The MeBo seafloor drill uses H-size HQ wireline coring tools. The outer diameter of the drill rods is 98 mm. During routine drilling operations, these pipes are used to extend the drill string downhole, and are recovered once terminal depth of a given hole is reached (Freudenthal and Wefer, 2013). Each HQ rod has a male end with a conical thread at the base and a female conical thread at the top end.

If an observatory is to be set with MeBo, procedures change in the sense that some drill pipes (i.e. several of the 2.35 m long rods) remain in the ground and serve as casing to stabilise the upper portion of the hole where the soft sediments would otherwise close in. The final, uppermost drill rod is then prepared to provide the borehole seal. In addition, this rod has a widened outer diameter so that it will facilitate a seal at the seafloor where the drilling operation may have excavated the ground. Note that the eventual sealing of the system occurs via sediment infilling around the outside of the drill rods left in place, in addition to the widened upper part and the seals inside the drill rod string. This is different from the traditional CORK seals where concrete, packers, and o-ring seals are utilised (see Becker and Davis, 2005), but cementing is impossible for MeBo and packer systems for MeBo rods are not even in the prototype stage today.

The two main principles to have a sealed upper rod are to screw in an instrumented plug having the shape of the conical thread and additional o-ring seals, or to prepare an entirely new instrumented rod of up to 2.35 m length with a hot-stab seal in the middle. These two principles have been used in designs of the so-called MeBoPLUG and the MeBoCORK (Circulation Obviation Retrofit Kit)-A (A stands for autonomous) and MeBoCORK-B (B stands for bottom) (see Sect. 3.2 below).

In addition to the borehole seal, data mining in the borehole and/or formation is a prerequisite for the instrumented drilling rods. If the inner volume of the “casing” (i.e. MeBo drill pipes in the ground) is the target, a simple plug with a data logger and transducers is sufficient to monitor, e.g. the equilibration of the hole and ambient values thereafter. If coupling to the formation is desired, a mechanism is needed that couples the transducers to the open-hole formation. For this purpose, we designed a small device at the base of the instrumented uppermost drill rod that has a small coil with PVC tubing (1 mm inner diameter) which can be unrolled via an electrical motor upon command (acoustic signal or timer with preset date/time for lowering). At the tip of the tubing there is a stainless steel conical drop weight with a boring that will then provide access to the sediment at the base of the hole. The tubing may then be used to get ambient pore pressure readings, or extract porewater samples (in case an osmo-sampler is hooked to it at the seafloor). We have realised designs of both in the MeBoPLUG and the MeBoCORK-A and MeBoCORK-B (see Sect. 3.2 below).

Retrieval of CORK data has always been a crucial issue in the past, with the majority of ODP-CORKs and IODP-CORKs relying on a remotely operated underwater vehicle (ROV) visits during regular third party cruises in the years after deployment. This is a costly endeavour and further bears the difficulty of vagueness in planning because of dependency on weather, ship time providers, etc. Only very few CORK systems were connected to seafloor cabled networks (e.g. ODP Hole 1026B to NEPTUNE, or ODP Site C0002 to DONET), so the requirement for ROV visits is reduced to instances of maintenance, not data transfer. In any case, one of the key requirements in developing MeBo observatories was hence to start with a simple instrument to be recovered by a ROV and then evolve into systems which make ROV visits or even visits of a vessel obsolete.

As a consequence, the MeBoCORK systems aimed at including acoustic modems so that the successful deployment and health of the system after the MeBo took off can be attested. Data transfer is moderately fast, so that practically only limited data sets are to be downloaded that way, while a final visit to recover the instruments by a ROV seems still advisable. In case the deployments take place in an area where a return cruise is not anticipated for longer periods of time, it may be better to release a pop-up unit with a mirror data set of the main data logger as well as an iridium telemetry to transfer the data without the necessity of retrieving the instrument. If the latter is sufficiently cheap, net cost may be saved since ship time is also precious. We have realised designs of both in the MeBoCORK-A and MeBoCORK-B and the MeBoPUPPI (Pop-Up Pore Pressure Instrument) (see Sect. 3.2 below).

Given the wealth of critical parameters that govern seafloor processes, scientist want to deploy multi-sensor instruments that often consume considerable energy. One main objective in MeBoCORK installation was hence to have a modular seafloor unit where space for additional devices and batteries play a minor role. Such a unit has to exchange the initial MeBoCORK instrument (with a limited lifetime) without affecting the borehole seal. This was achieved by a hot-stab connector that can transport fluids, power, or both. At the seafloor end, capabilities may be increased by not only adding osmo-samplers with long tubing coils for fluid sampling over 24 months and longer but also providing power for energy-demanding systems, e.g. geophones. We have realised this by inventing the MeBoCORK-B (see Sect. 3.2 below).

