Assessing Response of Bats to Emissions from Quarry Blasting

David Mitchell-Melcann Pty Ltd operates a quarry in a large outcrop of Permian limestone at Sherwood, near Kempsey in the Macleay Valley. As a result of mounting concern regarding the possible adverse effects of blasting within the limestone quarry on the bat colony, Heggies monitored the emission levels and the bat responses associated with a production blast at the quarry.

The objectives of the blast monitoring exercise were to:

1. Assess and quantify, where possible, the physical effect of the blast on the nearby bat colony
2. Measure the structural vibration in their cave and assess the extent of any possible effect on the structural integrity of the cave formation
3. Extrapolate the findings from 1. and 2. above to establish how close the limestone quarry can be extended towards the Bat Cave without affecting its structural integrity and the lifestyle and habitat of the bats

Summary of Results

Rifling of explosive gases from the blasthole collars was excessive and a blowout occurred in the face of the blast. Despite the high level of gas emitted to the atmosphere, the measured level of airblast at the Bat Cave entrance was nearly 6 dB lower than the level calculated using generally accepted prediction formulae.

Video film of the mouth of the Bat Cave clearly showed that no bats flew from the cave before, during or after the blast. The video technique was incorporated in the monitoring procedure as recognised experts in the behaviour of this species advised that the bats were likely to fly out of the cave if they became sufficiently alarmed.

The levels of ground-borne vibration measured at the foot of the limestone outcrop and inside the cave were lower than levels calculated using generally accepted prediction formulae by 12% and 54% respectively. This was almost certainly due to variations in geological conditions along the two vibration travel paths. The resultant particle velocity vibration trace measured on the wall of the cave nearest to the blast had two peaks of the same amplitude, namely 6.5 mm/s. This vibration level was significantly lower than the conservative "structural" criterion of 200 mm/s adopted for the project.

The measured level of airborne airblast in the cave was 110 dBLinear, with a dominant frequency of 2 Hz. The "human audible frequency" noise level generated by the re-radiation of the ground-borne vibration in the walls of the cave was 112 dBLinear, with a dominant frequency of 69 Hz. These airblast levels clearly comply with the maximum human comfort and structural damage levels of 120 dBLinear and 133 dBLinear respectively.

Subjectively, there was no noticeable change in the level of flight activity or of behavioural noise emitted by the bats before and after the blast. Quantitative analysis of the bat noise levels in the octave bands of centre frequency 1 kHz, 2 kHz, 4 kHz and 8 kHz (the twittering and chirping noise frequencies) showed no discernible change for the 60 second periods immediately before and immediately after the blast. There was also no significant change in the results of statistical analysis of the bat noise level emission (behavioural and flight noise) during the five minute periods before and then after the blast.

Remotely triggered infra-red photography revealed no obvious increase or decrease in the degree of bat flight activity either immediately after the blast or for several minutes after the blast, relative to the bat activity photographed before the blast.

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