Pre-mining seafloor topography determination over the Phalen mine workings, Sydney Coalfield, Nova Scotia

Mining Science and Technology, 13 (1991) 25-35 Elsevier Science Publishers B.V., A m s t e r d a m

25

Pre-mining seafloor topography determination over the Phalen mine workings, Sydney Coalfield, Nova Scotia T.R.C. Aston

a, W.D.

Gallant

b, A.

Lapierre c and J. McG. Stewart c

a Canadian Mining Technology Laboratory, CANMET-MRL, Energy Mines and Resources Canada, 555 Booth Street, Ottawa, Ont. K1A OG1, Canada b Cape Breton Coal Research Laboratory, CANMET-CRL, Energy Mines and Resources Canada, 210 George Street, Sydney, N.S. B1P 1J3, Canada c McGregor Geoscience Ltd., P.O. Box 1604, Station M, Halifax, N.S. B3J 2Y3, Canada (Received October 24, 1990; accepted November 12, 1990)

ABSTRACT Aston, T.R.C., Gallant, W.D., Lapierre, A. and Stewart, J. McG., 1991. Pre-mining seafloor topography determination over the Phalen Mine workings, Sydney Coalfield, Nova Scotia. Min. Sci. Technol., 13: 25-35. In 1987, a detailed marine geophysics survey consisting of precision echosounder and sub-bottom profiling was undertaken to establish the pre-mining seafloor topography over the Phalen mine workings in the Sydney Coalfield, Nova Scotia. If the same survey lines and monitoring techniques are subsequently used to establish the post-mining seafloor topography, a comparison of the two data sets may allow an evaluation of the resultant seafloor subsidence profiles. After a brief review of the progress of the longwall seafloor subsidence monitoring program, the paper examines the marine survey undertaken to establish the pre-mining seafloor topography. In addition to outlining the instrumentation and survey techniques used, the data processing methodology is described and the survey results discussed.

Introduction

In 1987, an opportunity occurred to establish the pre-mining seafloor topography above the proposed workings of the new Phalen mine in the Sydney Coalfield, Nova Scotia (Figs. 1 and 2). A detailed marine geophysics survey was therefore undertaken using precision echosounder and sub-bottom seismic techniques [1]. Once the pre-mining seafloor topography is established, subsequent surveys using the same survey lines (or gridlines) and monitoring techniques can be undertaken to compare the pre- and post-mining topographies and evaluate the magnitude of the seafloor subsidence profiles present (if any). The current work represents part of a longer term program to establish site specific guidelines for

the undersea longwall workings in the Sydney Coalfield, Nova Scotia. After reviewing the progress of the seafloor subsidence monitoring program currently being undertaken in the Sydney Coalfield, the paper examines the pre-mining survey undertaken in August 1987. In addition to describing the equipment and survey techniques used, the data processing methodology is examined and the results discussed.

Longwail mining subsidence Ground m o v e m e n t

Subsidence above longwall workings in a flat seam is characterised by the formation of a "trough" at the surface which is symmetri-

26

T.R.C.

cal and centrally located over the axis of the panel (Fig. 3). The magnitude of the trough is determined by the mining panel depth, width and length as well as the extracted seam height and overlying geology. Theoretically, the subsidence trough for an isolated longwall panel in an inclined seam is similar to that for one in a horizontal seam except that the profile axis migrates down dip. The subsidence effects of individual panels becomes cumulative when they are superimposed on adjacent or overlying profiles and this results in the formation of much deeper troughs than those expected for single seam panels. The occurrence of adjacent longwall panels separated by pillars would result in the formation of a sinusoidal seafloor topography. ..........

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Current mining operations Currently, three mines operated by the Cape Breton Development Corporation (CBDC) are operating in the Sydney Coalfield: Lingan mine (single entry advance longwall) and the Prince and Phalen mines (single entry retreat longwall). Within the longwall operations, the extracted seam height varies between 1.5 and 3.0 m, with face lengths between 100 and 250 m at depths between 200 and 700 m. At Prince and Lingan, the mining is single seam, while at Phalen the workings are currently undermining the historical Lingan workings, Use of existing prediction methods suggests that between 0.4 and 1.7 m of seafloor subsidence may have developed over the existing mine workings, while at the Phalen mine between 0.5 and 2.0 m of seafloor subsidence is predicted.

