b'Minerals geophysics Minerals geophysicsImaging: Vertical layers and boundariesseismic surveys to environments where cannot be imaged directly bylayering is to be expected, what can be conventional, straight wave-pathdone to address this?reflection seismics. With the energyFrom an interpretation viewpoint, source and the geophone receivers alla change of mind-set is needed to located on a level ground surface, itsacknowledge that an apparent paucity just not possible - it is simply a matter ofof sub-vertical structures in a seismic geometry (see Figure 1). Unless surveysection may merely be a function of procedures and arrays are speciallylimitations in the seismic method itself. adapted, this non-imaging problemPerhaps we need to be open to accepting effectively applies to steeply dippingsubtler evidence for such structures. layers and boundaries too. The dipWe could also draw on the considerable Terry Harveyof layers and boundaries that can beeffort that has gone into interpreting Associate Editor directly detected can be expandedthe steep dip environments associated for Minerals geophysics by displacing the geophone array inwith salt tectonics and diapirism, and the terry.v.harvey@glencore.com.au the down dip direction, but there arework the oil and gas industry is doing on practical limitations to how far this canrecognition and significance of different be taken. Also, as dip angles increase,seismic textures. Steeply dipping Hard rock seismics azimuthal direction effects increase,boundaries between regions of different implying that 3D arrays are essential forrock types may manifest as contrasts in The application of the seismic methodcomplex hard rock environments.to hard rock mineral exploration is astyles of seismic textures, and thus be developing field, and Preview PetroleumRecognition: Oil and gas explorationmapped.geophysics Associate Editor Michaelseismic section patterns typicallyFrom an acquisition viewpoint, the Micenko has devoted two recent articles tocomprise pervasive sub-horizontalsurvey array needs to be expanded it (see Seismic Window in Preview Issues 210reflectors mapping out the layered,into the third dimension, with some and 213). Mineral exploration geophysicsrelatively flat-lying stratigraphy. Steeplygeophones or shot points located has benefited greatly from experiencedipping structures are then inferred byat depth below the ground surface, gained in oil and gas explorationtracing out the disruptions, dislocationsand with the survey procedures and geophysics, and the seismic methodand truncations of these sub-horizontalprocessing regimes modified accordingly. is definitely the domain of oil and gasreflectors. When most of these reflectorsGiven that many hard rock seismic exploration. But this experience has beenare absent, as is often the case in hardsurveys are carried out at mine sites, with almost entirely gained in sub-horizontallyrock environments, recognition andthe option to position geophone strings layered soft rock environments. Here I looktracing of sub-vertical structures is notin boreholes or mine openings, this is at some aspects of seismics from a hardpossible. not as far-fetched as it sounds. Specialist rock mineral exploration perspective. The end result of these factors is thatseismic survey companies are already The seismic method utilises boundariessub-vertical reflectors will not normallyworking on this.between rocks of contrasting acousticbe present in the seismic results. AndAs I wrote at the beginning, this is a very impedance (acoustic impedance = s eismicin a complex hard rock, non-layeredbasic look at hard rock seismics. The velocity x density). The reflection seismicenvironment, in the absence of extensivemethod is far, far more sophisticated method has resolution at depth superiorsub-horizontal reflectors, inferring thethan a mere mapping of reflectors. Would to the other geophysical techniques usedpresence of any sub-vertical structuresI consider using 3D seismics? Absolutely! in mineral exploration, has excellent depthwill be that much more difficult. ThisBut Id make sure that everyone involved penetration, and offers the opportunity todisparity can bias the interpretation. was fully aware of the difficulties that characterise hitherto unused rockThe question then is, short of acceptancesteeply dipping boundaries in non-properties in situ. Furthermore, metallicof this limitation and putting it in thelayered, complex and disordered hard mineralisation and altered rock types cantoo hard basket, or restricting hard rockrock environments can pose.exhibit strong acoustic impedance contrasts with country rock. However, hard rock environments, typically those with steeply dipping (750-900) irregular boundaries and disruptive structures, can be far more complex and disordered than the layered earth environments commonly encountered in soft-rock exploration. It is these steeply dipping features which pose particular challenges to the seismic method.To me, there appear to be two aspects to the problem of steeply dipping featuresimaging and recognition. Figure 1.Sample wave-paths for horizontal, dipping and vertical boundaries.35 PREVIEW FEBRUARY 2022'