In the next section we will introduce three general instruments of the MeBo observatory family: the PLUGs, CORKs, and PUPPIs. Each of them is employing several (if not all) of the tasks listed above. In addition to those observatory instruments realised, many other combinations of transducers and data transfer principles, but also implementation of new transducers. This could be either in the instrumented drill pipe (if sufficiently slim) or inside the seafloor unit. With its generic and modular design MeBo observatories may become a versatile and affordable way to monitor hydrogeochemical and geological processes in the shallow sub-seafloor (< 200 m).

In regular-size industry boreholes, the casing of the upper portion on the well is a couple of decimetres, and re-entry into the hole is facilitated by a conical seafloor structure, often complemented by a platform to land a ROV. For the seafloor drill holes, all of this is miniaturised and in the case of MeBo a diameter of only 98–110 mm is available. Hence, space is most seriously governed the design of the MeBo observatories, and re-entry is not feasible in such narrow drill pipes. In essence, the entire hole completion was initially designed as a stand-alone instrument, which does not differ from the geometry of a standard MeBo drill pipe (approx. 235 cm long and 10 cm in diameter).

The name of the instrument is extended from the SmartPlug and GeniusPlug observatories, which also represent simple types of CORKs designed as an extension to mechanical bridge plugs, but with IODP drill hole-sized (Kopf et al., 2001b). Instead of a bridge plug or CORK seal, we make use of the conical threads of the MeBo drill pipes and seal the pipe with a pair of additional o-rings (Fig. 3). The pressure housing is machined from a thermoplastic synthetic polymer, polyoxymethylene (POM), which tolerates moderate pressure and temperature and has successfully been used to 25 MPa confining pressure. At the top, a handle designed to specification of the ROV MARUM-Quest manipulator was added. In the borehole-facing section, a thermistor as well as the downward-looking P port are situated (Fig. 3, inset on right).

Inside the housing we hosted standard “RBR Duo” data loggers (see www.rbr-global.com for detailed specifications) with Keller PD10 differential pressure transducers (500 kPa differential P range) for monitoring pressure transients in the boreholes (as a proxy for strain; Davis et al., 2006) relative to the seafloor pressure signal. With this approach we omit de-tiding of our data because all signals, e.g. wave action, occur more or less simultaneously at the seafloor and in the shallow sub-seafloor formation. Given that only one thermistor was fitted into the so-called MeBoPLUGs, we are lacking a temperature record from the seafloor (i.e. upward-looking side of the MeBoPlug; see Fig. 3). In order to overcome this shortcoming, a simple self-contained device for sea bottom temperature monitoring, a so-called MTL (Miniature Temperature Logger by Antares; Pfender and Villinger, 2002) was deployed by a ROV next to the MeBoPLUG (see next section). The sampling rate of the MeBoPLUGs was set to 10 s on the RBR data loggers, which is providing them with an estimated lifespan of about 4 years (anticipated end of recording is February 2016).

The two versions of the MeBoCORK, as seen in Figs. 4, 5, 6, and 7, are both self-contained with power, data logging, data transmission, and transducers. They are recoverable at all times and hence minimise the risk of losing the investment. In the following paragraphs, the deployment mechanism is explained in some detail. First, the MeBo hole has to be prepared for long-term instrumentation after coring is completed. This is achieved by leaving several pieces of MeBo outer drilling rods in the ground after wireline core retrieval. These rods, approx. 30–40 m in total length, act as a casing and stabilise the upper sub-seafloor portion where the deposits are poorly consolidated and otherwise may close in. Only the lowermost part of the hole is free of “casing” and provides direct access to the formation. The first prerequisite to keep the MeBoCORK simple follows the first hydrological observatory in ODP: only hydraulic tubing is lowered into the hole to access fluid pressure (or fluids) at depth (Wheat et al., 2011), and all electronics remain at the wellhead. For MeBo, a string of two armoured PTFE (polytetrafluoroethylene) tubes is coiled up in the lower part of the MeBo set observatory unit (termed MeBoCORK-A, where A stands for “autonomous”, i.e. MeBo by itself is capable of placing a stand-alone observatory), namely in the lower portion of the “adapter” hosting the receptacle for the hot stab. Once the drilling device has set this piece, the coil of tubing is unlocked and a dead weight favours the tubing's descent towards terminal depth where the hole is open. The upper end of the individual tubing connects to the borings of the hot-stab receptacle. The lower unit also hosts battery packs. For the installations during cruise SO222, we pre-installed 45 m long tubes because we never aimed to exploit the full depth of MeBo in the mud volcanoes with fairly homogeneous mud breccia, although space would allow for up to 70 m of tubing.