Program development

Fig. 1. Location of Cape Breton and the Sydney Coalfield, N o v a Scotia.

In 1983, the CANMET Cape Breton Coal Research Laboratory (CBCRL) initiated a long-term research program to monitor actual seafloor subsidence profiles,produce effective

27

PRE-MINING SEAFLOOR DETERMINATION

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prediction techniques and establish safe, sitespecific mining guidelines for the longwall coal mine workings in the offshore portion of the Sydney Coalfield, Nova Scotia [2]. Currently, the National Coal Board Subsidence Engineers' Handbook (NCB SEH) [3] is used to determine the theoretical maximum subsidence and strain values at the sea bed, since geological and mining conditions closely resemble those found in the United Kingdom and in particular the East Midlands Coalfield. However, a difference exists between the guidelines used in various countries for the theoretical maximum tensile strain values that can be induced at seabed (or the base of a major aquifer system) before the risk of water ingress to the mine workings becomes unacceptable. In the USA a value of 8.75 m m / m [4] is used, while in the United Kingdom the value is 10 m m / m [51. In the Sydney Coalfield the value is 8.5 m m / m [6]. To date, although no subsidence-related water occurrences have been experienced in the Sydney Coalfield, uncertainty exists regarding the applicability of the current guidelines. An over-cautious value effectively

sterilises large amounts of otherwise economically viable coal, while an over-optimistic value could significantly increase the risk of a major water occurrence within the mine workings. In 1984, a preliminary study was undertaken to examine potential data collection methods for monitoring seafloor subsidence profiles. The work revealed that any seafloor subsidence monitoring scheme would be influenced in both its design and development by atmospheric, sea state and socio-economic conditions, as well as the subsea geology. In addition, any potential system would result in problems regarding its deployment, monitoring and recovery and it should therefore contain a degree of built-in redundancy. Four basic monitoring schemes were identified: seasurface, subsea, geophysics and direct monitoring [7,8]. Further studies revealed that geophysical methods showed the most promise, but before undertaking a field measurement program, it was decided to undertake a review of archival geophysical data collected from within the Sydney Coalfield.

28

T.R.C. ASTON ET AL.

panels. However, of the 13 sections examined, only two revealed the presence of a clearly identifiable seafloor trough coincident with the longwall workings and these may or may not have been mining-induced. A comparison of the vertical seafloor trough dimensions revealed that the actual measured profiles (or troughs) were between 25 and 30% greater than the theoretically predicted troughs for the same mining configuration. However, both of the measured troughs were located over areas of double seam mining and the lack of detectable troughs over any of the single seam

Historical data review

In 1986, a review of historical geophysical data collected in the offshore portion of the Sydney Coalfield was undertaken [9,10], since current theories predict that seafloor subsidence profiles should already exist above the old longwall workings and a study might confirm both their presence and magnitude. The review revealed 13 sections where previous geophysical survey lines could be directly compared, on the basis of data quality and location with existing longwall mining

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PRE-MIN1NG

SEAFLOOR

29

DETERMINATION

workings made the subsidence explanation for the double seam extraction troughs tenuous. Analysis of the archival data revealed that a considerable number of potential errors and limitations precluded the identification of seafloor subsidence troughs over the single seam extraction panels. The most noticeable limitation was the absence of any pre-mining data against which the post-mining profiles could be compared. If these problems were addressed during future surveys, it was considered that subsidence-induced seafloor troughs could be identified and evaluated using hydrographic techniques.

