Research on visible-near infrared spectrum features of typical coal measures rocks

In order to study coal-rock identification method based on visible-near infrared spectrum technology, 11 types of typical coal measures rocks including three main categories namely shale, sandstone and limestone were collected from four coal mines of Shanxi and Shandong provinces. Their reflectance spectra in visible-near infrared band (400-2 450 nm) were measured and influences of mineral and elemental compositions on spectrum features were studied. Differences between reflectance spectrum curves of coal measures rocks and the ones of coals from the four coal mines were analyzed, and influence laws of carbonaceous matter content on characteristic parameters of reflectance spectra curves of coal measures shale were obtained. The study results show that reflectance spectrum curves of the most coal measures rocks show multiple absorption valleys with increasing wavelength in visible light band (400-780 nm) and shortwave near infrared band (780-1 100 nm). In longwave near infrared band(1 100-2 450 nm), distinct absorption valleys are mainly located near 1 400 nm, 1 900 nm, 2 200 nm and 2 350 nm wave length. The wave length of absorption valleys of shale and limestone are relatively fixed while the ones of different sandstones show a variety of changes.Except for carbonaceous shale without distinct absorption features, absorption features of reflectance spectra of coal measures rocks and coals from the same coal mine are obviously different in visible-near infrared band. With increase of carbonaceous matter content of coal measures shale, a common feature is presented that spectral slope of reflectance spectrum curves and depth of each distinct absorption valley in visible-near infrared band both decrease rapidly at first and then tend to be flat.