Borehole measurements are an indispensable part of scientific drilling projects to gain physical and chemical parameters continuously and in situ (Goldberg, 1997). They are usually carried out using the well-established wireline logging method, in which the measuring sondes are moved on a logging cable in the borehole. This method has the advantages of power supply to the sondes, online data transfer to the recording unit at surface, and very precise and reliable depth measurements. Furthermore, it is easy and flexible to use, comprises the largest range of sonde types, and is readily available through a large number of academic and commercial providers. In the framework of the International Continental Scientific Drilling Program (ICDP), we have conducted more than 37 wireline logging campaigns (Hodell et al., 2006; Koeberl et al., 2007; Gebhardt et al., 2013; Jackson et al., 2019; Jerram et al., 2019; Abbott and Rodbell, 2020) using slimhole tools. For the majority of these missions, most demands could be covered. However, there is also a significant proportion of projects in which unintentional or technically necessary downhole conditions adverse to wireline logging occur, making the use of logging sondes on a cable risky, extremely difficult, and, therefore, very time-consuming or even impossible. The most common adversities are partially blocked boreholes (e.g., large-scale wall collapse or local bridging) and borehole paths that deviate strongly from vertical.

In lacustrine sediment coring projects, operations are performed without casing; thus, unconsolidated sediments occurring over long sections often rapidly narrow boreholes and make them impassable for lightweight logging sondes. Mostly, however, a machine-powered drill string can pass through without problems or is used to redrill the well quickly. In such cases, the usual wireline logging procedure is to position the drill string at a depth with the drill bit below the suspected unstable section and to run the wireline sondes through the string into the open hole section below the bit. After measuring this section with all desired sondes, the drill string is pulled higher, leaving the unstable section uncased. These steps are repeated as often as the formation instabilities require (Baumgarten and Wonik, 2015). Overall, this is a very time-consuming procedure and can take up to 10 times longer than normal, unimpeded wireline logging in a several-hundred-meter-deep hole. Another obstacle is that wireline sondes are always run through the drill bit into the open hole. This causes a high risk of getting stuck at the drill bit when reentering.

These limitations of wireline logging are currently hampering scientific and commercial operations; hence, there is a need for the invention of novel methods to perform downhole logging, especially under adverse conditions. The most prominent technique is the integration of logging sensors into the drill string (logging while drilling, LWD), which is widely used in complex hydrocarbon exploration (Hansen and White, 1991) and in scientific ocean drilling (e.g., Moore, 2000). Other deployment options include pipe-conveyed logging or stiff wireline logging, but these techniques are mostly associated with significantly higher costs compared with wireline logging. Another method has also frequently been used in scientific drilling involving robust wireless memory sondes that are mounted atop the inner core barrel, typically delivering parameters such as temperature, pressure, and acceleration, among others, which can be measured from inside a drill pipe (Guerin and Goldberg, 2002).

For the logging of geophysical parameters, one cost-effective and minimum-risk method for drilling projects is the application of so-called memory-logging sondes (Singh et al., 2018). These sondes are not run on a logging cable, but measure autonomously at the end of the drill string after they have been dropped into the drill string (very much like an inner core barrel), and land on the landing shoulder in the outer core barrel at the drill bit. When the drill string is pulled out of the borehole (trip-out), the sondes are pulled along with it and the data are stored in the sonde, which is why this method is commonly referred to as “logging while tripping” (LWT; Figs. 1, 2; (Matheson and West, 2000).

The depth of each measuring point is determined at the surface as in wireline logging, but the assignment of the depth value to the measured value does not simultaneously take place during the measuring process; rather, the depth value is assigned to the measured value after the sonde data have been read out back on the surface. The combination of each individual downhole measured value with the associated surface depth value takes place by means of the measurement time (time stamp) registered in parallel at surface and down the hole. In other words, a downhole value with the same time as a depth value is assigned to this depth value. The depth and the downhole values are both deleted for times when the drill string is not moved upward, resulting a continuous, evenly decreasing depth profile of the downhole parameter. The LWT depth measurement in the drill rig is similar to the LWD depth determination; however, in the latter, the entire running-in of the drill string is measured from its very beginning at the derrick floor. LWT depth measurements can only begin when the last core has been drilled and brought to surface, i.e., the drill string has already been in the borehole for many core runs.

For LWT systems, the risks described above for wireline sondes are virtually negligible when running through unstable zones, because the tools only partially stick out of the core bit into the open hole. Moreover, the upper part of the tools always remains secure inside the drill string, which, on the one hand, completely eliminates the reentry risk and, on the other hand, minimizes exposure to the unstable hole. Should the LWT nevertheless get stuck, drillers can circulate drill mud through the drill bit in order to remove the blockage, and the tools in the drill string can easily be pulled with the high force of the drill rig. Under regular working conditions, the tool string can be retrieved at any time using the core retrieval device (coring wire line and overshot) – for example, if only a partial section of the borehole is to be logged or if technical drilling problems should arise.

Different variants of the logging while tripping method are used commercially worldwide (e.g., Aivalis et al., 2012; Beal, 2019). In the most widespread method, a special modified drill collar above the drill bit allows for the logging of some parameters from entirely within the pipe so that the logging tools never run into the open hole. Since 2012, a set of specially designed memory sondes have been deployed with the MeBo ocean floor drilling robot (Freudenthal et al., 2020), with most of the logging tools actually protruding out of the drill bit into the open hole. Having been involved in the very early development of the MeBo memory sondes, we used this successful example to develop, in cooperation with the same sonde manufacturer (ANTARES GmbH, Germany), a new and improved memory-logging sonde system motivated by a high demand for LWT capabilities in scientific drilling and a lack of affordable commercial LWT services.

