b'2.5D AEM inversionFeatureSystem complexity may be helpful in separating an inductive from an IP response. A high level of AEM processing experience is often necessary toThe presence of IP effects commonly results in poor misfits in resolve the complexity of some of the problems encounteredan inductive inversion, and is a pointer to their presence. The when processing data from both old and modern AEM survey2.5D software can also forward model a complex inductive and systems. A lack of experience can lead to poor outcomes. InIP response, which may be helpful in understanding the geo-particular, considerable care is necessary when reviewing theelectric section geometry in such a situation.system setup, as recorded system parameters can vary from surveySPM effectsto survey. The more modern helicopter suspended loop systems have the ability to vary transmitter waveforms, receiver filtersSuper paramagnetic (SPM) effects are much less common and time gate positions depending on survey conditions. Errorsthan IP effects. They manifest as late time positive anomalies sometimes occur in documentation, and detecting these errorswith a characteristic slow decay rate (1/time). Due to their requires a combination of experience and multiple test runs priorslow late-time decay, SPM responses can be confused with the to commencing full survey inversions. Well-designed work flowsresponses of deep conductors and vice versa. SPM effects are that formalise a series of checks are essential for a good outcome. best recognised by their decay rate, and also by their fast falloff Geological noise with increased survey elevation. The latter attribute means they are less commonly observed in fixed wing AEM surveys. Typical Conductive cover SPM decays are shown in red, green and blue, and compared with more shallow inductive decays in yellow and pink, in Conductive cover can pose serious limitations on an AEMFigure 23.systems ability to detect a buried conductor. In simple terms, a thicker and/or more conductive surficial cover (regolith) limits the detection depths of an AEM system. It is important to understand this attribute of a survey environment before planning and conducting an AEM survey, particularly in Australia. Existing AEM surveys can be informative, and forward modelling particular geological targets within known regolith and geological environments, such as demonstrated by the Elura example, can be very useful, saving time and money.IP effectsIP effects can be serious problem in AEM surveys, and in some cases can completely mask an AEM systems ability to detect a late time conductive response. This is often caused by the response of near surface clays to the AEM systems transmitted signal. A more powerful transmitted signal can increase the IP response. The IP response is opposite in sign to the inductive response and causes pulldown (a decrease in the measured response) at mid to late decay times where the positive response of a deep conductive target is expected. Hence, it can completely overpower theFigure 23.Typical SPM decays in red, green and blueinductive response and in this case, there is no reliable way to recover the target. A typical IP decay is shown in Figure 22.It is important to try to identify whether this might be aSPM effects are usually caused by very fine magnetite problem in a survey area. The 2.5D inversion software is capableaccumulations in surface soils or rocks (magnetite deposits). The of inverting for the inductive and IP responses jointly, which2.5D inversion software does not fit this type of anomaly due its slow decay rate. Care needs to be taken that the misfit failure is not caused by the inversion being run at too low a resolution.Correlated noiseCorrelated noise at late time can be a serious problem for 2.5D inversions. The 2.5D inversion sees relatively long wavelength (300 to 500 m) correlated noise at late time as signal, and fits it accordingly. This can produce a series of conductive blobs instead of either a deeper flat lying conductor or, alternatively, no conductor at all. This late time noise can have amplitudes well above noise levels, and has a negative impact on inversion quality. It can be removed by lateral smoothing, but this runs the risk of removing real late time anomalies of similar wavelength. This has been seen in some older surveys where it has been removed in 1D inversions by strong lateral smoothing. The source of the noise appears to be related to loop motion or swing. An example of this type of noise before and after lateral Figure 22.A typical IP decay curve. smoothing appears in Figure 24.45 PREVIEW AUGUST 2020'