DEBI-T was developed via collaboration between Photon Systems, Inc., Caltech’s Jet Propulsion Laboratory, the University of Southern California and the Lamont-Doherty Earth Observatory at Columbia University under National Science Foundation Funding. The primary use of this unique new instrument is to provide a quick means for characterizing the distribution of microbes in the deep subsurface. DEBI-t was deployed aboard the JOIDES Resolution as part of the Integrated Ocean Drilling Program (IODP) Expedition 336 into several bore holes at North Pond site in the western flank of the Mid-Atlantic Ridge with a sea floor depth about 5 km and into 4-inch diameter bore holes with depths up to about 799 m below the sea floor.
The instrument utilizes a 224 nm excitation source coupled to a detection system composed of 7 photomultiplier tubes (PMTs); each PMT is outfitted with a gated boxcar integrator and filters that isolate fluorescence features from specific regions of the electromagnetic spectrum. DEBI-T is also equipped with a pinhole camera that will record video of the logging operation to provide spatial context to the fluorescence information. Light for the camera is provided by white light LEDs located on the downhole end of the instrument. A deep UV transmissive sapphire window with a 20mm clear aperture is seated in the center of the front housing. This window allows for transmission of the excitation source to the sample and of the fluorescence signals to the detectors, as well as visible light for the camera. The light exits the housing down the center axis and a turning mirror seated in the bumper assembly directs the signal to and from the sidewall. The control module will record acceleration in X, Y and Z, allowing for greater contextual information on the relative orientation of DEBI-T as well as information concerning inclination and displacement as the tool logs the borehole. In addition, temperature inside DEBI-T will be logged to understand how the internal environment changes through the course of a logging operation.
224 nm laser source
The unique capabilities of DEBI-T stem from the ability to excite below 250nm, producing fluorescence signals capable of distinguishing microbes and organics from the underlying mineral matrix. The excitation source is a long pulse width (100 s) helium-argon hollow cathode laser developed at Photon Systems, Inc.. The lasers produce a narrow line-width emission at 224.3 nm, and are capable of reaching full power in less than 20 us from a cold start at a temperature range of -200 to 100 0C. The lasers are considered “soft-touch,” as the long pulse width and low average power (5 uJ) produce negligible thermal damage to the target.
DEBI-t was designed and built specifically for Expedition 336 to better our understanding of the nature of microbial communities harbored in young ridge flanks. In order to place the fluorescence data into the appropriate context, DEBI-t is deployed as part of what is known as the microbiology combination tool string. The microbiology combo tool obtains a number of parameters in addition to native fluorescence: 3-axis downhole acceleration, 3-axis magnetic field, temperature and total and spectral (Th, U and K) gamma-ray measurements. DEBI-t operates with a “fire and forget” methodology, in which the control software directs the instrument to fire the laser, collect data and transmit information uphole as soon as power is supplied. Power is supplied via a 24 V power supply in the multi-function telemetry module (MFTM), developed by LDEO. The MFTM also acts as the communications interface between DEBI-t and the Schlumberger wireline tools. During logging operations, DEBI-t transmits real-time clock, laser power and detector status to provide the operators on the ship with information regarding the health of the system. Transmission of this data is initiated by a request from the MFTM. There is no real time shipboard control over any of the instrument parameters. However, the ability to monitor instrument status in real time allows the operator to power cycle DEBI-t while still in the borehole, should the need arise. DEBI-t is rated to operate at a maximum pressure of 10,000 psi, while the electronics and optics in analogous systems have been operated successfully at temperatures between 0 and 40+ 0C. For Expedition 336, a single pass with the microbiology combo tool string consisted of one down and one uplog.