1987 survey equipment and techniques The location of the 1987 survey lines in relation to the Phalen mine workings is shown in Fig. 4. Horizontal onshore control Initially, using information supplied by the Land Registration Information Service, four control points were identified on the shoreline in the proximity of the survey area. However, a reconnaissance revealed that all the sites had been either modified, destroyed or masked and it was therefore necessary to re-survey four alternative control points. In each case, the point had to provide the necessary geometry and an unobstructed line of sight to the survey area. Finally, in addition to establishing the control points, it was also necessary to tie them into the CBDC survey grid used for the underground mine workings. Horizontal vessel control The Del Norte Trisponder Model 540 was used as the primary vessel positioning system. Four trisponders, one located at each of the

onshore control points, were used along with a Nortech computerised positioning system. The trisponders were each powered by two 12 V auto batteries, which provide 12 V DC, and use a microwave-based system operating at a frequency of 9400 MHz. The theoretical accuracy of the system is _+1 m. In practice, the system was found to give an accuracy of between 4 and 6 m, when weather and sea state conditions were taken into account. The Nortech positioning system uses a least squares computation and Kalman filtering technique to optimise the vessel location. Ongoing recording of the positional data, subsequently allowed the creation of survey track plots at a scale of 1 • 5000. Depth control An ATLAS DESO-20 precision echosounder with dual frequency operation (at 210 kHz and 33 kHz) was chosen as the depth measuring device. The high frequency component provides a high resolution profile of the seafloor, while the low frequency component is capable of penetrating soft, surficial sediments and therefore acts as a very high resolution shallow sub-bottom profiler and seabed classification system. In addition, the system has a wide range of vertical recording ranges and paper feed rates to optimise horizontal resolution. The graphical precision at 210 kHz is + 0.12% using the narrow beam (9 o) transducer. The echosounder transducers were located on a gimballed "over the side" mount which compensated for modest vessel motion. It should be noted that most of the survey lines were run under good sea state conditions, although some of the data is of poorer quality due to less than ideal sea state conditions. Tidal corrections were applied using data from the North Sydney permanent tidal station operated by the Canadian Hydrographic Service.

30

T.R.C. ASTON ET AL.

Seismic reflection system

survey lines are differentiated on the track plots by the following notation:

Definition of both the surficial sediment thickness and buried rock surface was considered beneficial to the project, since it would assist with the identification of stable bedrock areas. A F e r r a n t i / O R E Geopulse System was therefore used for the seismic profiling work. It consisted of a c a t a m a r a n - s u p p o r t e d " b o o m e r " transducer and high resolution, multi-hydrophone array towed behind the vessel. A high voltage power supply, located on the vessel, was capable of producing five energy output levels with a range between 105 and 455 J. The profiles were recorded on both EPC 3200 and EPC 1600 graphic recorders as well as a Hewlett Packard 9638, 8 track tape recorder. The quality of the survey results varied from good to poor depending on the sea state conditions. Most of the " b o o m e r " records show a broad reverberation pulse, both in areas of bedrock outcrop and sediment cover. Significant penetration of up to 20 msec (25 m) was achieved in some areas of bedrock (bedrock velocity 2500 ms-a), while in other areas with suspected channelling, up to 20 msec (15 m) of sediment appeared to occur (sediment velocity 1500 ms-a).

First r u n - - l i n e 1, line 2 . . . Second r u n - - l i n e 1.1, line 2 . 1 . . . The track plots were digitized using Supertech software to provide digital files of event mark locations (those points w h e r e the echosounder and boomer profiles are simultaneously marked along with the navigation position).

Echosounder profiles Tracings of the seafloor profiles were made on overlays prior to digitising, which allowed the position of the seafloor to be selected by the interpreter and this resulted in greater digitising efficiency, improved quality control, flexibility in editing and non-defacement of the original records. Using this technique, the effects of vessel pitch and roll were "filtered out" by drawing a line of "best fit" through the sinuous seafloor trace. A point digitising mode was used with a high sample rate, in order to compensate for changes in the horizontal scale, which occurred as the vessel's speed relative to the seafloor varied, thus allowing a direct comparison of the duplicated line data.

Tidal and water velocity correction

Data processing and analysis Track plots A total of 35 line kilometers of survey were run over the offshore portion of the Phalen mine workings, Fig. 4. Each gridline was run in two directions: (1) n o r t h - s o u t h and south-north, or (2) eastwest and west-east. The purpose of duplicating each gridline was to test the repeatability of the vessel guidance and positioning systems as well as the echosounder profile. The

The digital files of water depth were corrected for tidal variations using data from the Canadian Hydrographic Service permanent tide station at North Sydney and this was prepared by the Marine Environmental Data Service in Ottawa. The short time duration over which each line was run, allowed single bulk shifts in tidal elevation to be applied. The tidal correction calculated for the line midpoint was then applied to the entire line. The range of tidal variations during the survey period was approximately 0.84 m m a x i m u m (between high

31

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and low tide) and since the single line recording time was only 15 min, this simple correction was considered appropriate. Initial problems were encountered with measuring the velocity of sound in seawater using the bar check method. A velocity of 1400 ms -1 was therefore assumed for the first 2 days. However, after revising the bar check methodology, an accurate value of 1457 ms -1 was obtained. As a result, the raw echosounder data differs depending upon when it was collected. To correct for this phenomena, the appropriate digital values of water depth were adjusted to a velocity of 1457 ms -~. All processing and editing of the geographical coordinates and water depth data was undertaken using DBASE III software.

Plan and profile development The digitised survey track and echosounder data files were used to produce individual line plan and profile drawings. The drawings were produced by transferring the digital data from the Supertech software to an AutoCad system. Figures 5 and 6 show examples of the resultant track plan and bathymetric profiles, originally displayed at scales of 1 : 1 0 0 0 (horizontal) and 1 : 5 0 (vertical). Figure 5 shows the trackline P H N 2 and P H N 2.1 and Fig. 6 trackline P H N 5 and P H N 5.1. The establishment of AutoCad based premining bathymetric profiles, across the proposed Phalen mine west side workings, represents a major milestone in the overall project, since future post-mining profiles can now be readily added and compared. In addition, use of the AutoCad system also offers considerable potential for further editing, data manipulation and statistical analysis of the profile data.

Seismic boomer records

The analysis of pre- and post-mining seafloor topographies to detect seafloor subsi-

dence profiles, necessitates that the seafloor must be stable and unaffected by other destabilising mechanisms such as erosion or surficial sediment mobilisation. To establish the degree of bedrock and sediment distribution over the survey area, an O R E Geopulse Boomer System was used with the profiles being collected along each of the tracklines, coincident with the echosounder data. In the echosounder profiles, the seafloor bedrock topography is characterised by an asymmetrical sawtooth profile. On the " B o o m e r " profiles, intermittent, planar northward-dipping bedrock reflectors are commonly seen in the subsurface. In general, the results indicate that bedrock outcrops occur across the seafloor over the eastern portion of the Phalen Mine workings. In addition, east of 2200 E and immediately north of the 3 west panel, some channelling appears to exist. These features are apparent on the data from the P H N 11 and 12 lines, although the resolution is poor. U p to 15 m of sediment appears to exist, but further work is recommended to better define the features.

Discussion

Initially, steering problems were encountered on the P H N 1/1.1 and 2/2.1 lines and this occurred due to the helmsman overcompensating, while steering the vessel using the line-steering monitor. This device is used extensively for conventional geophysical surveys and can be invaluable in the hands of an experienced operator. However, during this near-shore survey, it was found to be more effective to have the helmsman bring the vessel to the start of the line via the monitor and then steer the line using a compass bearing. The navigator constantly monitored the vessels position and guided the helmsman with minor course corrections. Using this method, the line steering and repeatability was found to improve significantly.

34

The degree to which the re-run line tracks coincide, directly affects the degree of seafloor profile repeatability. This is particularly evident in the dip lines (those perpendicular to the longwall panels). Figure 5 represents a track line with poor repeatability, while Fig. 6 shows a track line with a good degree of repeatability. Where the track lines are almost superimposed, the profiles are nearly identical in both morphology and depth, with all the major and minor features preserved. Contrasts between the first and second line runs are typically 0.25 and 0.5 m and these variations can be explained by vessel heave (the degree of smoothing applied to the echosounder profile by the interpreter), inherent inaccuracies in the navigation equipment and the microtopography of the seafloor. In Fig 4, the n o r t h - s o u t h lines represent bedrock dip lines, while the east-west lines represent strike lines. In contrast to the dip lines, PHN 1-8, the strike line profiles, P H N 9-12, exhibit a much lower degree of repeatability even when the lines coincide and are therefore much more sensitive to the degree of line coincidence. Topographic highs and ridges on the seafloor, caused by bedrock outcrops, tend to be parallel to strike, Slight offsets in the vessel position can therefore represent differences between a profile within a trough or along the crest of a ridge. By comparison, the dip lines represent cross-sectional profiles consisting of ridges and troughs, which have considerable lateral continuity and are consequently not as sensitive to line offsets.

