One woman's quest for geological understanding!

Geophysics training: (time-lapse) micro-gravity survey along the Marston canal, Northwich, Cheshire

Subsidence as a result of mining in the UK (and abroad) has occurred throughout history. This varies in severity, with evidence of subsidence ranging from topographic depressions to catastrophic surface collapse. In Northwich, and the surrounding areas, the long history of Salt mining and brine pumping has taken its toll. In order to detect / characterise / monitor areas (susceptible) of subsidence, a combination of remote sensing and near-surface geophysics can be applied.

Jamie Pringle and his team at Keele university have conducted much research and monitoring around the Marston Canal over more than 20 years (Pringle, Styles et al. 2012)

“In the village of Marston, the Trent and Mersey Canal crosses several abandoned salt mine workings and previously subsiding areas, the canal being breached by a catastrophic subsidence event in 1953. This canal section is the focus of a long-term monitoring study by conventional geotechnical topographic and microgravity surveys. Results of 20 years of topographic time-lapse surveys indicate specific areas of local subsidence that could not be predicted by available site and mine abandonment plan and shaft data. Subsidence has subsequently necessitated four phases of temporary canal bank remediation. Ten years of microgravity time-lapse data have recorded major deepening negative anomalies in specific sections that correlate with topographic data. Gravity 2D modelling using available site data found upwardly propagating voids, and associated collapse material produced a good match with observed microgravity data. Intrusive investigations have confirmed a void at the major anomaly. The advantages of undertaking such long-term studies for near-surface geophysicists, geotechnical engineers, and researchers working in other application areas are discussed.” (Pringle, Styles et al. 2012)

The fieldwork which was conducted, is a continuation of Jamie’s research.

Fieldwork

Fieldwork was conducted 27/7/15 & 28/7/15 along the Marston Canal, in the Survey area indicated in Figure 1.

Figure 1. Location map of survey area: Marston, Northwich.

 

Aim

Monitor and measure relict salt mines using geophysical methods

Objectives

  • Become familiar with measuring micro-gravity geophysical data using a Scintrex CG-5 micro-gravity meter, Leica Pinpoint R100 and Prism and pole.
  • Survey data collection area, where gravity points are taken
  • Conduct a survey

Methodology

At the beginning, and end, of both days, readings were taken using the Scintrex CG-5 micro-gravity meter at the base station (See Fig. 1). This is important, as all data collected must be corrected for Instrumental Drift (see below). The base station has been chosen because it sits on an Ordnance Survey reference point (bench mark). These are marks made by the Ordnance Survey to record height above Ordnance Datum. They are often found on buildings or other semi-permanent features, e.g. St Helen’s Church, Northwich.

Three cycles of readings, of 90 seconds each per station, were taken for data reliability. Base station readings consisted of 2 sets of 3 cycles, in rapid succession each time, to further ensure reliability.

Then readings were taken along the survey area (Indicated in Fig 1).

In total, and over the two days, 62 stations were surveyed, along the Marston canal, using the microgravity meter. In order to ensure consistency, the 1st station commenced at a specific point along the canal (a concrete block on the edge of the tow path, where before there was none) and stations were marked (with biodegradable spray paint), and readings taken, at regular intervals thereafter:

• Stations 1-17 were spaced at 20m intervals
• Stations 18-51 were spaced at 5m intervals
• Stations 52-32 were spaced at 20m intervals

Station Readings Station Readings

Umbrella

 

 

 

 

 

If there was any doubt about the readings at a particular station, i.e. the mGal figures were “off” or the readings were disturbed by something (weather / pedestrians), then they were delayed and/or repeated. A golf umbrella was used to protect the equipment against such elements.

10% of the microgravity stations were revisited and readings taken to assess data quality and repeatability.

