3.10 The EVENT entity

A wide range of measurements, observations or interpretations can be made relating to a groundwater_feature. Figure 3.5 shows the more common types of data that are collected from a groundwater_feature.

During construction of a bore, the lithology may be logged by a geologist inspecting the cuttings collected at defined intervals down hole by the driller. Cuttings from a few of these intervals may be selected and bagged by the geologist and dispatched for more detailed palynological analysis to provide information on stratigraphy or depositional environment. A suite of geophysical wireline logs may also be run to support the stratigraphic interpretation and to optimise the position of the well screen. All these types of data are similar in the sense that space (eg. the down hole sampling position) is important and the time of data collection is less critical. They are typically called data logs. What does differentiate these data sets is spatial resolution - for the geophysical logs the sampling frequency is high, for the palynological samples the sampling frequency is low.

For groundwater measurements derived from a constructed bore, the relationship between time and space is reversed. As the position of the groundwater_source (eg. well screen) typically remains constant during data collection, space is not so critical but time is. These types of measurements are typically called time series data. This includes measurements of the groundwater level within a bore over time (ie. a bore hydrograph) as well as periodic sampling of groundwater for detailed chemical analysis. Resolution is the feature that differentiates the datasets, but not in space as in the case of data logs but in time. The output from an automatic water level recorder has a high sampling frequency through time, while ad-hoc groundwater chemistry samples have a low sampling frequency.

Somewhere between these two end members of time series data and data logs is the example of a down hole survey of an open hole using a salinity probe. This type of data collection can be used in the investigation of seawater intrusion in coastal aquifers. Under equilibrium conditions, changes in salinity down the hole are assumed to reflect the vertical salinity profile in the aquifer. Hence, space defined by the position of the probe in the hole is critical. The survey may be conducted on a routine basis (eg. weekly), to monitor fluctuations in the position of the seawater-groundwater interface. Hence, time is also important.

The following entities of event, sample, parameter and result (Sections 3.10 to 3.13) have been defined to accommodate this spectrum of data that can be collected in both space and time from a groundwater_feature.

Measurements or observations can be made throughout the history of a groundwater_feature. Each discrete episode of data collection is termed an event, and a groundwater_feature can have many such events. For example, a water cut intersected during drilling of a bore can be tested and information on yield, salinity and water levels commonly collected. A lithological description and a stratigraphic interpretation from the drill cuttings can be made by the geologist. A suite of geophysical wireline logs may be run down the hole to determine optimum placement of the well screen. An aquifer test on the constructed bore gives valuable information in terms of storativity, transmissivity or optimum pumping depth from interpretation of flow and water level data. The water level in a piezometer may be measured and a water sample taken for chemical analysis. All these episodes of data collection are termed events.

An event involves the collection of data. This data may:

• be about properties of the groundwater_feature itself, the surrounding hydrogeological material, the in-situ groundwater or the ambient conditions evident during the event; and
• include actual observations or measurements made in the field during the event, later analyses undertaken in the laboratory, or parameters interpreted from subsequent data analysis.

Table 3.11 Attributes of the event entity

Data can be collected from groundwater_features as part of a particular work program designed with a particular program_objective in mind. Many investigation bores may be drilled and tested during the assessment of the suitability of an aquifer for a town water supply. A network of piezometers may be constructed and used to monitor groundwater levels in a catchment. An investigation of a contaminated site may involve a concentrated effort of drilling, sampling, monitoring and analysis of many test holes. Hence, an event may be part of a larger program of work. A program may involve many groundwater_features with many events occurring for each groundwater_feature. In some cases, a data collection event is associated with a construction_activity, linked by the activity_number.

Each event is uniquely defined by the combination of event_feature_identifier and event_number. An event involves a person from an organisation who is responsible for measurements or observations on behalf of a client. For bailer tests conducted during bore construction, the person would be the driller from the drilling company who is finding a water supply for a farmer. At a contaminated site the person would be the geologist from a consultancy with the role of onsite management and undertaking an assessment on behalf of the site owners. The event occurs over a time period defined by the start_date and finish_ date. This time period may be relatively short such as an ad-hoc water level measurement from a piezometer, or it may be quite long, or even on-going, as in the case of the duration of data collection from an automatic water level recorder established in a monitoring bore.

The nominal interval in both space and time that data is collected during an event can be recorded using the resolution and frequency attributes, respectively. For example, the resolution of a lithological log may be one-metre intervals down the hole. A water level monitoring event may have a frequency of data collection of one measurement every hour. In other cases, monitoring may be irregular or triggered by special circumstances such as rainfall. Various attributes (bibliographic_source, custodian, archive, format, and access) are used to describe the nature and availability of data that has been generated from the event.

Figure 3.5 Common types of data collected from a groundwater_feature