The LWT system that we designed consists (from top to bottom) of a landing unit which can be adapted to different drill string dimensions, five combinable single memory sondes, and a precise, robust, and field-application-friendly depth-measuring device at the surface (Figs. 2, 3, 4).

The technical functionality with respect to memory and wireline operation, pressure and temperature resistance, and handling of all iMLS sondes was verified in the 4000 m deep test borehole of the KTB Depth Laboratory (Harms and Kück, 2020).

The goal of the first full field utilization of the entire iMLS system was twofold: on the one hand, we aimed to check and improve the procedure that we had designed; on the other hand, we wished to examine the accuracy of the iMLS depth measurement method. The key operational objective was to exercise the complete LWT procedure for the first time, involving all components, and to compare the log depths obtained with those from subsequent wireline logging runs of the same memory sondes.

The field performance test was conducted during the active drilling operations at the 1000 m deep I-EDDA Test Center well (Almqvist et al., 2018). In cooperation with the drill rig operator, iMLS tests were carried out under realistic conditions in the depth range from 0 to 500 m. At the beginning of the test, the HQ core drill string was already installed in the hole with the coring bit at 500 m. First, a mechanical test was carried out with a dummy sonde, consisting of the landing unit and a solid steel bar with the dimensions and weight of an iMLS sonde string. After confirming the resilience and retrievability of the landing unit, the actual iMLS logging in memory mode was carried out along the depth section from 500 to 390 m with the long sonde combination of LU–MemBat–mSGR–mBCS–BCS–Cent–mMS (Fig. 3). A wheel-based depth counter prototype and the draw-wire device were used one after the other. The iDMD-W described above was not yet available at that time. The large size of the wheel-prototype only allowed for a mounting position between the rod holder and the top drive. Unfortunately, the prototype was rammed by the downward-moving top drive after only 8 m of pipe tripping and became unusable, whereupon the iDMD-R was used. After the complete trip-out of the drill string, we ran the iMLS sondes in wireline mode in the open hole using a 600 m logging winch. We ran the same sonde stack as that used in LWT memory mode for the wireline mode logging. As the sondes and the logging speed were identical in memory and wireline logging, the logs show the same values and amplitudes, although their depth positions may have differed due to the very unequal depth-measuring method. The direct comparison of memory and wireline logs was then used to estimate the accuracy of the memory depth. The depth of the wireline log can be assumed to be almost equal to true depth because the TC1 hole is vertical and has a very smooth wall surface at these shallow depths.

When comparing wireline and memory logs, the memory depth first had to be shifted by a constant value using simple visual correlation due to the inexact starting depth information (drill bit depth) of the memory registrations. The magnetic susceptibility log (MSUS) was best suited to this task due to the almost complete reproduction of its profile in repeat measurements and its high vertical resolution of less than 0.1 m. Depth offsets for correction to the wireline depth were calculated as +4.6 m for the wheel prototype and +8.8 m for the iDMD-R. After this correction of the starting depth, the comparison of the wireline and the memory logs shows a remarkably good agreement of the depth of the measured curves (Fig. 5). The logs are only used here for the purpose of depth correlation. A detailed evaluation of these and additional geophysical logs along with core-derived data is the subject of ongoing investigations.

The excellent depth match is most evident in the high-depth-resolution curves of the magnetic susceptibility (MSUS), which almost overlay each other. The average depth difference is 0.2 m, which is in the order of the sample spacing of 0.1 m. Furthermore, the four parameters of the natural gamma ray spectrum sonde mSGR (total SGR; potassium, K; uranium, U; and thorium, Th) show a very good depth correlation of memory and wireline curves. All mSGR curves are corrected for the small hole size, and the memory curves are additionally corrected for the attenuation by the drill pipe. Slight differences in the curve shapes are due to the statistical nature of the natural gamma radiation as a product of radioactive decay. The P-wave velocity curves (Vp) of the sonic tool also show a good log–log correlation except for small divergences in a few short zones resulting from the difficult-to-process sonic waveform data.

The depth differences are less than 0.1 m along the 100 m long section. This demonstrates the ability of the iMLS depth devices to determine the movement of the sonde string in the borehole with the same accuracy and resolution as the classic wireline method. However, it also shows how immensely important the reliability of the exact starting depth reported by the driller is for LWT, without which an absolute LWT depth determination is impossible; for example, in lake drilling projects, the drop in the total GR at the lake floor can sometimes be used as a reference, whereas in onshore drilling situations, this is not the case. As described above, differences of up to several meters between the bit depth notified by drillers and the wireline logger depth are common. For this reason, whenever possible, a wireline log of total gamma ray and magnetic susceptibility should be carried out before an iMLS run. Even if only a short MSUS log length in the open hole is possible, the gained short profile can be used for depth correlation. A single total gamma ray would allow for a correlation, albeit a coarser one, in a case where a wireline sonde cannot run out of the bit at all. Logistically, a wireline pre-measurement will be possible in cases where the iMLS serves as a backup for the primary preferred wireline measurements. This procedure is not possible in strongly inclined bores. Ultimately, even without an absolute depth reference (e.g., wireline depth), the LWT depth will instantaneously match the core depth, as both refer to the driller's depth.

Although our system is now basically ready for regular use in HQ and PQ holes, we will use both our iMLS and wireline logging systems in upcoming projects to carry out further tests of the depth-recording device in comparison to wireline logging in order to improve the iMLS performance and to verify the observed accuracy of the memory depth measurement on a broader statistical basis.