Conclusions A long-term research program was initiated in 1983 to monitor actual seafloor profiles, produce effective prediction techniques and establish safe, site-specific mining guidelines for the longwall coal mine workings in the

T.R.C. A S T O N E T AL.

offshore portion of the Sydney Coalfield, Nova Scotia. In 1987, as part of this program, 35 line kilometers of marine geophysics survey were run over the western portion of the Phalen mine workings to establish the premining seafloor topography. Each survey line was run twice in order to establish whether a high degree of repeatability could be obtained with reference to the vessel guidance and positioning systems as well as the echosounder profiles. Initial problems encountered in exactly re-running the survey lines were overcome by making simple changes to the navigation procedure. Where the re-run survey lines are almost coincident, the echosounder profiles are nearly identical in both morphology and depth. Contrasts between the first and second line runs are typically between 0.25 and 0.5 m and these variations can be explained by vessel heave, errors in the navigation system and variations in seafloor microtopography. The capability to accurately re-run historical survey lines at a future date is critical in order to establish the post-mining seafloor topography across a known area and therefore evaluate the presence of seafloor longwall subsidence profiles.

Acknowledgements The authors would like to acknowledge the assistance provided by the Cape Breton Development Corporation in the preparation of this work. The paper represents work undertaken by MacGregor Geoscience Ltd. of Halifax, Nova Scotia under research contract (DSS File No. 15SQ.23440-7-9018) for the E M R C A N M E T - C R L Cape Breton Coal Research Laboratory. At the time this work was undertaken, Dr T. Aston was employed as a Research Scientist at the E M R C A N M E T - C R L Cape Breton Coal Research Laboratory in Sydney, Nova

PRE-MINING SEAFLOOR DETERMINATION

Scotia. T h i s p a p e r is p r i n t e d w i t h p e r m i s s i o n of the M i n i s t e r o f S u p p l y a n d Services Canada.

References 1 McGregor Geoscience Ltd., Determination of seafloor topography over the new Phalen mine, Sydney coalfield, Nova Scotia. Rep. for CANMET, Ottawa, DSS File No. 15SQ.23440-7-9018 (1988). 2 Aston, T.R.C., Discussion paper: Subsidence research in the Sydney coalfield. Div. Rep. ERP/CRL 83-10 (IR), CANMET, Energy, Mines Resour., Can., Ottawa (1983). 3 National Coal Board, Subsidence Engineers Handbook. Mining Dep. Natl. Coal Board, London, 2nd Ed. (1975). 4 Wardell, K. and Partners, Guidelines for mining under surface water, phase III, Final report. Pub. PB264-729, US Bur. Mines, Washington, DC (1976). 5 National Coal Board, Working under the Sea. Production Dep. Instr., PI/1968/8, Natl. Coal Board, London (1968).

35

6 Wardell, K. and Partners. Report to the Commision of Inquiry 1974 on the question of submarine mining in Cape Breton. (1974) (unpublished). 7 Aston, T.R.C., Steeves, D. and Amirault, J.A., Instrumentation schemes capable of measuring seafloor subsidence over longwall mining operations. In: Proc. Int. Mine Surveying Congr. 6th (Harrogate, UK), Balkema, Rotterdam, Vol. 2 (1985), pp. 758768. 8 Forrester, D.J. and Aston, T.R.C., A review of mining subsidence instrumentation and its potential application for seabed monitoring. Min. Sci. Technol., 4 (1987): 225-240. 9 McGregor Geoscience Ltd., An examination of archival geophysical data from the Sydney coalfield, Nova Scotia to identify seafloor longwall subsidence profiles. Rep. for CANMET, Ottawa, DSS File No. 03SQ.23440-6-9026 (1987). 10 Aston, T.R.C., Lapierre, A. and Stewart, J.McG., A review of archival geophysics data to identify seafloor longwall subsidence profiles. J. Inst. Min. Eng., 148 (328) (1989) 336-340.