The upper portion of the MeBoCORK-A hosts the data transmission unit, data logger, and transducers, the latter of which are connected to the borings of the male hot-stab end (Figs. 4a, 5). The hot stab is mated with its female counterpart, and the two halves of CORK-A are further secured by a bayonet connector that allows coupling of the MeBo70 top drive and hence torque being transmitted (in clockwise direction only). During installation MeBo fully screws in the entire CORK instrument, which in total is the exact length of a regular MeBo drill rod and which sits on the magazine with the other rods. Once this piece is properly set, MeBo pushes the unit to a depth so that only the titanium part (ca. 70 cm long) sticks out of the seafloor and then disconnects. In the initial design, this simple, MeBo set CORK monitors pressure and temperature, which are both indicators for deep-seated fluid flow; pore pressure is additionally valuable as a strain proxy (see above). Depending on the sampling rate, the batteries will allow monitoring for many months to a few years (in case of the system deployed during leg SO222A, this was 7 months at a sampling rate of 1 Hz; see below).

If monitoring as well as parameters other than pressure and temperature is desired, the instrumented MeBo70 rod (CORK-A) is too small and a ROV dive is required to recover the CORK-A and deploy an external, more sophisticated observatory unit (Figs. 4b, 6). Since the ROV dives to the seafloor and connects a seafloor (0 sea bottom) unit to the MeBo rod with a hot-stab receptacle, this system was termed MeBoCORK-B (i.e. bottom). The ROV is able to unlatch the bayonet connector in an anticlockwise direction and can transport the MeBo set CORK back to the ship, because its weight is low and the diameter is suitable for the manipulator claw. Before this, a seafloor unit will be placed next to the MeBo drill site (Fig. 6). This system consists of a pressure housing with an attached hood in which a male hot-stab adapter plus the umbilical of armoured tubing strings is coiled up. The ROV takes the hot stab and places it into the lower portion of the MeBoCORK, which remains connected to the outer drill string at all times. Hot-stab operations are also common in IODP CORKs to enhance the capabilities for hydrogeological testing (e.g. Kopf et al., 2011a). The pressure housing at the seafloor can be equipped to the mission's/scientists' specifications, and in the case of this proposal will host the pressure and temperature transducers (same as in the instrumented rod) plus an osmo-sampler (Fig. 7; see also Jannasch et al., 2003).

Both MeBoCORK instruments are programmed to measure the pressure at 1 Hz for a period of 30 s and then record the average pressure value. Seafloor reference and borehole pressure are offset by 15 s, so that the data are written to the disk alternately.

In a recent effort, we have taken the MeBo long-term borehole observatory science to the next level and built pop-up borehole instruments that can be released from the casing string after a predefined period. An underwater connector, which is Capable of being mated, further enables the user to also “manually” release the unit with a ROV. Once the unit has ascended to sea level, the pop-up units (Fig. 8) send their data via satellite link, rendering a second visit and additional ship time unnecessary. Such instruments would in a first iteration monitor pressure and temperature, but could be equipped with geophones, seismometers, and other components depending on the scientific demands of a given mission, the duration of monitoring, the amount of time series data, etc.

In contrast to the MeBoCORK systems, the MeBoPUPPIs do not rely on a hot-stab connector and tubing that is lowered towards the terminal depth of the borehole where direct access to the formation exists. Instead, the monitoring is restricted to pressure and temperature inside and outside of the borehole as well as tilt using a triaxial array of accelerometers. If desired, an additional unit that samples simultaneously one hydrophone and three geophone channels (type SM6, with 4.5 Hz lower frequency band, but no mechanical gimbaling system) can be mounted. Either sensing package has to remain in the borehole string for reasons of limited space and weight of the pop-up unit. This may be equally the case for part of the battery packs when very long monitoring periods are desired. In these cases, almost all of the total of 235 cm length of a MeBo70 drill rod segment, excluding a 53 cm long adapter to protect the glass dome (Fig. 8, righthand side drawing), are available.