GeoVista
Unit 6,
Cae Ffwt Business Park
Glan Conwy
Conwy,  LL28 5SP  
United Kingdom
T: +44 (0) 1492 57 33 99
F: +44 (0) 1492 58 11 77
E: geovista@geovista.co.uk

News and Views

• July, 2004 - 1550 mm Calliper for Pile Borehole Logging • April, 2004 - 3000 m  logging system delivered for deep aquifer exploration programme. • March, 2003 - New Sub allows for simultaneous Resistivity + Calliper  Logging and an early return to base • February, 2002 - Completion of first Micro-Velocity probe (MVP) • September, 2001 - First order In-line Temperature & Conductivity Sonde • August, 2001 - User Functions While Logging • July, 2001 - New improved 7-electrode Conductivity Sonde • May, 2001 - Scott Polar Research Institute chooses Geovista logger for Arctic borehole logging • April, 2001 - Dual Guard Log Sonde • January, 2001 - Fully Modula Sonic Sonde • November, 2000 - High Reliability Bridle • February, 2000 - Water & Gas sampler • April, 1999 - Borehole Video Imager • February, 1999 - Downhole Temperature Measurements • January, 1998 - On Water Quality Logging • November, 1998 - An affordable logger for shallow boreholes • September, 1998 - New generation Logging Systems
July, 2004 1550 mm Calliper for Pile Borehole Logging Geovista did not shy away from the challenge when they were asked by a client for a version of their 4-arms calliper with an extended range to measure diameters of up to 1550 mm. The calliper, with 4-independent arms measuring radii x1, x2, y1 and y2 was supplied with reinforced springs to resist the increased loading at the arm tips.
April, 2004 3000 m   logging system delivered for deep aquifer exploration programme. A logging system with 3000 m depth capability was delivered to log exploration boreholes to evaluate deep groundwater resources in the desert. According to the client, the technical evaluation of competing systems left them "pretty much with the Geovista logging system on its own". Winning points included sonde combination possibilities and an exceptionally good winch with useful safety features. Top March, 2003 New Sub allows for simultaneous Resistivity + Calliper  Logging and an early return to base A new "in-line" isolator sub now allows Geovista Logging System users to have a Calliper together with Resistivity in the same sonde stack.  Previously, the difficulty with preventing the calliper from acting as an electrode made this particularly important combination impracticable. A calliper log is almost invariably requested together with a Natural Gamma Ray, SP and Resistivity logs in newly drilled water wells. This meant that logging services providers had to make a minimum of two two trips in the hole. With this new device, Geovista Logging System users can do the job in one run, save their clients precious rig time and to base even earlier. Top February, 2002 Completion of first Micro-Velocity probe (MVP) The first Geovista Micro-Velocity probe was delivered to INERIS School of Mines. The sonde will be used to interval sonic velocities within boreholes  in rocks or concrete structures as an aid to safety analysis. Both P- and S-wave velocity measurements are are performed to allow for the analysis of rock microfracture density, saturation and stress state  over very short ray paths of 10 to 30 cm. These measurements are particularly useful in analysing damaged zones around excavations, or through rock pillars, and in monitoring changes in these zones over time. September, 2001 First order In-line Temperature & Conductivity Sonde Geovista received from Hydro-Assistance of France an order for the first In-line Temperature & Conductivity sonde. This would allow Hydro-assistance to log these parameters simultaneously with a flowmeter during pump testing. Top August, 2001 User Functions While Logging Version 3.0 and version 4.0 (USB comms) of Geovista Platform Logging data acquisition software now includes the option of applying user functions to produce and record computed log traces in real time, while logging. For example one can conceivably produce such log traces as normalised (to 25ºC) conductivity values, borehole volume, linear or volumetric flow rates, etc.. The operator is naturally free to enter their own formulae to suit their data requirement. Top July, 2001 New improved 7-electrode Conductivity Sonde A straight forward method of measuring fluid conductivity in boreholes is to use a sonde with 3 circular electrodes. The upper and lower electrodes are connected to chassis potential (the same as the cable armour) and the centre electrode has a constant alternating polarity voltage applied to it. The electronics measure resulting current that flows from the centre electrode. This current is proportional to the fluid conductivity. The problem with this approach is the electrode contact resistance whose value would change due to fouling Change in electrode resistance would cause an change in measured current and hence an error in measured conductivity. A better method as adopted by Geovista consists of using two additional pairs of "monitoring" electrodes. In this configuration, the monitoring electrodes are placed between the centre drive electrode and each of the two grounded electrodes. The voltage between the monitoring electrodes is measured using a high input resistance amplifier. This measurement is then used to control the drive applied to the centre electrode so that the voltage measured across the monitor electrodes is kept constant. The very high input resistance of the monitor amplifier means that we can tolerate quite a high electrode resistance at the monitoring electrodes. For example, adding a series resistor of 1kW in series with a 1MW measure circuit will introduce an error of 1 part in 1,000. This sort of error in the 3 electrode cell would simply produce meaningless results. This 7 electrode configuration allows for reliable measurement of fluid conductivity under a wide range of non-ideal conditions.
Top May, 2001 Scott Polar Research Institute chooses Geovista logger for Arctic borehole logging Most climate model simulations predict enhanced warming in the Arctic as a result of increased concentration of greenhouse gases. The Scott Polar Research Institute (Cambridge -UK) chose a Geovista logger to help them collect complementary data from ice borehole on Devon Island Ice Cap in the Arctic as part of a study of the variations of surface mass balance.   Variations in surface mass balance are linked to climatic changes. Data derived from the ice density profile of the upper 70m of the ice cap is used to measure the changing state of the Devon ice cap, monitor the effect of climate change and determine its contribution to sea level change.
April, 2001 Dual Guard Log Sonde A Dual Guard log is the ideal answer to the 64" Long Normal resistivity reading aberrations against thin beds (sounds familiar?). It offers both a shallow and a deep focussed resistivity reading with a superior vertical resolution making resistivity log interpretation that mush easier. As to whether this spells the end of the Long & Short Normal Resistivity sonde, remains to be seen. Results from field trials under various conditions are eagerly awaited
January, 2001 Fully Modular Sonic Sonde The Geovista sonic sonde is now reduced to a set of spacer and transducer modules to be connected at will. This is a welcome development, away from fixed configurations and weighty, oversize sondes   November, 2000 High Reliability Bridle Geovista are pleased to announce the introduction of a new high reliability bridle, developed with the assistance of Vector-Schlumberger cables. This bridle will be supplied with all new resistivity sondes. It is also available for sale to existing Geovista resistivity sonde owners Top
February, 2000 Water & Gas Sampler The Geovista sampler sonde comes with a motorised valve system section that couples with a variety of sample vessels. Another valve system allows for samples to be retrieved under downhole conditions. The sonde is available in three models according to the purpose of sampling, be it water hydrochemistry, trace metals or dissolved gases. Care is taken in the design to avoid specific sample contamination as required by the end-user. Being a Geovista sonde, the sampler is naturally combinable with other logging sondes. In practices, this means that within the same run, one can conceivably record other logs and take multiple samples at different depths. Samples can also be taken with confidence, with exceptional depth accuracy by correlating.
April, 1999 Borehole Video Imager We are pleased to announce that addition of a Borehole Video Imager (BVI) to our range of sondes. The tool provides continuous logs of oriented, unwrapped images of borehole walls. Further processing allows for the computation and display of standard information on fractures and other geological features. The software set includes MakiCam for data acquisition, MakiCad for image log processing, and MakiVision for image viewing, generation of 3D cores and printing. Unlike the acoustic televiewer, the BVI can be run in both air filled and (clear) water filled holes.   Top

