Derivative Financial Instruments - Additional Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2013
Derivative Financial Instruments [Line Items]
Accrued unrealized foreign exchange losses	$ 0.4
Foreign Currency Forward Contract
Derivative Financial Instruments [Line Items]
Foreign currency forward contract to hedge	20.0
Canadian Subsidiary
Derivative Financial Instruments [Line Items]
Denominated intercompany accounts payable	$ 28.3

Foreign currency transaction gain loss unrealized accrued.

No definition available.

Name:	geos_ForeignCurrencyTransactionGainLossUnrealizedAccrued
Namespace Prefix:	geos_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant