FM256 Technical Support - Page 1

Page 1
What do I do if the gradiometer will not zero correctly on the 0.1 nT range?
How do I prevent defects that are visible in my data?
Why does tilting the gradiometer make the reading change?
Why does the displayed reading flicker?

Page 2
How do I download data if I do not have the latest Geoplot?
How do I download data if I only have an FM18/36 download cable?
What do I do if I am having difficulty downloading the data?
Why is the data scrambled?
What do I do if there is water in the FM256?
Why does the NiMH battery pack not hold its charge?
What can I do in an emergency if I cannot obtain the correct size memory backup battery?
What do I do if a Start-Stop button stops working?

What do I do if the gradiometer will not zero correctly on the 0.1 nT range?
This should not occur with an FM256 due to the increased range of the zero control. In the unlikely event of this occurance please see the FM9/18/36 technical support page.

How do I prevent defects that are visible in my data?
There are several defects that can arise in a fluxgate gradiometer survey data. These include : slope in the grid data, discontinuities at the edges of grids, banding, traverse striping, stagger errors and periodic errors. Most of these errors can be corrected for by using software process functions specifically tailored to tackle each problem, as provided in Geoplot 3. Since many of these errors have their origin in the way the instrument is carried by the operator, you can minimise their appearance in future surveys by getting an observer to be critical of your surveying stance. A good walking style will use the body as a stabilising system that maintains the gradiometer at a constant height above ground, keeps the sensor tube positioned vertically and in the same heading all the time.
Slope Errors
Slope errors in grid data show as a very small and slow drift in the average data value throughout a grid, leading to a small difference in the background levels between the first and last traverses. However, it is essentially constant during the time required to scan an individual traverse. You can use the FM36 Log Zero Drift correction facility to help reduce this effect. However, there is usually no need to use Log Zero Drift since the Zero Mean Traverse function, which will almost always be used in routine data processing automatically removes any slope in the data.
Grid Mismatch
Grid edge discontinuities can arise for a number of reasons:

  Poor choice of the zero reference station.
  Inconsistent positioning at the Zero Reference station.
  Failure to periodically check alignment, balance and re-zero the instrument.
  Traverse direction different from that adopted when zeroing the instrument at the Reference station.

Grid mismatch is usually present to some extent in most surveys since the zero reference station is rarely perfect. Zero Mean Grid and/or Zero Mean Traverse process functions can correct for this error.
This may be observed running at right angles to the traverse direction at the start and/or end of a grid. It arises due to the operator adopting a slightly different posture at the start and/or end of the traverse. Be sure to start walking at the start of a traverse at least a metre before the grid edge, and ensure that when the Start/Stop switch is pressed you are careful not to change posture at that point. Likewise, continue walking normally for at least a metre past the end point before relaxing to avoid a change of posture introducing an error signal. Techniques do exist for removing these errors though there are no straightforward process functions to achieve this - see 'Processing Techniques' in the Geoplot manual for further details.
Traverse Striping
Traverse striping is where alternate traverses have a slightly different background level. It shows up in graphics plots as a series of stripes orientated in the traverse direction, and is especially noticeable if the plotting parameters are set to look for very weak features. The error can occur for a number of reasons:

  Failure to maintain the instrument at the same height on alternate traverses.
  A tendency to twist the body slightly for each alternate traverse, resulting in a slight change of instrument   orientation.
  Tilting of the instrument forwards or backwards from vertical, which becomes compounded with zig-zag traverses and improper alignment.
  Poor choice of Alignment and Balance Station which leads to improper alignment and noticeable tilt errors.
  High walking speeds leading to poor angular control of the instrument.