February, 1999
Downhole Temperature Measurements
Formation temperature increases predictably with depth. This geothermal gradient G is usually in the range of 1 to 3°C per 100m. Thus, the temperature at any depth can be estimated from Tform = Tsurf + Gx Depth where Tsurf = mean annual surface temperature (Typical range is 16 to 2l° C). Under steady conditions, the temperature in a borehole tends to assume the ambient temperature of the surrounding formation. Thus, a log of Temperature against Depth should indicate the local geothermal gradient. Any departures from the geothermal gradient are picked out as "anomalies". These usually require further interpretation.
Borehole temperature measurements are important in several areas of underground resource investigation and management. In mineral exploration, it allows for the detection of masive sulphide mineralisation. In hydrogeology, temperature variations can be a key element to the understanding of groundwater flow. Departures from the geothermal gradient are usually indicators of fluid movement. They tend to be proportional to the mass flow rate of the liquid present. The Differential Temperature curve is used to accentuate the occurrences of changes.

Temperature sensors
There are mainly three types of devices that can be used in downhole logging sondes:

1- Semiconductor devices
These are special circuits that have a known and predictable relationship between their output (either DC voltage or current) and temperature. An accuracy of better that 1° C is usually difficult to achieve with these. Their mass gives them a modest time response to temperature changes.

2- Thermocouple devices
These have a known and predictable relationship between the DC voltage generated at the junction and temperature. Achieving the required accuracy is complicated because a "cold junction" reference is required. This can be difficult in a downhole sonde where the ambient temperature changes. Their low mass allows for a rapid response to temperature changes.

Note: All dissimilar metallic junctions are thermocouples. This includes the connections where wires join a printed circuit board and where the integrated circuit pins make contact with the board. In a DC measurement system the only way to minimize these effects is to ensure that all such junctions are at the same temperature and that the number and type of connections is the same in both sides of the measurement circuit. The alternative method is to use AC excitation. In this scenario, the thermocouple voltages cancel themselves out between the positive and negative excursions of the measurement cycle.

3- Platinum resistance thermometer devices (PRT).
These thin-film Platinum resistive elements have a resistance that follows a well defined (non-linear) relationship with temperature. They are available conforming to IEC 751:1983 (BS EN 60751:1996) Class A or Class B. The class A will give an absolute error of less than 0.45° C over the range of interest. Class B will be accurate within 0.8° C. A further standard, known as 1/10DIN, will give an error of less than 0.2° C over the range of interest.The long term stability of the PRT is better than that achieved with other devices, certainly better than that of thermocouples.

GeoVista Temperature probe design
The Geovista probe uses a PRT with AC excitation and a ratiometric measurement system. The users can select the PRT sensor grades to suit their needs. The 1KHz AC excitation signal is applied to the PRT device and to a series combination of a 0.01% resistances with a temperature coefficient of 1ppm/° C. A single integrated circuit then acquires the signals and computes the ratio of the resistance values to a resolution of 20 bits.

The absolute value of the AC excitation is unimportant, as long as it stays within a quite broad tolerance band. Any drift with temperature is cancelled out in the ratio circuit and any parasitic thermocouple voltages will cancel out due to the AC excitation. The only important factors are the accuracy of the PRT, the stability of the reference resistance and the stability of the ADC circuits. Factory calibration, easily reproduced in the field, is by means of a set of precision 0.01% resistances inserted in place of the PRT. Resistors designed to reproduce specific temperatures are available, including standard values of 100 Ohm, equivalent to 0° C, and 138.5 Ohm, equivalent to 100° C.

Electronic measurement system drift with temperature
With a 100 Ohm resistance installed in place of the PRT, representing 0°C and the electronics heated independently, the error curve presented in Fig 1 was achieved over the temperature range.

Speed of response.
This was measured by placing the PRT into a water bath with a temperature difference from ambient. The time constant (time required to reach 63% of the final value) was measured as 3.5 seconds.

Summary
The above tests and descriptions show how the Geovista temperature sonde can achieve a temperature accuracy of +/- 0.5 ° C using a PRT Class A measurement sensor, and an accuracy better than +/- 0.3° C using the 1/10DIN sensor.
Semiconductor probes were excluded as the absolute temperature accuracy required was not readily achieved with this technology, due in part to the fact that it is a DC measurement method. Thermocouples were excluded mainly because of the parasitic thermocouples that are created and the need for a cold junction reference.

Top

January, 1999
On Water Quality Logging
More often than not, water quality probes are used for monitoring changes in water quality over time, or for spot readings at discrete locations.These operations tend to be quite separate from traditional "geophysical" logging . Instrumentation is usually supplied by companies specialising in water chemistry. With increasing concerns about underground water quality and protection strategies, hydrogeologists now seek more understanding from continuous logs, profiling the changes in water quality and flow against depth and formation in boreholes. This implies a combination of both traditional wireline logs and water chemistry logs.
In response to this requirement, Geovista now supplies an adapted version of the Hydrolab™ MiniSonde™ that supports a wide variety of water quality sensors for among others, Nitrates, Ammonium, Salinity, pH, ORP, Dissolved Oxygen, Total Dissolve Gas, Turbidity, Conductivity, Temperature, etc. Rated to 20 bars, this sonde can typically accommodate up to 4 different sensors at a time.For greater pressures, we also supply the Idronaut 302™ probe rated to 150 bars. This sonde can typically accommodate up to 6 different sensors at a time.


November, 1998
An affordable logger for shallow boreholes
The new GV110 Junior is a high specification logger with a manual winch, initially developed for shallow site investigation and mineral exploration boreholes. It subsequently found additional applications in logging the large number of low budget shallow water boreholes which hitherto, were not logged for cost reasons. A complete system with a 38 mm or 28mm O.D. Natural Gamma sonde costs only £6,850.00

Top

September, 1998
New generation Logging Systems
Final conclusive tests were completed during the last two months on our new generation logging systems. These latest systems stem from our extensive experience both in the oil industry and the water\mineral\geotechnical fields.The innovative variable measurement resolution (8, 16, 24 or 32 bit) makes optimal usage of cable bandwidth, allowing for the provision of a low cost surface data acquisition system when compared with older fixed resolution telemetry systems. An important benefit of this is the elegant ability to interface to most modern digital mono-cable sondes and older pulse analogue sondes from other manufacturers.
The loggers use an RS232 PC interface and can either be packaged in a separate housing or integrated in a rack system. A variety of winches and stackable sondes respectively allow for a choice of depth capability and a flexible make up of sensors. The option of stacking sondes allows for cost benefits both when purchasing the equipment and when operating it. In deep boreholes, time is saved as multiple measurements can be made in a single logging run. In shallower boreholes, there is the added benefit of the generally smaller physical size.

Cost savings are a common thread throughout our thinking. We are committed to providing reliable and affordable downhole data acquisition equipment to our customers.

Top