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Supplemental Cash Flow Information and Non-cash Activity (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Cash paid during the period for:    
Interest $ 6,620 $ 7,075
Income taxes 1,354 2,364
Non-cash items from investing and financing activities:    
Investment in multi-client data library in accounts payable and accrued expenses $ 0 $ 4,561