The Zero Mean Traverse process function can be used to reduce this error in most cases.
Stagger Errors
Stagger defects arise due to the sensor tube not being directly adjacent to a guide marker when a 1m marker 'beep' is sounded. When using zig-zag traverses data will be slewed or displaced backwards or forwards for successive traverses. The result is that a linear anomaly running perpendicular to the traverse direction will not show as a clean linear response, but as a chevron type pattern, with the maximum of the response being displaced first forward and then backwards in each alternate traverse. It usually occurs when zig-zag traverses are made too fast and sample interval is small. The Destagger process function can be used to correct for these errors to some extent. However, it cannot fully compensate for poor marker alignment, and there will inevitably be some subsequent loss of signal in the correction process.
Periodic Errors
Periodic errors show up as a series of linear bands perpendicular to the traverse direction, with a period usually approximately equal to one or two walking paces (1 c/m or 0.5 c/m). They usually arise because the operator changes his stance or elevation slightly whenever the left or right foot is placed on the ground, or he launches himself forward for the next pace. It is more likely to be a problem if the speed of walking is high, the ground has a higher than normal magnetic susceptibility, the terrain is uneven, if the alignment of the gradiometer sensors are not checked often enough, or any combination of these factors. It is also most likely to be noticeable if you are setting plotting parameters to look at very weak responses. When it occurs the typical strength is 1nT but can be up to 5nT in extreme cases.
Other factors that can cause periodic errors include the following:

  A poor choice of Alignment and Balance Station, or failure to align and balance periodically, can cause angular motion of the gradiometer to introduce periodic errors.
  'Regimental' style of walking.
  'Bouncy' style of walking.
  High susceptibility soils and/or rough terrain, with mud sticking to boots causing an effect as each foot moves past the bottom sensor.
  The trimmer tool is slightly magnetic and should not be carried on the person, e.g. back pocket.
  Magnetic wellington boots - although usually non-magnetic some boots have been found to contain small quantities of ferrous material, sufficient to cause periodic errors.
  Magnetic clothing. Double check every piece of clothing for small items of metal by holding against a static gradiometer.
  Some coins are
extremely magnetic.
  Keys, wallets, credit cards etc carried in the pocket.

Why does tilting the gradiometer make the reading change?
When you tilt a gradiometer a small change in reading will occur even when properly aligned and balanced - this is known as a tilt error and will introduce in effect a small amount of noise to the observed or logged readings. Operators try to minimise this error by holding the gradiometer tube as vertical as possible. However, the magnitude of the tilt error is dependant on the direction or heading in which the gradiometer is pointed, with respect to magnetic north (heading direction is defined as the direction in which the electronics housing and handle axis is pointed). Since each instrument behaves differently in this respect there are unique optimum headings for each instrument that will minimise these tilt errors. You should ideally survey with the instrument pointing in one of these directions for lowest tilt error. Take care to remember that optimum heading direction refers to the direction in which the instrument is pointing - this may not necessarily be the same as the traverse direction, since it depends on the instrument holding technique you use.
The optimum heading will vary depending on your geomagnetic latitude. You can find the optimum headings at your latitude as follows:

  Align and zero the instrument normally over a high quality alignment and balance / zero reference station.
  Hold the instrument about 1m above the ground.
  Observe the shift in reading as the instrument is tilted about 10 degrees from vertical in all four directions, with it facing in turn North, South, East and West.
  The optimum directions are those that show the smallest change in reading.

Why does the displayed reading flicker?
Some flickering of the reading will be observed with 0.1 nT resolution in normal circumstances in the field. However, if flickering is greater than normal, say greater than 1 nT then this may be caused by external interference. On urban sites this is likely to be due to the magnetic fields from underground mains power earth currents, telephone earth return currents, or currents conducted along underground pipes. On rural sites interference can occur when operating in the vicinity of radar, radio or television transmitters or booster stations. Interference will increase as you get nearer the source, though it is difficult to give precise indications of the level of interference. As an example, you may observe fluctuations of about 3 nT at distances less than 50m from a radio booster station, though the distance and degree of fluctuation will vary from situation to situation. Some radar stations including those of air traffic control, ship harbour and military establishments can transmit at very large signal strengths, causing interference at up to 15-100 Km distances.
You can try to improve the signal to noise ratio by using digital averaging. If this does not effect a cure then either move further away from the interfering source or reduce the sensitivity of the gradiometer.
Overhead mains cables, suspended from pylons, do not in general cause any interference problems, though the pylons themselves, being made of steel will show as a very large response.
Note that the instrument is not designed for laboratory use so if you try to operate in such an environment you may observe interference in the form of flickering readings. This is usually due to the magnetic fields generated by mains operated devices, in particular, mains transformers such as those found in computers, televisions etc.

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