All stations were then surveyed using a Leica Pinpoint R100 and Prism and pole (AKA “Gandalf’s Staff” LOL) in order to determine their absolute positions for the surveys and the data elevation corrections in microgravity data post processing. Due to the canal bend, several positions were used in order to cover the full length of the survey area. A note of changes required in the pole height (which had to be altered to clear canal boats/bridges) was also made.

Accuracy and reliability of data is of paramount importance when conducting a geophysical survey. I particularly like the following quote:

As Oscar Wilde might have said (had he opted for a career in field geophysics), to spend a few hours recording rubbish might be accounted a misfortune. To spend anything more than a day doing so looks suspiciously like carelessness” (Milsom and Eriksen, 2011)

Geological Notebook

In order to ensure that no data is lost, e.g. by failure of the meter to record, notes of all readings, in addition to any associated notes (e.g., canal boat /cyclist etc. passing / change in weather) were made.

The information was recorded as follows:

Date: 27/7/15
Station Number: 31

Reading Number Reading in microgals (µGal) Standard Deviation (SD or σ) Time of reading
3 7134.468 0.083 14:13
2 7134.470 0.085 14:11
1 7134.462 0.088 14:09

The next steps?

All data collected must be corrected for Instrumental Drift (Reynolds 2011):

  1. Tides : The effects of the Earth Tides (which account for a magnitude of up to 250 microgals (µGal)), will need to be removed and the data corrected based upon the latitude and longitude of the site the precise time at which the gravity observation was made.
  2. Free-air anomaly / Free-air gravity – the quantity obtained after applying:
    • Latitude correction: observed or absolute gravity MINUS normal gravity (calculated from the International Gravity Formula)
    • Free-air correction: The remainder left after latitude correction is partially due to the height of the gravity station above the sea-level (SL) reference surface. The greater the height above SL, the greater the distance from the Earth’s centre of mass, therefore the effect is negative (and therefore the air correction is positive) for stations, like those along the Marston Canal, which are above SL.
  3. Bouguer anomaly: a gravity anomaly corrected for the height at which it is measured and the attraction of terrain. The height correction alone gives a free-air gravity anomaly. The Bouguer effect is positive (and therefore the Bouguer correction is negative).

After these steps have been completed, the data will be calibrated with past data to determine any changes in the density/movement of the underlying ground over time:

  1. The absolute x, y positions must be established to in order to ensure that the survey sample positions are consistent with past data
  2. Identify and differences between previous surveys conducted and quantify any changes

Further research to be undertaken

  • I have access to a canal boat, and plan to take depth readings along the Survey area (Fig. 1) at the same points as gravity readings were taken. Given that it is impossible to completely halt a canal boat, a more instantaneous reading of depth may be obtained by using a hand held sonar device, although this is to be tested by comparing measurements (n=20) in a stationary setting along the canal. The Sonar device only has an accuracy of .1m, however I feel that measurements taken with rope and weight, or even a stick, would be equally inaccurate, if not more so, due to the boat movements. This will take place during the second week in September.
  • Undertake conductivity tests of the water at each of the base points, in order to establish any changes in salinity – planned for the second week of September.
  • Obtain and digitalise mine plans for the area and on water tables / change in water tables in the area.

Follow this research, and more on Twitter:

  • @KeeleGeophysics : Applied & Environmental Geophysics at Keele University
  • @KeeleGeology : Geology and Geoscience at Keele University, Staffordshire, United Kingdom.
  • @DaisysGeology : Student and lover of Earth Sciences and related topics (PhD student, Keele).

Published Article Links:

A copy of Jamie’s Paper, Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K (Pringle et al. 2012) can be found here.

References

  • Milsom, J. and A. Eriksen (2011). Field Geophysics, Wiley.
  • Pringle, J. K., et al. (2012). “Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K.” GEOPHYSICS 77(6): B287-B294.  DOI:10.1190/GEO2011-0491.1
  • Reynolds, J. M. (2011). An Introduction to Applied and Environmental Geophysics, Wiley.

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