions as you can’t simply place markers on the ground to keep you on track as you could during land surveys.

 

On the other hand, if a survey covers a relatively small area than your average GPS will not have the positional accuracy needed to locate small objects within that small grid. For example, archeology surveys are often done over small grids (for example, 20m by 20 m), which are later combined in post-processing to see the big picture of the entire survey area.  In this case, one can simply place markers on the ground and follow the markers to stay on your survey line and within the survey grid.